Equality (==) operator in Java with Examples

== operator is a type of Relational Operator in Java used to check for relations of equality. It returns a boolean result after the comparison and is extensively used in looping statements and conditional if-else statements. 

Syntax: 

LHS value == RHS value

But, while comparing these values, three cases arise generally:

Case 1: When both LHS and RHS values are primitive 

This is the most simple among the cases. As primitive data is stored in the stack memory, in this case, the actual value of both sides is fetched from the stack memory and compared. It returns true if they both are equal, else false is returned.

Syntax: 

Actual value == Actual value

Example: 

// Java program for using == operator

import java.io.*;

public class GFG {
    public static void main(String[] args)
    {
        // Declaring primitive values
        int a = 4;
        int b = 4;
        int c = 5;

        // Comparing a and b using == operator
        System.out.println("Are " + a + " and " + b
                           + " equal? " + (a == b));

        // Comparing b and c using == operator
        System.out.println("Are " + b + " and " + c
                           + " equal? " + (b == c));
    }
}

Are 4 and 4 equal? true
Are 4 and 5 equal? false

Case 2: When one of the LHS and RHS values is primitive, and the other is a reference 

In this scenario, for the primitive side, the actual value is taken for comparison from the stack memory. But for the reference side, when an array is declared and initialized, the data is stored in the heap memory and the reference pointer in the stack memory. So all that is in the stack memory is the memory address. 

Syntax: 

Actual value == Address value
    OR
Address value == Actual value

So when the comparison between a primitive value and a reference value is taken, the program doesn't compile and throws an error:

Compilation Error in java code:- 

prog.java:20: error: incomparable types: int and int[]
                           + (a == b));
                                ^
1 error

This is because the value for the primitive side is easily fetched from the stack memory, but for the reference side, the value cannot be fetched as the value is in heap memory. Hence the error.

Example: 

// Java program for using == operator

import java.io.*;

public class GFG {
    public static void main(String[] args)
    {

        // Declaring primitive value
        int a = 4;

        // Declaring reference value
        int[] b = { 1, 2, 3, 4 };

        // Comparing a and b using == operator
        System.out.println("Are " + a + " and " + b
                           + " equal? " + (a == b));
    }
}

prog.java:17: error: bad operand types for binary operator '=='
                           + " equal? " + (a == b));
                                             ^
  first type:  int
  second type: int[]
1 error

Case 3: When both of the LHS and RHS values are reference 

In this scenario, for both sides, when an array is declared and initialized, the data is stored in the heap memory and the reference pointer in the stack memory. So both the variables, their address is checked. If both the variables point to the same memory address, this operator returns true. Else it returns false.

Syntax: 

Address value == Address value

Example: 

// Java program for using == operator

import java.io.*;

public class GFG {
    public static void main(String[] args)
    {

        // Declaring reference value
        int[] a = { 1, 2, 3, 4 };
        int[] b = { 1, 2, 3, 4 };
        int[] c = b;

        // Comparing a and b using == operator
        // Though they both have the same value
        // the output will be false because
        // they both have a different address in the memory
        System.out.println("Are a and b equal? "
                           + (a == b));

        // Comparing b and c using == operator
        // Though they both have the same value
        // the output will be true because
        // they both have same address in the memory
        System.out.println("Are b and c equal? "
                           + (b == c));
    }
}

Are a and b equal? false
Are b and c equal? true