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Thursday, May 23, 2019

Java Collection Interview Questions and Answers For Freshers & Experienced

Java Collections Interview Questions and Answers
What are Collection related features in Java 8?

Java Stream API for collection classes for supporting sequential as well as parallel processing

Iterable interface is extended with forEach() default method that we can use to iterate over a collection. It is very helpful when used with lambda expressions because it’s argument Consumer is a function interface.

Miscellaneous Collection API improvements such as forEachRemaining(Consumer action)method in Iterator interface, Map replaceAll(), compute(), merge() methods.

What is Java Collections Framework? List out some benefits of Collections framework?

Reduced development effort by using core collection classes rather than implementing our own collection classes.

Code quality is enhanced with the use of well-tested collections framework classes.

Reduced effort for code maintenance by using collection classes shipped with JDK.

Reusability and Interoperability.

What is the benefit of Generics in the Collections Framework?
What are the basic interfaces of the Java Collections Framework?
Why Collection doesn’t extend Cloneable and Serializable interfaces?
Why Map interface doesn’t extend Collection interface?
What is an Iterator?
What is the difference between Enumeration and Iterator interface?
Why there is not method like Iterator.add() to add elements to the collection?
Why Iterator doesn’t have a method to get the next element directly without moving the cursor?

What is different between Iterator and ListIterator?

We can use Iterator to traverse Set and List collections whereas ListIterator can be used with Lists only.

Iterator can traverse in forwarding direction only whereas ListIterator can be used to traverse in both the directions.

ListIterator inherits from Iterator interface and comes with extra functionalities like adding an element, replacing an element, getting index position for previous and next elements.

What are different ways to iterate over a list?

List<String> strList = new ArrayList<>();

//using for-each loopfor(String obj : strList){ System.out.println(obj);}

//using iteratorIterator<String> it = strList.iterator();while(it.hasNext()){ String obj = it.next(); System.out.println(obj);}

What do you understand by iterator fail-fast property?
What is the difference between fail-fast and fail-safe?
How to avoid ConcurrentModificationException while iterating a collection?
Why there are no concrete implementations of the Iterator interface?
What is UnsupportedOperationException?
How HashMap works in Java?

What is the importance of hashCode() and equals() methods?

If o1.equals(o2), then o1.hashCode() == o2.hashCode()should always be true.

If o1.hashCode() == o2.hashCode is true, it doesn’t mean that o1.equals(o2) will be true.

Can we use any class as Map key?

If the class overrides equals() method, it should also override hashCode() method.

The class should follow the rules associated with equals() and hashCode() for all instances. Please refer an earlier question for these rules.

If a class field is not used in equals(), you should not use it in the hashCode() method.

Best practice for user defined key class is to make it immutable, so that hashCode() value can be cached for fast performance. Also, immutable classes make sure that hashCode() and equals() will not change in future that will solve any issue with mutability.

For example, let’s say I have a class MyKey that I am using for HashMap key.

//MyKey name argument passed is used for equals() and hashCode()MyKey key = new MyKey("Pankaj"); //assume hashCode=1234myHashMap.put(key, "Value");

// Below code will change the key hashCode() and equals()// but it's location is not changed.key.setName("Amit"); //assume new hashCode=7890

//below will return null, because HashMap will try to look for key//in the same index as it was stored but since key is mutated, //there will be no match and it will return null.myHashMap.get(new MyKey("Pankaj"));

Java Collections Interview Questions
What are different Collection views provided by Map interface?

Set<K> keySet(): Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator’s own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator. remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

Collection<V> values(): Returns a Collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. If the map is modified while an iteration over the collection is in progress (except through the iterator’s own remove operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator. remove, Collection.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

Set<Map.Entry<K, V>> entrySet(): Returns a Set view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator’s own remove operation, or through the setValue operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator. remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

What is the difference between HashMap and Hashtable?

HashMap allows null key and values whereas Hashtable doesn’t allow null key and values.

Hashtable is synchronized but HashMap is not synchronized. So HashMap is better for the single-threaded environment, Hashtable is suitable for a multi-threaded environment.

LinkedHashMap was introduced in Java 1.4 as a subclass of HashMap, so in case you want iteration order, you can easily switch from HashMap to LinkedHashMap but that is not the case with Hashtable whose iteration order is unpredictable.

HashMap provides Set of keys to iterate and hence it’s fail-fast but Hashtable provides Enumeration of keys that doesn’t support this feature.

Hashtable is considered to be legacy class and if you are looking for modifications of Map while iterating, you should use ConcurrentHashMap.

How to decide between HashMap and TreeMap?
What are the similarities and difference between ArrayList and Vector?

Both are index based and backed up by an array internally.

Both maintain the order of insertion and we can get the elements in the order of insertion.

The iterator implementations of ArrayList and Vector both are fail-fast by design.

ArrayList and Vector both allow null values and random access to an element using index number.

Vector is synchronized whereas ArrayList is not synchronized. However, if you are looking for a modification of list while iterating, you should use CopyOnWriteArrayList.

ArrayList is faster than Vector because it doesn’t have any overhead because of synchronization.

ArrayList is more versatile because we can get synchronized list or read-only list from it easily using Collections utility class.

What is the difference between Array and ArrayList? When will you use Array over ArrayList?

If the size of the list is fixed and mostly used to store and traverse them.

For a list of primitive data types, although Collections use autoboxing to reduce the coding effort still it makes them slow when working on fixed size primitive data types.

If you are working on the fixed multi-dimensional situation, using [][] is far easier than List<List<>>

Java Collections Interview Questions
What is the difference between ArrayList and LinkedList?

ArrayList is an index based data structure backed by Array, so it provides random access to its elements with performance as O(1) but LinkedList stores data as a list of nodes where every node is linked to it’s previous and next node. So even though there is a method to get the element using an index, internally it traverse from start to reach the index node and then return the element, so performance is O(n) that is slower than ArrayList.

Insertion, addition or removal of an element is faster in LinkedList compared to ArrayList because there is no concept of resizing array or updating the index when an element is added in middle.

LinkedList consumes more memory than ArrayList because of every node in LinkedList stores reference of previous and next elements.

Which collection classes provide random access to its elements?
What is EnumSet?
Which collection classes are thread-safe?
What are concurrent Collection Classes?

Avoid ConcurrentModificationException

CopyOnWriteArrayList Example

HashMap vs ConcurrentHashMap

What is BlockingQueue?
What is Queue and Stack, list their differences?
What is Collections Class?
What is the Comparable and Comparator interface?
What is the difference between Comparable and Comparator interface?
How can we sort a list of Objects?
While passing a Collection as an argument to a function, how can we make sure the function will not be able to modify it?
How can we create a synchronized collection from the given collection?
What are common algorithms implemented in Collections Framework?
What is Big-O notation? Give some examples?
What are the best practices related to the Java Collections Framework?

Choosing the right type of collection based on the need, for example, if the size is fixed, we might want to use Array over ArrayList. If we have to iterate over the Map in order of insertion, we need to use TreeMap. If we don’t want duplicates, we should use Set.

Some collection classes allow to specify the initial capacity, so if we have an estimate of a number of elements we will store, we can use it to avoid rehashing or resizing.

Write a program in terms of interfaces, not implementations, it allows us to change the implementation easily at later point of time.

Always use Generics for type-safety and avoid ClassCastException at runtime.

Use immutable classes provided by JDK as key in Map to avoid implementation of hashCode() and equals() for our custom class.

Use Collections utility class as much as possible for algorithms or to get read-only, synchronized or empty collections rather than writing own implementation. It will enhance code-reuse with greater stability and low maintainability.

Java Collections Interview Questions
What is Java Priority Queue?
Why can’t we write code as List<Number> numbers = new ArrayList<Integer>();?

List<Long> listLong = new ArrayList<Long>();listLong.add(Long.valueOf(10));List<Number> listNumbers = listLong; // compiler errorlistNumbers.add(Double.valueOf(1.23)); Why can’t we create generic array? or write code as List<Integer>[] array = new ArrayList<Integer>[10];

List<Integer>[] intList = new List<Integer>[5]; // compile errorObject[] objArray = intList;List<Double> doubleList = new ArrayList<Double>();doubleList.add(Double.valueOf(1.23));

objArray[0] = doubleList; // this should fail but it would pass because at runtime intList and doubleList both are just List

Java 8 has brought major changes in the Collection API. Some of the changes are:

Collections are used in every programming language and initial java release contained few classes for collections: Vector, Stack, Hashtable, Array. But looking at the larger scope and usage, Java 1.2 came up with Collections Framework that group all the collections interfaces, implementations and algorithms.
Java Collections have come through a long way with usage of Generics and Concurrent Collection classes for thread-safe operations. It also includes blocking interfaces and their implementations in java concurrent package.
Some of the benefits of collections framework are;

Java 1.5 came with Generics and all collection interfaces and implementations use it heavily. Generics allow us to provide the type of Object that a collection can contain, so if you try to add any element of other type it throws compile time error.
This avoids ClassCastException at Runtime because you will get the error at compilation. Also Generics make code clean since we don’t need to use casting and instanceof operator. I would highly recommend to go through Java Generic Tutorial to understand generics in a better way.

Collection is the root of the collection hierarchy. A collection represents a group of objects known as its elements. The Java platform doesn’t provide any direct implementations of this interface.

Set is a collection that cannot contain duplicate elements. This interface models the mathematical set abstraction and is used to represent sets, such as the deck of cards.

List is an ordered collection and can contain duplicate elements. You can access any element from it’s index. List is more like array with dynamic length.

A Map is an object that maps keys to values. A map cannot contain duplicate keys: Each key can map to at most one value.

Some other interfaces are `Queue`, `Dequeue`, `Iterator`, `SortedSet`, `SortedMap` and `ListIterator`.

Collection interface specifies group of Objects known as elements. How the elements are maintained is left up to the concrete implementations of Collection. For example, some Collection implementations like List allow duplicate elements whereas other implementations like Set don’t.
A lot of the Collection implementations have a public clone method. However, it does’t really make sense to include it in all implementations of Collection. This is because Collection is an abstract representation. What matters is the implementation.
The semantics and the implications of either cloning or serializing come into play when dealing with the actual implementation; so concrete implementation should decide how it should be cloned or serialized, or even if it can be cloned or serialized.
So mandating cloning and serialization in all implementations is actually less flexible and more restrictive. The specific implementation should make the decision as to whether it can be cloned or serialized.

Although Map interface and it’s implementations are part of Collections Framework, Map are not collections and collections are not Map. Hence it doesn’t make sense for Map to extend Collection or vice versa.
If Map extends Collection interface, then where are the elements? Map contains key-value pairs and it provides methods to retrieve list of Keys or values as Collection but it doesn’t fit into the “group of elements” paradigm.

Iterator interface provides methods to iterate over any Collection. We can get iterator instance from a Collection using iterator() method. Iterator takes the place of Enumeration in the Java Collections Framework. Iterators allow the caller to remove elements from the underlying collection during the iteration. Java Collection iterator provides a generic way for traversal through the elements of a collection and implements Iterator Design Pattern.

Enumeration is twice as fast as Iterator and uses very less memory. Enumeration is very basic and fits to basic needs. But Iterator is much safer as compared to Enumeration because it always denies other threads to modify the collection object which is being iterated by it.
Iterator takes the place of Enumeration in the Java Collections Framework. Iterators allow the caller to remove elements from the underlying collection that is not possible with Enumeration. Iterator method names have been improved to make it’s functionality clear.

The semantics are unclear, given that the contract for Iterator makes no guarantees about the order of iteration. Note, however, that ListIterator does provide an add operation, as it does guarantee the order of the iteration.

It can be implemented on top of current Iterator interface but since it’s use will be rare, it doesn’t make sense to include it in the interface that everyone has to implement.

We can iterate over a list in two different ways – using iterator and using for-each loop.

Using iterator is more thread-safe because it makes sure that if underlying list elements are modified, it will throw `ConcurrentModificationException`.

Iterator fail-fast property checks for any modification in the structure of the underlying collection everytime we try to get the next element. If there are any modifications found, it throws `ConcurrentModificationException`. All the implementations of Iterator in Collection classes are fail-fast by design except the concurrent collection classes like ConcurrentHashMap and CopyOnWriteArrayList.

Iterator fail-safe property work with the clone of underlying collection, hence it’s not affected by any modification in the collection. By design, all the collection classes in `java.util` package are fail-fast whereas collection classes in `java.util.concurrent` are fail-safe.
Fail-fast iterators throw ConcurrentModificationException whereas fail-safe iterator never throws ConcurrentModificationException.
Check this post for CopyOnWriteArrayList Example.

We can use concurrent collection classes to avoid `ConcurrentModificationException` while iterating over a collection, for example CopyOnWriteArrayList instead of ArrayList.
Check this post for ConcurrentHashMap Example.

Iterator interface declare methods for iterating a collection but it’s implementation is responsibility of the Collection implementation classes. Every collection class that returns an iterator for traversing has it’s own Iterator implementation nested class.
This allows collection classes to chose whether iterator is fail-fast or fail-safe. For example ArrayList iterator is fail-fast whereas CopyOnWriteArrayList iterator is fail-safe.

`UnsupportedOperationException` is the exception used to indicate that the operation is not supported. It’s used extensively in JDK classes, in collections framework `java.util.Collections.UnmodifiableCollection` throws this exception for all `add` and `remove`operations.

HashMap stores key-value pair in `Map.Entry` static nested class implementation. HashMap works on hashing algorithm and uses hashCode() and equals() method in `put` and `get` methods.

When we call `put` method by passing key-value pair, HashMap uses Key hashCode() with hashing to find out the index to store the key-value pair. The Entry is stored in the LinkedList, so if there are already existing entry, it uses equals() method to check if the passed key already exists, if yes it overwrites the value else it creates a new entry and store this key-value Entry.

When we call `get` method by passing Key, again it uses the hashCode() to find the index in the array and then use equals() method to find the correct Entry and return it’s value. Below image will explain these detail clearly.

The other important things to know about HashMap are capacity, load factor, threshold resizing. HashMap initial default capacity is 16 and load factor is 0.75. Threshold is capacity multiplied by load factor and whenever we try to add an entry, if map size is greater than threshold, HashMap rehashes the contents of map into a new array with a larger capacity. The capacity is always power of 2, so if you know that you need to store a large number of key-value pairs, for example in caching data from database, it’s good idea to initialize the HashMap with correct capacity and load factor.

Java Collections Interview Questions

HashMap uses Key object hashCode() and equals() method to determine the index to put the key-value pair. These methods are also used when we try to get value from HashMap. If these methods are not implemented correctly, two different Key’s might produce same hashCode() and equals() output and in that case rather than storing it at different location, HashMap will consider them same and overwrite them.

Similarly all the collection classes that doesn’t store duplicate data use hashCode() and equals() to find duplicates, so it’s very important to implement them correctly. The implementation of equals() and hashCode() should follow these rules.

We can use any class as Map Key, however following points should be considered before using them.

This is the reason why String and Integer are mostly used as HashMap keys.

Map interface provides three collection views:

HashMap and Hashtable both implements Map interface and looks similar, however there are following difference between HashMap and Hashtable.

For inserting, deleting, and locating elements in a Map, the HashMap offers the best alternative. If, however, you need to traverse the keys in a sorted order, then TreeMap is your better alternative. Depending upon the size of your collection, it may be faster to add elements to a HashMap, then convert the map to a TreeMap for sorted key traversal.

ArrayList and Vector are similar classes in many ways.

These are the differences between ArrayList and Vector.

Arrays can contain primitive or Object whereas ArrayList can contain only Objects.
Arrays are fixed size whereas ArrayList size is dynamic.
Arrays don’t provide a lot of features like ArrayList, such as addAll, removeAll, iterator, etc.

Although ArrayList is the obvious choice when we work on the list, there are a few times when an array is good to use.

Java Collections Interview Questions

ArrayList and LinkedList both implement List interface but there are some differences between them.

ArrayList, HashMap, TreeMap, Hashtable classes provide random access to it’s elements. Download java collections pdf for more information.

`java.util.EnumSet` is Set implementation to use with enum types. All of the elements in an enum set must come from a single enum type that is specified, explicitly or implicitly, when the set is created. EnumSet is not synchronized and null elements are not allowed. It also provides some useful methods like copyOf(Collection c), of(E first, E… rest) and complementOf(EnumSet s).

Check this post for java enum tutorial.

Vector, Hashtable, Properties and Stack are synchronized classes, so they are thread-safe and can be used in multi-threaded environment. Java 1.5 Concurrent API included some collection classes that allows modification of collection while iteration because they work on the clone of the collection, so they are safe to use in multi-threaded environment.

Java 1.5 Concurrent package (`java.util.concurrent`) contains thread-safe collection classes that allow collections to be modified while iterating. By design Iterator implementation in `java.util`packages are fail-fast and throws ConcurrentModificationException. But Iterator implementation in `java.util.concurrent` packages are fail-safe and we can modify the collection while iterating. Some of these classes are `CopyOnWriteArrayList`, `ConcurrentHashMap`, `CopyOnWriteArraySet`.

Read these posts to learn about them in more detail.

`java.util.concurrent.BlockingQueue` is a Queue that supports operations that wait for the queue to become non-empty when retrieving and removing an element, and wait for space to become available in the queue when adding an element.

BlockingQueue interface is part of java collections framework and it’s primarily used for implementing producer consumer problem. We don’t need to worry about waiting for the space to be available for producer or object to be available for consumer in BlockingQueue as it’s handled by implementation classes of BlockingQueue.

Java provides several BlockingQueue implementations such as ArrayBlockingQueue, LinkedBlockingQueue, PriorityBlockingQueue, SynchronousQueue etc.
Check this post for use of BlockingQueue for producer-consumer problem.

Both Queue and Stack are used to store data before processing them. `java.util.Queue` is an interface whose implementation classes are present in java concurrent package. Queue allows retrieval of element in First-In-First-Out (FIFO) order but it’s not always the case. There is also Deque interface that allows elements to be retrieved from both end of the queue.
Stack is similar to queue except that it allows elements to be retrieved in Last-In-First-Out (LIFO) order.
Stack is a class that extends Vector whereas Queue is an interface.

`java.util.Collections` is a utility class consists exclusively of static methods that operate on or return collections. It contains polymorphic algorithms that operate on collections, “wrappers”, which return a new collection backed by a specified collection, and a few other odds and ends.

This class contains methods for collection framework algorithms, such as binary search, sorting, shuffling, reverse etc.

Java provides Comparable interface which should be implemented by any custom class if we want to use Arrays or Collections sorting methods. Comparable interface has compareTo(T obj) method which is used by sorting methods. We should override this method in such a way that it returns a negative integer, zero, or a positive integer if “this” object is less than, equal to, or greater than the object passed as argument.

But, in most real life scenarios, we want sorting based on different parameters. For example, as a CEO, I would like to sort the employees based on Salary, an HR would like to sort them based on the age. This is the situation where we need to use `Comparator` interface because `Comparable.compareTo(Object o)` method implementation can sort based on one field only and we can’t chose the field on which we want to sort the Object.

Comparator interface `compare(Object o1, Object o2)` method need to be implemented that takes two Object argument, it should be implemented in such a way that it returns negative int if first argument is less than the second one and returns zero if they are equal and positive int if first argument is greater than second one.

Check this post for use of Comparable and Comparator interface to sort objects.

Comparable and Comparator interfaces are used to sort collection or array of objects.

Comparable interface is used to provide the natural sorting of objects and we can use it to provide sorting based on single logic.
Comparator interface is used to provide different algorithms for sorting and we can chose the comparator we want to use to sort the given collection of objects.

If we need to sort an array of Objects, we can use `Arrays.sort()`. If we need to sort a list of objects, we can use `Collections.sort()`. Both these classes have overloaded sort() methods for natural sorting (using Comparable) or sorting based on criteria (using Comparator).
Collections internally uses Arrays sorting method, so both of them have same performance except that Collections take sometime to convert list to array.

We can create a read-only collection using `Collections.unmodifiableCollection(Collection c)`method before passing it as argument, this will make sure that any operation to change the collection will throw `UnsupportedOperationException`.

We can use `Collections.synchronizedCollection(Collection c)` to get a synchronized (thread-safe) collection backed by the specified collection.

Java Collections Framework provides algorithm implementations that are commonly used such as sorting and searching. Collections class contain these method implementations. Most of these algorithms work on List but some of them are applicable for all kinds of collections.
Some of them are sorting, searching, shuffling, min-max values.

The Big-O notation describes the performance of an algorithm in terms of number of elements in a data structure. Since Collection classes are actually data structures, we usually tend to use Big-O notation to chose the collection implementation to use based on time, memory and performance.

Example 1: ArrayList `get(index i)` is a constant-time operation and doesn’t depend on the number of elements in the list. So it’s performance in Big-O notation is O(1).
Example 2: A linear search on array or list performance is O(n) because we need to search through entire list of elements to find the element.

PriorityQueue is an unbounded queue based on a priority heap and the elements are ordered in their natural order or we can provide Comparator for ordering at the time of creation. PriorityQueue doesn’t allow null values and we can’t add any object that doesn’t provide natural ordering or we don’t have any comparator for them for ordering. Java PriorityQueue is not thread-safe and provided O(log(n)) time for enqueing and dequeing operations. Check this post for java priority queue example.

Generics doesn’t support sub-typing because it will cause issues in achieving type safety. That’s why List<T> is not considered as a subtype of List<S> where S is the super-type of T. To understanding why it’s not allowed, let’s see what could have happened if it has been supported.

As you can see from above code that IF generics would have been supporting sub-typing, we could have easily add a Double to the list of Long that would have caused `ClassCastException` at runtime while traversing the list of Long.

We are not allowed to create generic arrays because array carries type information of its elements at runtime. This information is used at runtime to throw if elements type doesn’t match to the defined type. Since generics type information gets erased at compile time by Type Erasure, the array store check would have been passed where it should have failed. Let’s understand this with simple example code.

Arrays are covariant by nature i.e S[] is a subtype of T[] whenever S is a subtype of T but generics doesn’t support covariance or sub-typing as we saw in the last question. So if we would have been allowed to create generic arrays, because of type erasure we would not get array store exception even though both types are not related

Java Interview Questions and Answers

1) What is Java?

Java is the high-level, object-oriented, robust, secure programming language, platform-independent, high performance, Multithreaded, and portable programming language. It was developed by James Gosling in June 1991. It can also be known as the platform as it provides its own JRE and API.

Java interview questions

2) What are the differences between C++ and Java?

The differences between C++ and Java are given in the following table.

Comparison Index	C++	Java
Platform-independent	C++ is platform-dependent.	Java is platform-independent.
Mainly used for	C++ is mainly used for system programming.	Java is mainly used for application programming. It is widely used in window, web-based, enterprise and mobile applications.
Design Goal	C++ was designed for systems and applications programming. It was an extension of C programming language.	Java was designed and created as an interpreter for printing systems but later extended as a support network computing. It was designed with a goal of being easy to use and accessible to a broader audience.
Goto	C++ supports the goto statement.	Java doesn't support the goto statement.
Multiple inheritance	C++ supports multiple inheritance.	Java doesn't support multiple inheritances through class. It can be achieved by interfaces in java.
Operator Overloading	C++ supports operator overloading.	Java doesn't support operator overloading.
Pointers	C++ supports pointers. You can write pointer program in C++.	Java supports pointer internally. However, you can't write the pointer program in java. It means java has restricted pointer support in java.
Compiler and Interpreter	C++ uses compiler only. C++ is compiled and run using the compiler which converts source code into machine code so, C++ is platform dependent.	Java uses compiler and interpreter both. Java source code is converted into bytecode at compilation time. The interpreter executes this bytecode at runtime and produces output. Java is interpreted that is why it is platform independent.
Call by Value and Call by reference	C++ supports both call by value and call by reference.	Java supports call by value only. There is no call by reference in java.
Structure and Union	C++ supports structures and unions.	Java doesn't support structures and unions.
Thread Support	C++ doesn't have built-in support for threads. It relies on third-party libraries for thread support.	Java has built-in thread support.
Documentation comment	C++ doesn't support documentation comment.	Java supports documentation comment (/** ... */) to create documentation for java source code.
Virtual Keyword	C++ supports virtual keyword so that we can decide whether or not override a function.	Java has no virtual keyword. We can override all non-static methods by default. In other words, non-static methods are virtual by default.
unsigned right shift >>>	C++ doesn't support >>> operator.	Java supports unsigned right shift >>> operator that fills zero at the top for the negative numbers. For positive numbers, it works same like >> operator.
Inheritance Tree	C++ creates a new inheritance tree always.	Java uses a single inheritance tree always because all classes are the child of Object class in java. The object class is the root of the inheritance tree in java.
Hardware	C++ is nearer to hardware.	Java is not so interactive with hardware.
Object-oriented	C++ is an object-oriented language. However, in C language, single root hierarchy is not possible.	Java is also an object-oriented language. However, everything (except fundamental types) is an object in Java. It is a single root hierarchy as everything gets derived from java.lang.Object.

3) List the features of Java Programming language.

There are the following features in Java Programming Language.

Simple: Java is easy to learn. The syntax of Java is based on C++ which makes easier to write the program in it.

Object-Oriented: Java follows the object-oriented paradigm which allows us to maintain our code as the combination of different type of objects that incorporates both data and behavior.

Portable: Java supports read-once-write-anywhere approach. We can execute the Java program on every machine. Java program (.java) is converted to bytecode (.class) which can be easily run on every machine.

Platform Independent: Java is a platform independent programming language. It is different from other programming languages like C and C++ which needs a platform to be executed. Java comes with its platform on which its code is executed. Java doesn't depend upon the operating system to be executed.

Secured: Java is secured because it doesn't use explicit pointers. Java also provides the concept of ByteCode and Exception handling which makes it more secured.

Robust: Java is a strong programming language as it uses strong memory management. The concepts like Automatic garbage collection, Exception handling, etc. make it more robust.

Architecture Neutral: Java is architectural neutral as it is not dependent on the architecture. In C, the size of data types may vary according to the architecture (32 bit or 64 bit) which doesn't exist in Java.

Interpreted: Java uses the Just-in-time (JIT) interpreter along with the compiler for the program execution.

High Performance: Java is faster than other traditional interpreted programming languages because Java bytecode is "close" to native code. It is still a little bit slower than a compiled language (e.g., C++).

Multithreaded: We can write Java programs that deal with many tasks at once by defining multiple threads. The main advantage of multi-threading is that it doesn't occupy memory for each thread. It shares a common memory area. Threads are important for multi-media, Web applications, etc.

Distributed: Java is distributed because it facilitates users to create distributed applications in Java. RMI and EJB are used for creating distributed applications. This feature of Java makes us able to access files by calling the methods from any machine on the internet.

Dynamic: Java is a dynamic language. It supports dynamic loading of classes. It means classes are loaded on demand. It also supports functions from its native languages, i.e., C and C++.

4) What do you understand by Java virtual machine?

Java Virtual Machine is a virtual machine that enables the computer to run the Java program. JVM acts like a run-time engine which calls the main method present in the Java code. JVM is the specification which must be implemented in the computer system. The Java code is compiled by JVM to be a Bytecode which is machine independent and close to the native code.

5) What is the difference between JDK, JRE, and JVM?

JVM

JVM is an acronym for Java Virtual Machine; it is an abstract machine which provides the runtime environment in which Java bytecode can be executed. It is a specification which specifies the working of Java Virtual Machine. Its implementation has been provided by Oracle and other companies. Its implementation is known as JRE.

JVMs are available for many hardware and software platforms (so JVM is platform dependent). It is a runtime instance which is created when we run the Java class. There are three notions of the JVM: specification, implementation, and instance.

JRE

JRE stands for Java Runtime Environment. It is the implementation of JVM. The Java Runtime Environment is a set of software tools which are used for developing Java applications. It is used to provide a runtime environment. It is the implementation of JVM. It physically exists. It contains a set of libraries + other files that JVM uses at runtime.

JDK

JDK is an acronym for Java Development Kit. It is a software development environment which is used to develop Java applications and applets. It physically exists. It contains JRE + development tools. JDK is an implementation of any one of the below given Java Platforms released by Oracle Corporation:

Standard Edition Java Platform.
Enterprise Edition Java Platform.
Micro Edition Java Platform.

Java interview questions

6) How many types of memory areas are allocated by JVM?

Many types:

Class(Method) Area: Class Area stores per-class structures such as the runtime constant pool, field, method data, and the code for methods.
Heap: It is the runtime data area in which the memory is allocated to the objects
Stack: Java Stack stores frames. It holds local variables and partial results, and plays a part in method invocation and return. Each thread has a private JVM stack, created at the same time as the thread. A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes.
Program Counter Register: PC (program counter) register contains the address of the Java virtual machine instruction currently being executed.
Native Method Stack: It contains all the native methods used in the application.

7) What is JIT compiler?

Just-In-Time(JIT) compiler: It is used to improve performance. JIT compiles parts of the bytecode that have similar functionality at the same time, and hence reduces the amount of time needed for compilation. Here the term “compiler” refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.

8) What is the platform?

A platform is the hardware or software environment in which a piece of software is executed. There are two types of platforms, software-based and hardware-based. Java provides the software-based platform.

9) What are the main differences between the Java platform and other platforms?

There are the following differences between the Java platform and other platforms.

Java is the software-based platform whereas other platforms may be the hardware platforms or software-based platforms.
Java is executed on the top of other hardware platforms whereas other platforms can only have the hardware components.

10) What gives Java its 'write once and run anywhere' nature?

The bytecode. Java compiler converts the Java programs into the class file (Byte Code) which is the intermediate language between source code and machine code. This bytecode is not platform specific and can be executed on any computer.

11) What is classloader?

Classloader is a subsystem of JVM which is used to load class files. Whenever we run the java program, it is loaded first by the classloader. There are three built-in classloaders in Java.

Bootstrap ClassLoader: This is the first classloader which is the superclass of Extension classloader. It loads the rt.jar file which contains all class files of Java Standard Edition like java.lang package classes, java.net package classes, java.util package classes, java.io package classes, java.sql package classes, etc.
Extension ClassLoader: This is the child classloader of Bootstrap and parent classloader of System classloader. It loads the jar files located inside $JAVA_HOME/jre/lib/ext directory.
System/Application ClassLoader: This is the child classloader of Extension classloader. It loads the class files from the classpath. By default, the classpath is set to the current directory. You can change the classpath using "-cp" or "-classpath" switch. It is also known as Application classloader.

Java interview questions

12) Is Empty .java file name a valid source file name?

Yes, Java allows to save our java file by .java only, we need to compile it by javac .java and run by java classname Let's take a simple example:

//save by .java only  
class A{  
public static void main(String args[]){  
System.out.println("Hello java");  
}  
}  
//compile by javac .java  
//run by     java A  

compile it by javac .java

run it by java A

13) Is delete, next, main, exit or null keyword in java?

No.

14) If I don't provide any arguments on the command line, then what will the value stored in the String array passed into the main() method, empty or NULL?

It is empty, but not null.

15) What if I write static public void instead of public static void?

The program compiles and runs correctly because the order of specifiers doesn't matter in Java.

16) What is the default value of the local variables?

The local variables are not initialized to any default value, neither primitives nor object references.

17) What are the various access specifiers in Java?

In Java, access specifiers are the keywords which are used to define the access scope of the method, class, or a variable. In Java, there are four access specifiers given below.

Public The classes, methods, or variables which are defined as public, can be accessed by any class or method.
Protected Protected can be accessed by the class of the same package, or by the sub-class of this class, or within the same class.
Default Default are accessible within the package only. By default, all the classes, methods, and variables are of default scope.
Private The private class, methods, or variables defined as private can be accessed within the class only.

18) What is the purpose of static methods and variables?

The methods or variables defined as static are shared among all the objects of the class. The static is the part of the class and not of the object. The static variables are stored in the class area, and we do not need to create the object to access such variables. Therefore, static is used in the case, where we need to define variables or methods which are common to all the objects of the class.

For example, In the class simulating the collection of the students in a college, the name of the college is the common attribute to all the students. Therefore, the college name will be defined as static.

Java interview questions

19) What are the advantages of Packages in Java?

There are various advantages of defining packages in Java.

Packages avoid the name clashes.
The Package provides easier access control.
We can also have the hidden classes that are not visible outside and used by the package.
It is easier to locate the related classes.

20) What is the output of the following Java program?

class Test   
{  
    public static void main (String args[])   
    {  
        System.out.println(10 + 20 + "Javatpoint");   
        System.out.println("Javatpoint" + 10 + 20);  
    }  
}  

The output of the above code will be

30Javatpoint
Javatpoint1020

Explanation

In the first case, 10 and 20 are treated as numbers and added to be 30. Now, their sum 30 is treated as the string and concatenated with the string Javatpoint. Therefore, the output will be 30Javatpoint.

In the second case, the string Javatpoint is concatenated with 10 to be the string Javatpoint10 which will then be concatenated with 20 to be Javatpoint1020.

21) What is the output of the following Java program?

class Test   
{  
    public static void main (String args[])   
    {  
        System.out.println(10 * 20 + "Javatpoint");   
        System.out.println("Javatpoint" + 10 * 20);  
    }  
}  

The output of the above code will be

200Javatpoint
Javatpoint200

Explanation

In the first case, The numbers 10 and 20 will be multiplied first and then the result 200 is treated as the string and concatenated with the string Javatpoint to produce the output 200Javatpoint.

In the second case, The numbers 10 and 20 will be multiplied first to be 200 because the precedence of the multiplication is higher than addition. The result 200 will be treated as the string and concatenated with the string Javatpointto produce the output as Javatpoint200.

22) What is the output of the following Java program?

class Test   
{  
    public static void main (String args[])   
    {  
        for(int i=0; 0; i++)   
        {  
            System.out.println("Hello Javatpoint");  
        }  
    }  
}  

The above code will give the compile-time error because the for loop demands a boolean value in the second part and we are providing an integer value, i.e., 0.

Core Java - OOPs Concepts: Initial OOPs Interview Questions

There is given more than 50 OOPs (Object-Oriented Programming and System) interview questions. However, they have been categorized in many sections such as constructor interview questions, static interview questions, Inheritance Interview questions, Abstraction interview question, Polymorphism interview questions, etc. for better understanding.

23) What is object-oriented paradigm?

It is a programming paradigm based on objects having data and methods defined in the class to which it belongs. Object-oriented paradigm aims to incorporate the advantages of modularity and reusability. Objects are the instances of classes which interacts with one another to design applications and programs. There are the following features of the object-oriented paradigm.

Follows the bottom-up approach in program design.
Focus on data with methods to operate upon the object?s data
Includes the concept like Encapsulation and abstraction which hides the complexities from the user and show only functionality.
Implements the real-time approach like inheritance, abstraction, etc.
The examples of the object-oriented paradigm are C++, Simula, Smalltalk, Python, C#, etc.

24) What is an object?

The Object is the real-time entity having some state and behavior. In Java, Object is an instance of the class having the instance variables as the state of the object and the methods as the behavior of the object. The object of a class can be created by using the new keyword.

25) What is the difference between an object-oriented programming language and object-based programming language?

There are the following basic differences between the object-oriented language and object-based language.

Object-oriented languages follow all the concepts of OOPs whereas, the object-based language doesn't follow all the concepts of OOPs like inheritance and polymorphism.
Object-oriented languages do not have the inbuilt objects whereas Object-based languages have the inbuilt objects, for example, JavaScript has window object.
Examples of object-oriented programming are Java, C#, Smalltalk, etc. whereas the examples of object-based languages are JavaScript, VBScript, etc.

26) What will be the initial value of an object reference which is defined as an instance variable?

All object references are initialized to null in Java.

Core Java - OOPs Concepts: Constructor Interview Questions

27) What is the constructor?

The constructor can be defined as the special type of method that is used to initialize the state of an object. It is invoked when the class is instantiated, and the memory is allocated for the object. Every time, an object is created using the new keyword, the default constructor of the class is called. The name of the constructor must be similar to the class name. The constructor must not have an explicit return type.

More Details.

28) How many types of constructors are used in Java?

Based on the parameters passed in the constructors, there are two types of constructors in Java.

Default Constructor: default constructor is the one which does not accept any value. The default constructor is mainly used to initialize the instance variable with the default values. It can also be used for performing some useful task on object creation. A default constructor is invoked implicitly by the compiler if there is no constructor defined in the class.
Parameterized Constructor: The parameterized constructor is the one which can initialize the instance variables with the given values. In other words, we can say that the constructors which can accept the arguments are called parameterized constructors.

Java interview questions

29) What is the purpose of a default constructor?

The purpose of the default constructor is to assign the default value to the objects. The java compiler creates a default constructor implicitly if there is no constructor in the class.

class Student3{  
int id;  
String name;  
  
void display(){System.out.println(id+" "+name);}  
  
public static void main(String args[]){  
Student3 s1=new Student3();  
Student3 s2=new Student3();  
s1.display();  
s2.display();  
}  
}  

Output:

0 null
0 null

Explanation: In the above class, you are not creating any constructor, so compiler provides you a default constructor. Here 0 and null values are provided by default constructor.

30) Does constructor return any value?

Ans: yes, The constructor implicitly returns the current instance of the class (You can't use an explicit return type with the constructor).

31)Is constructor inherited?

No, The constructor is not inherited.

32) Can you make a constructor final?

No, the constructor can't be final.

33) Can we overload the constructors?

Yes, the constructors can be overloaded by changing the number of arguments accepted by the constructor or by changing the data type of the parameters. Consider the following example.

class Test   
{  
    int i;   
    public Test(int k)  
    {  
        i=k;  
    }  
    public Test(int k, int m)  
    {  
        System.out.println("Hi I am assigning the value max(k, m) to i");  
        if(k>m)  
        {  
            i=k;   
        }  
        else   
        {  
            i=m;  
        }  
    }  
}  
public class Main   
{  
    public static void main (String args[])   
    {  
        Test test1 = new Test(10);  
        Test test2 = new Test(12, 15);  
        System.out.println(test1.i);  
        System.out.println(test2.i);  
    }  
}  
      

In the above program, The constructor Test is overloaded with another constructor. In the first call to the constructor, The constructor with one argument is called, and i will be initialized with the value 10. However, In the second call to the constructor, The constructor with the 2 arguments is called, and i will be initialized with the value 15.

34) What do you understand by copy constructor in Java?

There is no copy constructor in java. However, we can copy the values from one object to another like copy constructor in C++.

There are many ways to copy the values of one object into another in java. They are:

By constructor
By assigning the values of one object into another
By clone() method of Object class

In this example, we are going to copy the values of one object into another using java constructor.

//Java program to initialize the values from one object to another  
class Student6{  
    int id;  
    String name;  
    //constructor to initialize integer and string  
    Student6(int i,String n){  
    id = i;  
    name = n;  
    }  
    //constructor to initialize another object  
    Student6(Student6 s){  
    id = s.id;  
    name =s.name;  
    }  
    void display(){System.out.println(id+" "+name);}  
   
    public static void main(String args[]){  
    Student6 s1 = new Student6(111,"Karan");  
    Student6 s2 = new Student6(s1);  
    s1.display();  
    s2.display();  
   }  
}  

Output:

111 Karan
111 Karan

35) What are the differences between the constructors and methods?

There are many differences between constructors and methods. They are given below.

Java Constructor	Java Method
A constructor is used to initialize the state of an object.	A method is used to expose the behavior of an object.
A constructor must not have a return type.	A method must have a return type.
The constructor is invoked implicitly.	The method is invoked explicitly.
The Java compiler provides a default constructor if you don't have any constructor in a class.	The method is not provided by the compiler in any case.
The constructor name must be same as the class name.	The method name may or may not be same as class name.

36) What is the output of the following Java program?

public class Test   
{  
    Test(int a, int b)  
    {  
        System.out.println("a = "+a+" b = "+b);  
    }  
    Test(int a, float b)  
    {  
        System.out.println("a = "+a+" b = "+b);  
    }  
    public static void main (String args[])  
    {  
        byte a = 10;   
        byte b = 15;  
        Test test = new Test(a,b);  
    }  
}  

The output of the following program is:

a = 10 b = 15

Here, the data type of the variables a and b, i.e., byte gets promoted to int, and the first parameterized constructor with the two integer parameters is called.

37) What is the output of the following Java program?

class Test   
{  
    int i;   
}  
public class Main   
{  
    public static void main (String args[])   
    {  
        Test test = new Test();   
        System.out.println(test.i);  
    }  
}  

The output of the program is 0 because the variable i is initialized to 0 internally. As we know that a default constructor is invoked implicitly if there is no constructor in the class, the variable i is initialized to 0 since there is no constructor in the class.

38) What is the output of the following Java program?

class Test   
{  
    int test_a, test_b;  
    Test(int a, int b)   
    {  
    test_a = a;   
    test_b = b;   
    }  
    public static void main (String args[])   
    {  
        Test test = new Test();   
        System.out.println(test.test_a+" "+test.test_b);  
    }  
}  

There is a compiler error in the program because there is a call to the default constructor in the main method which is not present in the class. However, there is only one parameterized constructor in the class Test. Therefore, no default constructor is invoked by the constructor implicitly.

Core Java - OOPs Concepts: static keyword Interview Questions

39) What is the static variable?

The static variable is used to refer to the common property of all objects (that is not unique for each object), e.g., The company name of employees, college name of students, etc. Static variable gets memory only once in the class area at the time of class loading. Using a static variable makes your program more memory efficient (it saves memory). Static variable belongs to the class rather than the object.

//Program of static variable  
  
class Student8{  
   int rollno;  
   String name;  
   static String college ="ITS";  
     
   Student8(int r,String n){  
   rollno = r;  
   name = n;  
   }  
 void display (){System.out.println(rollno+" "+name+" "+college);}  
  
 public static void main(String args[]){  
 Student8 s1 = new Student8(111,"Karan");  
 Student8 s2 = new Student8(222,"Aryan");  
   
 s1.display();  
 s2.display();  
 }  
}  

Output:111 Karan ITS
       222 Aryan ITS

Java interview questions

40) What is the static method?

A static method belongs to the class rather than the object.
There is no need to create the object to call the static methods.
A static method can access and change the value of the static variable.

41) What are the restrictions that are applied to the Java static methods?

Two main restrictions are applied to static methods.

The static method can not use non-static data member or call the non-static method directly.
this and super cannot be used in static context as they are non-static.

42) Why is the main method static?

Because the object is not required to call the static method. If we make the main method non-static, JVM will have to create its object first and then call main() method which will lead to the extra memory allocation.

43) Can we override the static methods?

No, we can't override static methods.

44) What is the static block?

Static block is used to initialize the static data member. It is executed before the main method, at the time of classloading.

class A2{  
  static{System.out.println("static block is invoked");}  
  public static void main(String args[]){  
   System.out.println("Hello main");  
  }  
}  

Output: static block is invoked
       Hello main

45) Can we execute a program without main() method?

Ans) Yes, one of the ways to execute the program without the main method is using static block.

46) What if the static modifier is removed from the signature of the main method?

Program compiles. However, at runtime, It throws an error "NoSuchMethodError."

47) What is the difference between static (class) method and instance method?

static or class method	instance method
1)A method that is declared as static is known as the static method.	A method that is not declared as static is known as the instance method.
2)We don't need to create the objects to call the static methods.	The object is required to call the instance methods.
3)Non-static (instance) members cannot be accessed in the static context (static method, static block, and static nested class) directly.	Static and non-static variables both can be accessed in instance methods.
4)For example: public static int cube(int n){ return nnn;}	For example: public void msg(){...}.

48) Can we make constructors static?

As we know that the static context (method, block, or variable) belongs to the class, not the object. Since Constructors are invoked only when the object is created, there is no sense to make the constructors static. However, if you try to do so, the compiler will show the compiler error.

49) Can we make the abstract methods static in Java?

In Java, if we make the abstract methods static, It will become the part of the class, and we can directly call it which is unnecessary. Calling an undefined method is completely useless therefore it is not allowed.

50) Can we declare the static variables and methods in an abstract class?

Yes, we can declare static variables and methods in an abstract method. As we know that there is no requirement to make the object to access the static context, therefore, we can access the static context declared inside the abstract class by using the name of the abstract class. Consider the following example.

abstract class Test  
{  
    static int i = 102;  
    static void TestMethod()  
    {  
        System.out.println("hi !! I am good !!");  
    }  
}  
public class TestClass extends Test   
{  
    public static void main (String args[])  
    {  
        Test.TestMethod();  
        System.out.println("i = "+Test.i);  
    }  
}  

Output

hi !! I am good !!
i = 102

Core Java - OOPs Concepts: Inheritance Interview Questions

51) What is this keyword in java?

The this keyword is a reference variable that refers to the current object. There are the various uses of this keyword in Java. It can be used to refer to current class properties such as instance methods, variable, constructors, etc. It can also be passed as an argument into the methods or constructors. It can also be returned from the method as the current class instance.

52) What are the main uses of this keyword?

There are the following uses of this keyword.

this can be used to refer to the current class instance variable.
this can be used to invoke current class method (implicitly)
this() can be used to invoke the current class constructor.
this can be passed as an argument in the method call.
this can be passed as an argument in the constructor call.
this can be used to return the current class instance from the method.

53) Can we assign the reference to this variable?

No, this cannot be assigned to any value because it always points to the current class object and this is the final reference in Java. However, if we try to do so, the compiler error will be shown. Consider the following example.

public class Test  
{  
    public Test()  
    {  
        this = null;   
        System.out.println("Test class constructor called");  
    }  
    public static void main (String args[])  
    {  
        Test t = new Test();  
    }  
}  

Output

Test.java:5: error: cannot assign a value to final variable this
        this = null; 
        ^
1 error

54) Can this keyword be used to refer static members?

Yes, It is possible to use this keyword to refer static members because this is just a reference variable which refers to the current class object. However, as we know that, it is unnecessary to access static variables through objects, therefore, it is not the best practice to use this to refer static members. Consider the following example.

public class Test   
{  
    static int i = 10;   
    public Test ()  
    {  
        System.out.println(this.i);      
    }  
    public static void main (String args[])  
    {  
        Test t = new Test();  
    }  
}  

Output

55) How can constructor chaining be done using this keyword?

Constructor chaining enables us to call one constructor from another constructor of the class with respect to the current class object. We can use this keyword to perform constructor chaining within the same class. Consider the following example which illustrates how can we use this keyword to achieve constructor chaining.

public class Employee  
{  
    int id,age;   
    String name, address;  
    public Employee (int age)  
    {  
        this.age = age;  
    }  
    public Employee(int id, int age)  
    {  
        this(age);  
        this.id = id;  
    }  
    public Employee(int id, int age, String name, String address)  
    {  
        this(id, age);  
        this.name = name;   
        this.address = address;   
    }  
    public static void main (String args[])  
    {  
        Employee emp = new Employee(105, 22, "Vikas", "Delhi");  
        System.out.println("ID: "+emp.id+" Name:"+emp.name+" age:"+emp.age+" address: "+emp.address);  
    }  
      
}  

Output

ID: 105 Name:Vikas age:22 address: Delhi

56) What are the advantages of passing this into a method instead of the current class object itself?

As we know, that this refers to the current class object, therefore, it must be similar to the current class object. However, there can be two main advantages of passing this into a method instead of the current class object.

this is a final variable. Therefore, this cannot be assigned to any new value whereas the current class object might not be final and can be changed.
this can be used in the synchronized block.

Java interview questions

57) What is Inheritance?

Inheritance is a mechanism by which one object acquires all the properties and behavior of another object of another class. It is used for Code Reusability and Method Overriding. The idea behind inheritance in Java is that you can create new classes that are built upon existing classes. When you inherit from an existing class, you can reuse methods and fields of the parent class. Moreover, you can add new methods and fields in your current class also. Inheritance represents the IS-A relationship which is also known as a parent-child relationship.

There are five types of inheritance in Java.

Single-level inheritance
Multi-level inheritance
Multiple Inheritance
Hierarchical Inheritance
Hybrid Inheritance

Multiple inheritance is not supported in Java through class.

58) Why is Inheritance used in Java?

There are various advantages of using inheritance in Java that is given below.

Inheritance provides code reusability. The derived class does not need to redefine the method of base class unless it needs to provide the specific implementation of the method.
Runtime polymorphism cannot be achieved without using inheritance.
We can simulate the inheritance of classes with the real-time objects which makes OOPs more realistic.
Inheritance provides data hiding. The base class can hide some data from the derived class by making it private.
Method overriding cannot be achieved without inheritance. By method overriding, we can give a specific implementation of some basic method contained by the base class.

59) Which class is the superclass for all the classes?

The object class is the superclass of all other classes in Java.

60) Why is multiple inheritance not supported in java?

To reduce the complexity and simplify the language, multiple inheritance is not supported in java. Consider a scenario where A, B, and C are three classes. The C class inherits A and B classes. If A and B classes have the same method and you call it from child class object, there will be ambiguity to call the method of A or B class.

Since the compile-time errors are better than runtime errors, Java renders compile-time error if you inherit 2 classes. So whether you have the same method or different, there will be a compile time error.

class A{  
void msg(){System.out.println("Hello");}  
}  
class B{  
void msg(){System.out.println("Welcome");}  
}  
class C extends A,B{//suppose if it were  
   
 Public Static void main(String args[]){  
   C obj=new C();  
   obj.msg();//Now which msg() method would be invoked?  
}  
}  

 Compile Time Error

61) What is aggregation?

Aggregation can be defined as the relationship between two classes where the aggregate class contains a reference to the class it owns. Aggregation is best described as a has-a relationship. For example, The aggregate class Employee having various fields such as age, name, and salary also contains an object of Address class having various fields such as Address-Line 1, City, State, and pin-code. In other words, we can say that Employee (class) has an object of Address class. Consider the following example.

Address.java

public class Address {  
String city,state,country;  
  
public Address(String city, String state, String country) {  
    this.city = city;  
    this.state = state;  
    this.country = country;  
}  
  
}  

Employee.java

public class Emp {  
int id;  
String name;  
Address address;  
  
public Emp(int id, String name,Address address) {  
    this.id = id;  
    this.name = name;  
    this.address=address;  
}  
  
void display(){  
System.out.println(id+" "+name);  
System.out.println(address.city+" "+address.state+" "+address.country);  
}  
  
public static void main(String[] args) {  
Address address1=new Address("gzb","UP","india");  
Address address2=new Address("gno","UP","india");  
  
Emp e=new Emp(111,"varun",address1);  
Emp e2=new Emp(112,"arun",address2);  
      
e.display();  
e2.display();  
      
}  
}  

Output

111 varun
gzb UP india
112 arun
gno UP india

62) What is composition?

Holding the reference of a class within some other class is known as composition. When an object contains the other object, if the contained object cannot exist without the existence of container object, then it is called composition. In other words, we can say that composition is the particular case of aggregation which represents a stronger relationship between two objects. Example: A class contains students. A student cannot exist without a class. There exists composition between class and students.

63) What is the difference between aggregation and composition?

Aggregation represents the weak relationship whereas composition represents the strong relationship. For example, the bike has an indicator (aggregation), but the bike has an engine (composition).

64) Why does Java not support pointers?

The pointer is a variable that refers to the memory address. They are not used in Java because they are unsafe(unsecured) and complex to understand.

65) What is super in java?

The super keyword in Java is a reference variable that is used to refer to the immediate parent class object. Whenever you create the instance of the subclass, an instance of the parent class is created implicitly which is referred by super reference variable. The super() is called in the class constructor implicitly by the compiler if there is no super or this.

class Animal{  
Animal(){System.out.println("animal is created");}  
}  
class Dog extends Animal{  
Dog(){  
System.out.println("dog is created");  
}  
}  
class TestSuper4{  
public static void main(String args[]){  
Dog d=new Dog();  
}  
}  

Output:

animal is created
dog is created

66) How can constructor chaining be done by using the super keyword?

class Person  
{  
    String name,address;   
    int age;  
    public Person(int age, String name, String address)  
    {  
        this.age = age;  
        this.name = name;  
        this.address = address;  
    }  
}  
class Employee extends Person   
{  
    float salary;  
    public Employee(int age, String name, String address, float salary)  
    {  
        super(age,name,address);  
        this.salary = salary;  
    }  
}  
public class Test   
{  
    public static void main (String args[])  
    {  
        Employee e = new Employee(22, "Mukesh", "Delhi", 90000);  
        System.out.println("Name: "+e.name+" Salary: "+e.salary+" Age: "+e.age+" Address: "+e.address);  
    }  
}  

Output

Name: Mukesh Salary: 90000.0 Age: 22 Address: Delhi

67) What are the main uses of the super keyword?

There are the following uses of super keyword.

super can be used to refer to the immediate parent class instance variable.
super can be used to invoke the immediate parent class method.
super() can be used to invoke immediate parent class constructor.

68) What are the differences between this and super keyword?

There are the following differences between this and super keyword.

The super keyword always points to the parent class contexts whereas this keyword always points to the current class context.
The super keyword is primarily used for initializing the base class variables within the derived class constructor whereas this keyword primarily used to differentiate between local and instance variables when passed in the class constructor.
The super and this must be the first statement inside constructor otherwise the compiler will throw an error.

69) What is the output of the following Java program?

class Person   
{  
    public Person()   
    {  
        System.out.println("Person class constructor called");  
    }  
}  
public class Employee extends Person   
{  
    public Employee()   
    {  
        System.out.println("Employee class constructor called");  
    }  
    public static void main (String args[])  
    {  
        Employee e = new Employee();  
    }  
}  

Output

Person class constructor called
Employee class constructor called

Explanation

The super() is implicitly invoked by the compiler if no super() or this() is included explicitly within the derived class constructor. Therefore, in this case, The Person class constructor is called first and then the Employee class constructor is called.

70) Can you use this() and super() both in a constructor?

No, because this() and super() must be the first statement in the class constructor.

Example:

public class Test{  
    Test()  
     {  
         super();   
         this();  
         System.out.println("Test class object is created");  
     }  
     public static void main(String []args){  
     Test t = new Test();  
     }  
}  

Output:

Test.java:5: error: call to this must be first statement in constructor

71)What is object cloning?

The object cloning is used to create the exact copy of an object. The clone() method of the Object class is used to clone an object. The java.lang.Cloneable interface must be implemented by the class whose object clone we want to create. If we don't implement Cloneable interface, clone() method generates CloneNotSupportedException.

protected Object clone() throws CloneNotSupportedException

Core Java - OOPs Concepts: Method Overloading Interview Questions

72) What is method overloading?

Method overloading is the polymorphism technique which allows us to create multiple methods with the same name but different signature. We can achieve method overloading in two ways.

Changing the number of arguments
Changing the return type

Method overloading increases the readability of the program. Method overloading is performed to figure out the program quickly.

Java interview questions

73) Why is method overloading not possible by changing the return type in java?

In Java, method overloading is not possible by changing the return type of the program due to avoid the ambiguity.

class Adder{  
static int add(int a,int b){return a+b;}  
static double add(int a,int b){return a+b;}  
}  
class TestOverloading3{  
public static void main(String[] args){  
System.out.println(Adder.add(11,11));//ambiguity  
}}

Output:

Compile Time Error: method add(int, int) is already defined in class Adder

74) Can we overload the methods by making them static?

No, We cannot overload the methods by just applying the static keyword to them(number of parameters and types are the same). Consider the following example.

public class Animal  
{  
    void consume(int a)  
    {  
        System.out.println(a+" consumed!!");  
    }  
    static void consume(int a)  
    {  
        System.out.println("consumed static "+a);  
    }  
    public static void main (String args[])  
    {  
        Animal a = new Animal();  
        a.consume(10);  
        Animal.consume(20);  
    }  
}    

Output

Animal.java:7: error: method consume(int) is already defined in class Animal
    static void consume(int a)
                ^
Animal.java:15: error: non-static method consume(int) cannot be referenced from a static context
        Animal.consume(20);
              ^
2 errors

75) Can we overload the main() method?

Yes, we can have any number of main methods in a Java program by using method overloading.

76) What is method overloading with type promotion?

By Type promotion is method overloading, we mean that one data type can be promoted to another implicitly if no exact matching is found.

As displayed in the above diagram, the byte can be promoted to short, int, long, float or double. The short datatype can be promoted to int, long, float or double. The char datatype can be promoted to int, long, float or double and so on. Consider the following example.

class OverloadingCalculation1{  
  void sum(int a,long b){System.out.println(a+b);}  
  void sum(int a,int b,int c){System.out.println(a+b+c);}  
  
  public static void main(String args[]){  
  OverloadingCalculation1 obj=new OverloadingCalculation1();  
  obj.sum(20,20);//now second int literal will be promoted to long  
  obj.sum(20,20,20);  
  }  
}  

Output

40
60

77) What is the output of the following Java program?

class OverloadingCalculation3{    
  void sum(int a,long b){System.out.println("a method invoked");}    
  void sum(long a,int b){System.out.println("b method invoked");}    
    
  public static void main(String args[]){    
  OverloadingCalculation3 obj=new OverloadingCalculation3();    
  obj.sum(20,20);//now ambiguity    
  }    
}    

Output

OverloadingCalculation3.java:7: error: reference to sum is ambiguous
obj.sum(20,20);//now ambiguity  
     ^ 
      both method sum(int,long) in OverloadingCalculation3 
      and method sum(long,int) in OverloadingCalculation3 match
1 error

Explanation

There are two methods defined with the same name, i.e., sum. The first method accepts the integer and long type whereas the second method accepts long and the integer type. The parameter passed that are a = 20, b = 20. We can not tell that which method will be called as there is no clear differentiation mentioned between integer literal and long literal. This is the case of ambiguity. Therefore, the compiler will throw an error.

Core Java - OOPs Concepts: Method Overriding Interview Questions

78) What is method overriding:

If a subclass provides a specific implementation of a method that is already provided by its parent class, it is known as Method Overriding. It is used for runtime polymorphism and to implement the interface methods.

Rules for Method overriding

The method must have the same name as in the parent class.
The method must have the same signature as in the parent class.
Two classes must have an IS-A relationship between them.

79) Can we override the static method?

No, you can't override the static method because they are the part of the class, not the object.

80) Why can we not override static method?

It is because the static method is the part of the class, and it is bound with class whereas instance method is bound with the object, and static gets memory in class area, and instance gets memory in a heap.

81) Can we override the overloaded method?

Yes.

82) Difference between method Overloading and Overriding.

Method Overloading	Method Overriding
1) Method overloading increases the readability of the program.	Method overriding provides the specific implementation of the method that is already provided by its superclass.
2) Method overloading occurs within the class.	Method overriding occurs in two classes that have IS-A relationship between them.
3) In this case, the parameters must be different.	In this case, the parameters must be the same.

83) Can we override the private methods?

No, we cannot override the private methods because the scope of private methods is limited to the class and we cannot access them outside of the class.

84) Can we change the scope of the overridden method in the subclass?

Yes, we can change the scope of the overridden method in the subclass. However, we must notice that we cannot decrease the accessibility of the method. The following point must be taken care of while changing the accessibility of the method.

The private can be changed to protected, public, or default.
The protected can be changed to public or default.
The default can be changed to public.
The public will always remain public.

85) Can we modify the throws clause of the superclass method while overriding it in the subclass?

Yes, we can modify the throws clause of the superclass method while overriding it in the subclass. However, there are some rules which are to be followed while overriding in case of exception handling.

If the superclass method does not declare an exception, subclass overridden method cannot declare the checked exception, but it can declare the unchecked exception.
If the superclass method declares an exception, subclass overridden method can declare same, subclass exception or no exception but cannot declare parent exception.

86) What is the output of the following Java program?

class Base  
{  
    void method(int a)  
    {  
        System.out.println("Base class method called with integer a = "+a);  
    }  
       
    void method(double d)  
    {  
        System.out.println("Base class method called with double d ="+d);  
    }  
}  
   
class Derived extends Base  
{  
    @Override  
    void method(double d)  
    {  
        System.out.println("Derived class method called with double d ="+d);  
    }  
}  
   
public class Main  
{      
    public static void main(String[] args)  
    {  
        new Derived().method(10);  
    }  
}  

Output

Base class method called with integer a = 10

Explanation

The method() is overloaded in class Base whereas it is derived in class Derived with the double type as the parameter. In the method call, the integer is passed.

87) Can you have virtual functions in Java?

Yes, all functions in Java are virtual by default.

88) What is covariant return type?

Now, since java5, it is possible to override any method by changing the return type if the return type of the subclass overriding method is subclass type. It is known as covariant return type. The covariant return type specifies that the return type may vary in the same direction as the subclass.

class A{  
A get(){return this;}  
}  
  
class B1 extends A{  
B1 get(){return this;}  
void message(){System.out.println("welcome to covariant return type");}  
  
public static void main(String args[]){  
new B1().get().message();  
}  
}  

Output: welcome to covariant return type

89) What is the output of the following Java program?

class Base   
{  
    public void baseMethod()  
    {  
        System.out.println("BaseMethod called ...");  
    }  
}  
class Derived extends Base   
{  
    public void baseMethod()  
    {  
        System.out.println("Derived method called ...");  
    }  
}  
public class Test   
{  
    public static void main (String args[])  
    {  
        Base b = new Derived();  
        b.baseMethod();  
    }  
}  

Output

Derived method called ...

Explanation

The method of Base class, i.e., baseMethod() is overridden in Derived class. In Test class, the reference variable b (of type Base class) refers to the instance of the Derived class. Here, Runtime polymorphism is achieved between class Base and Derived. At compile time, the presence of method baseMethod checked in Base class, If it presence then the program compiled otherwise the compiler error will be shown. In this case, baseMethod is present in Base class; therefore, it is compiled successfully. However, at runtime, It checks whether the baseMethod has been overridden by Derived class, if so then the Derived class method is called otherwise Base class method is called. In this case, the Derived class overrides the baseMethod; therefore, the Derived class method is called.

Core Java - OOPs Concepts: final keyword Interview Questions

90) What is the final variable?

In Java, the final variable is used to restrict the user from updating it. If we initialize the final variable, we can't change its value. In other words, we can say that the final variable once assigned to a value, can never be changed after that. The final variable which is not assigned to any value can only be assigned through the class constructor.

class Bike9{  
 final int speedlimit=90;//final variable  
 void run(){  
  speedlimit=400;  
 }  
 public static void main(String args[]){  
 Bike9 obj=new  Bike9();  
 obj.run();  
 }  
}//end of class  

Output:Compile Time Error

91) What is the final method?

If we change any method to a final method, we can't override it.

class Bike{  
  final void run(){System.out.println("running");}  
}  
     
class Honda extends Bike{  
   void run(){System.out.println("running safely with 100kmph");}  
     
   public static void main(String args[]){  
   Honda honda= new Honda();  
   honda.run();  
   }  
}  

Output:Compile Time Error

92) What is the final class?

If we make any class final, we can't inherit it into any of the subclasses.

final class Bike{}  
  
class Honda1 extends Bike{  
  void run(){System.out.println("running safely with 100kmph");}  
    
  public static void main(String args[]){  
  Honda1 honda= new Honda1();  
  honda.run();  
  }  
}  

Output:Compile Time Error

93) What is the final blank variable?

A final variable, not initialized at the time of declaration, is known as the final blank variable. We can't initialize the final blank variable directly. Instead, we have to initialize it by using the class constructor. It is useful in the case when the user has some data which must not be changed by others, for example, PAN Number. Consider the following example:

class Student{  
int id;  
String name;  
final String PAN_CARD_NUMBER;  
...  
}  

94) Can we initialize the final blank variable?

Yes, if it is not static, we can initialize it in the constructor. If it is static blank final variable, it can be initialized only in the static block.

95) Can you declare the main method as final?

Yes, We can declare the main method as public static final void main(String[] args){}.

96) What is the output of the following Java program?

class Main {  
 public static void main(String args[]){  
   final int i;  
   i = 20;  
   System.out.println(i);  
 }  
}  

Output

Explanation

Since i is the blank final variable. It can be initialized only once. We have initialized it to 20. Therefore, 20 will be printed.

97) What is the output of the following Java program?

class Base   
{  
    protected final void getInfo()  
    {  
        System.out.println("method of Base class");  
    }  
}  
   
public class Derived extends Base  
{  
    protected final void getInfo()  
    {  
        System.out.println("method of Derived class");  
    }  
    public static void main(String[] args)  
    {  
        Base obj = new Base();  
        obj.getInfo();  
    }  
}  

Output

 Derived.java:11: error: getInfo() in Derived cannot override getInfo() in Base
    protected final void getInfo()
                         ^
  overridden method is final
1 error

Explanation

The getDetails() method is final; therefore it can not be overridden in the subclass.

98) Can we declare a constructor as final?

The constructor can never be declared as final because it is never inherited. Constructors are not ordinary methods; therefore, there is no sense to declare constructors as final. However, if you try to do so, The compiler will throw an error.

99) Can we declare an interface as final?

No, we cannot declare an interface as final because the interface must be implemented by some class to provide its definition. Therefore, there is no sense to make an interface final. However, if you try to do so, the compiler will show an error.

100) What is the difference between the final method and abstract method?

The main difference between the final method and abstract method is that the abstract method cannot be final as we need to override them in the subclass to give its definition.

Thursday, May 23, 2019

Java 8 has brought major changes in the Collection API. Some of the changes are:

The semantics are unclear, given that the contract for Iterator makes no guarantees about the order of iteration. Note, however, that ListIterator does provide an add operation, as it does guarantee the order of the iteration.

It can be implemented on top of current Iterator interface but since it’s use will be rare, it doesn’t make sense to include it in the interface that everyone has to implement.

We can iterate over a list in two different ways – using iterator and using for-each loop. Using iterator is more thread-safe because it makes sure that if underlying list elements are modified, it will throw ConcurrentModificationException.

We can use concurrent collection classes to avoid ConcurrentModificationException while iterating over a collection, for example CopyOnWriteArrayList instead of ArrayList. Check this post for ConcurrentHashMap Example.

UnsupportedOperationException is the exception used to indicate that the operation is not supported. It’s used extensively in JDK classes, in collections framework java.util.Collections.UnmodifiableCollection throws this exception for all add and removeoperations.

We can use any class as Map Key, however following points should be considered before using them. This is the reason why String and Integer are mostly used as HashMap keys.

Map interface provides three collection views:

HashMap and Hashtable both implements Map interface and looks similar, however there are following difference between HashMap and Hashtable.

ArrayList and Vector are similar classes in many ways. These are the differences between ArrayList and Vector.

ArrayList and LinkedList both implement List interface but there are some differences between them.

ArrayList, HashMap, TreeMap, Hashtable classes provide random access to it’s elements. Download java collections pdf for more information.

We can create a read-only collection using Collections.unmodifiableCollection(Collection c)method before passing it as argument, this will make sure that any operation to change the collection will throw UnsupportedOperationException.

We can use Collections.synchronizedCollection(Collection c) to get a synchronized (thread-safe) collection backed by the specified collection.

2) What are the differences between C++ and Java?

3) List the features of Java Programming language.

4) What do you understand by Java virtual machine?

5) What is the difference between JDK, JRE, and JVM?

JVM

JRE

JDK

6) How many types of memory areas are allocated by JVM?

7) What is JIT compiler?

8) What is the platform?

9) What are the main differences between the Java platform and other platforms?

10) What gives Java its 'write once and run anywhere' nature?

11) What is classloader?

12) Is Empty .java file name a valid source file name?

13) Is delete, next, main, exit or null keyword in java?

14) If I don't provide any arguments on the command line, then what will the value stored in the String array passed into the main() method, empty or NULL?

15) What if I write static public void instead of public static void?

16) What is the default value of the local variables?

17) What are the various access specifiers in Java?

18) What is the purpose of static methods and variables?

19) What are the advantages of Packages in Java?

20) What is the output of the following Java program?

21) What is the output of the following Java program?

22) What is the output of the following Java program?

Core Java - OOPs Concepts: Initial OOPs Interview Questions

23) What is object-oriented paradigm?

24) What is an object?

25) What is the difference between an object-oriented programming language and object-based programming language?

26) What will be the initial value of an object reference which is defined as an instance variable?

Core Java - OOPs Concepts: Constructor Interview Questions

27) What is the constructor?

28) How many types of constructors are used in Java?

29) What is the purpose of a default constructor?

30) Does constructor return any value?

31)Is constructor inherited?

32) Can you make a constructor final?

33) Can we overload the constructors?

34) What do you understand by copy constructor in Java?

35) What are the differences between the constructors and methods?

36) What is the output of the following Java program?

37) What is the output of the following Java program?

38) What is the output of the following Java program?

Core Java - OOPs Concepts: static keyword Interview Questions

39) What is the static variable?

40) What is the static method?

41) What are the restrictions that are applied to the Java static methods?

42) Why is the main method static?

43) Can we override the static methods?

44) What is the static block?

45) Can we execute a program without main() method?

46) What if the static modifier is removed from the signature of the main method?

47) What is the difference between static (class) method and instance method?

48) Can we make constructors static?

49) Can we make the abstract methods static in Java?

50) Can we declare the static variables and methods in an abstract class?

Core Java - OOPs Concepts: Inheritance Interview Questions

51) What is this keyword in java?

52) What are the main uses of this keyword?

53) Can we assign the reference to this variable?

54) Can this keyword be used to refer static members?

55) How can constructor chaining be done using this keyword?

56) What are the advantages of passing this into a method instead of the current class object itself?

57) What is Inheritance?

58) Why is Inheritance used in Java?

59) Which class is the superclass for all the classes?

We can iterate over a list in two different ways – using iterator and using for-each loop.

Using iterator is more thread-safe because it makes sure that if underlying list elements are modified, it will throw `ConcurrentModificationException`.

We can use concurrent collection classes to avoid `ConcurrentModificationException` while iterating over a collection, for example CopyOnWriteArrayList instead of ArrayList.
Check this post for ConcurrentHashMap Example.

`UnsupportedOperationException` is the exception used to indicate that the operation is not supported. It’s used extensively in JDK classes, in collections framework `java.util.Collections.UnmodifiableCollection` throws this exception for all `add` and `remove`operations.

We can use any class as Map Key, however following points should be considered before using them.

This is the reason why String and Integer are mostly used as HashMap keys.

ArrayList and Vector are similar classes in many ways.

These are the differences between ArrayList and Vector.

We can create a read-only collection using `Collections.unmodifiableCollection(Collection c)`method before passing it as argument, this will make sure that any operation to change the collection will throw `UnsupportedOperationException`.

We can use `Collections.synchronizedCollection(Collection c)` to get a synchronized (thread-safe) collection backed by the specified collection.