JAVA INTERVIEW QUESTIONS AND ANSWERS

What is JAVA?

 Java is a high-level programming language and is platform independent.

Java is a collection of objects. It was developed by Sun Microsystems. There are a lot of applications, websites and Games that are developed using Java.

 What are the features in JAVA?

Features of Java:

  • Oops concepts
    • Object-oriented
    • Inheritance
    • Encapsulation
    • Polymorphism
    • Abstraction
  • Platform independent: A single program works on different platforms without any modification.
  • High Performance: JIT (Just In Time compiler) enables high performance in Java. JIT converts the bytecode into machine language and then JVM starts the execution.
  • Multi-threaded: A flow of execution is known as a Thread. JVM creates a thread which is called main thread. The user can create multiple threads by extending the thread class or by implementing Runnable interface.

How does Java enable high performance?

Java uses Just In Time compiler to enable high performance. JIT is used to convert the instructions into bytecodes.

What are the Java IDE’s?

Eclipse and NetBeans are the IDE’s of JAVA.

What do you mean by Constructor?

The points given below explain what a Constructor is in detail:

  • When a new object is created in a program a constructor gets invoked corresponding to the class.
  • The constructor is a method which has the same name as class name.
  • If a user doesn’t create a constructor implicitly a default constructor will be created.
  • The constructor can be overloaded.
  • If the user created a constructor with a parameter then he should create another constructor explicitly without a parameter.

What is meant by Local variable and Instance variable?

Local variables are defined in the method and scope of the variables that have existed inside the method itself.

An instance variable is defined inside the class and outside the method and scope of the variables exist throughout the class.

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.

  1. 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.
  2. 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.
  3. 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.

What are constructors in Java?

In Java, constructor refers to a block of code which is used to initialize an object. It must have the same name as that of the class. Also, it has no return type and it is automatically called when an object is created.

There are two types of constructors:

  1. Default constructor
  2. Parameterized constructor

Explain public static void main(String args[]).

  • public : Public is an access modifier, which is used to specify who can access this method. Public means that this Method will be accessible by any Class.
  • static : It is a keyword in java which identifies it is class based i.e it can be accessed without creating the instance of a Class.
  • void : It is the return type of the method. Void defines the method which will not return any value.
  • main: It is the name of the method which is searched by JVM as a starting point for an application with a particular signature only. It is the method where the main execution occurs.
  • String args[] : It is the parameter passed to the main method.

What is singleton class and how can we make a class singleton?

Singleton class is a class whose only one instance can be created at any given time, in one JVM. A class can be made singleton by making its constructor private.

What is the difference between Array list and vector?

Array ListVector
Array List is not synchronized.Vector is synchronized.
Array List is fast as it’s non-synchronized.Vector is slow as it is thread safe.
If an element is inserted into the Array List, it increases its Array size by 50%.Vector defaults to doubling size of its array.
Array List does not define the increment size.Vector defines the increment size.
Array List can only use Iterator for traversing an Array List.Except Hashtable, Vector is the only other class which uses both Enumeration and Iterator.

What is the difference between equals() and == ?

Equals() method is defined in Object class in Java and used for checking equality of two objects defined by business logic.
“==” or equality operator in Java is a binary operator provided by Java programming language and used to compare primitives and objects. public boolean equals(Object o) is the method provided by the Object class. The default implementation uses == operator to compare two objects. For example: method can be overridden like String class. equals() method is used to compare the values of two objects.

What is association?

Association is a relationship where all object have their own lifecycle and there is no owner. Let’s take an example of Teacher and Student. Multiple students can associate with a single teacher and a single student can associate with multiple teachers but there is no ownership between the objects and both have their own lifecycle. These relationship can be one to one, One to many, many to one and many to many.

What do you mean by aggregation?

Aggregation is a specialized form of Association where all object have their own lifecycle but there is ownership and child object can not belongs to another parent object. Let’s take an example of Department and teacher. A single teacher can not belongs to multiple departments, but if we delete the department teacher object will not destroy. 

What is composition in Java?

Composition is again specialized form of Aggregation and we can call this as a “death” relationship. It is a strong type of Aggregation. Child object dose not have their lifecycle and if parent object deletes all child object will also be deleted. Let’s take again an example of relationship between House and rooms. House can contain multiple rooms there is no independent life of room and any room can not belongs to two different house if we delete the house room will automatically delete.

What are the Oops concepts?

Ans: Oops concepts include:

  • Inheritance
  • Encapsulation
  • Polymorphism
  • Abstraction
  • Interface

What is Inheritance?

Ans: Inheritance means one class can extend to another class. So that the codes can be reused from one class to another class.

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++.

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.

What is method overloading and method overriding?

Method Overloading :

  • In Method Overloading, Methods of the same class shares the same name but each method must have different number of parameters or parameters having different types and order.
  • Method Overloading is to “add” or “extend” more to method’s behavior.
  • It is a compile time polymorphism.
  • The methods must have different signature.
  • It may or may not need inheritance in Method Overloading.

Let’s take a look at the example below to understand it better.

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class Adder {
Static int add(int a, int b)
{
return a+b;
}
Static double add( double a, double b)
{
return a+b;
}
public static void main(String args[])
{
System.out.println(Adder.add(11,11));
System.out.println(Adder.add(12.3,12.6));
}}

Method Overriding:  

  • In Method Overriding, sub class have the same method with same name and exactly the same number and type of parameters and same return type as a super class.
  • Method Overriding is to “Change” existing behavior of method.
  • It is a run time polymorphism.
  • The methods must have same signature.
  • It always requires inheritance in Method Overriding.

Let’s take a look at the example below to understand it better.

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class Car {
void run(){
System.out.println(“car is running”);
}
Class Audi extends Car{
void run()
{
System.out.prinltn(“Audi is running safely with 100km”);
}
public static void main( String args[])
{
Car b=new Audi();
b.run();
}
}
 
 
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