Most Important Java Interview Questions and Answers

1) what are static blocks and static initializers in Java?

In most of java interview questions commonly asked question, Static blocks or static initializers are used to initialize static fields in java.

we declare static blocks when we want to initialize static fields in our class.

Static blocks get executed exactly once when the class is loaded Static blocks are executed even before the constructors are executed.

2) How to call one constructor from the other constructor?

With in the same class if we want to call one constructor from other we use this() method.

Based on the number of parameters we pass appropriate this() method is called.
Restrictions for using this method :
1) this must be the first statement in the constructor
2)we cannot use two this() methods in the constructor

3) What is method overriding in java?

If we have methods with the same signature (same name, same signature, same return type) in the superclass and subclass then we say
subclass method is overridden by the superclass.

When to use overriding in java?
If we want the same method with different behavior in superclass and subclass then we go for overriding.
When we call overridden method with subclass reference subclass method is called hiding the superclass
method.

Method Overriding and Method overloading important java programming interview questions mostly asked in interviews

4) What is a super keyword in java?

Variables and methods of the superclass can be overridden in a subclass.

In case of overriding, a subclass object calls its own variables and methods.

A subclass cannot access the variables and methods of superclass because the overridden variables or methods hide the methods and variables of the superclass.

But still, java provides a way to access superclass members even if its members are overridden. Super is used to access superclass variables, methods, constructors.
Super can be used in two forms :
1) the First form is for calling the superclass constructor.
2) the Second one is to call superclass variables, methods.
Super if present must be the first statement.

5) Difference between method overloading and method overriding in java ?

Important Java interview questions

6) Difference between abstract class and interface ?

7) Why java is platform-independent?

The most unique feature of java is platform-independent.

In any programming language, source code is compiled into executable code. This cannot be run across all platforms.

When javac compiles a java program it generates an executable file called .class file. a class file contains byte codes.

Byte codes are interpreted only by JVM’s. Since these JVM’s are made available across all platforms by Sun Microsystems, we can execute this byte code on any platform.

Byte code generated in a windows environment can also be executed in a Linux environment. This makes the java platform independent.

8) What is method overloading in java?

A class having two or more methods with the same name but with different arguments then we say that those methods are overloaded.

Static polymorphism is achieved in java using method overloading. Method overloading is used when we want the methods to perform similar tasks but with different inputs or values. When an overloaded method is invoked java first checks the method name, the number of arguments, type of arguments; based on this compiler executes this method. The compiler decides which method to call at compile time. By using overloading static polymorphism or static binding can be achieved in java.
Note: Return type is not part of the method signature. we may have methods with different return types but return type alone is not sufficient to call a method in java.

9) What is the JIT compiler?

JIT compiler stands for Just in time compiler. JIT compiler compiles byte code into executable code. JIT is a part of JVM.JIT cannot convert a complete java program into executable code it converts as and when it is needed during execution.

10) What is bytecode in java?

When a javac compiler compiler compiles a class it generates a .class file. This .class file contains a set of instructions called byte code. Byte code is a machine-independent language and contains a set of instructions which are to be executed only by JVM. JVM can understand these byte codes

11) Difference between this() and super() in java ?

this() is used to access one constructor from another within the same class while super() is used to access the superclass constructor. Either this() or super() exists it must be the first statement in the constructor.

12) What is a class?

Classes are the fundamental or basic unit in Object-Oriented Programming . A class is a kind of blueprint or template for objects. Class defines variables, methods. A class tells what type of objects we are creating. For example, taking a Department class tells us we can create department-type objects. We can create any number of department objects. All programming constructs in java reside in class. When JVM starts running it first looks for the class when we compile. Every Java application must have at least one class and one main method. Class starts with the class keyword. A class definition must be saved in a class file that has the same class name. The file name must end with .java extension

public class FirstClass 
{
    public static void main(String[] args) 
    {
        System.out.println(“My First class”);
    }
}

If we see the above class when we compile JVM loads the FirstClass and generates a .class file(FirstClass.class). When we run the program we are running the class and then executes the main method

13) What is an object ?

It’s basics of java interview questions, An Object is an instance of the class. A class defines the type of object. Each object belongs to some class. Every object contains a state and behavior. The state is determined by the value of attributes and behavior is called
method. Objects are also called an instance. To instantiate the class we declare with the class type

public classFirstClass 
{
   public static voidmain(String[] args) 
   {
      FirstClass f=new FirstClass();
      System.out.println(“My First class”);
   }
}

To instantiate the FirstClass we use this statement
FirstClass f=new FirstClass();
f is used to refer FirstClass object.

14)What is method in java ?

It contains the executable body that can be applied to the specific object of the class.
Method includes method name, parameters or arguments and return type and a body of executable code.

Syntax : type methodName(Argument List){
}

ex : public float add(int a, int b, int c)
methods can have multiple arguments. Separate with commas when we have multiple arguments

15) What is encapsulation ?

The process of wrapping or putting up of data in to a single unit class and
keeps data safe from misuse is called encapsulation .
Through encapsulation we can hide and protect the data stored in java objects.Java supports encapsulation through access control. There are four access control modifiers in java public , private, protected and default level.
For example take a car class , In car we have many parts which is not required for driver to know what all it consists inside. He is required to know only about how to start and stop the car. So we can expose what all are required and hide the rest by using encapsulation

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16) Why main() method is public, static, and void in java?

public: “public” is an access specifier which can be used outside the class. When the main method is declared public it means it can be used outside the class.
static: To call a method we require an object. Sometimes it may be required to call a method without the help of an object. Then we declare that method as static. JVM calls the main() method without creating an object by declaring keyword static.
void: the void return type is used when a method doesn’t return any value. the main() method doesn’t return any value, so main() is declared as void.

Signature : public static void main(String[] args) {

17) Explain about main() method in java ?

Main() method is starting point of execution for all java applications.

public static void main(String[] args) {

}

String args[] are an array of string objects we need to pass from command line arguments.
Every Java application must have at least one main method

18)What is constructor in java ?

A constructor is a special method used to initialize objects in java.
we use constructors to initialize all variables in the class when an object is created. As and when an object
is created it is initialized automatically with the help of constructor in java.
We have two types of constructors
i) Default Constructor
ii) Parameterized Constructor

Constructor is one of the interview question asked in java interview questions

Signature : public classname()
{
}
Signature : public classname(parameters list)
{
}

19) What is difference between length and length() method in java ?

length() : In String class we have length() method which is used to return the number of characters in
string.
Ex : String str = “Hello World”;
System.out.println(str.length());
Str.length() will return 11 characters including space.

length : we have length instance variable in arrays which will return the number of values or objects in
array.
For example :
String days[]={” Sun”,”Mon”,”wed”,”thu”,”fri”,”sat”};
Will return 6 since the number of values in days array is 6

20) Difference between Character Constant and String Constant in java ?

Character constant is enclosed in single quotes. String constants are enclosed in double quotes. Character
constants are single digit or character. String Constants are collection of characters.
Ex :’2’, ‘A’
Ex : “Hello World”

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