Java Program to Swap Two Numbers

To swap two numbers in Java, use a temporary variable like temp = a; a = b; b = temp; to exchange their values.

Examples

Inputa = 5, b = 10

OutputAfter swap: a = 10, b = 5

Inputa = -3, b = 7

OutputAfter swap: a = 7, b = -3

Inputa = 0, b = 0

OutputAfter swap: a = 0, b = 0

How to Think About It

To swap two numbers, think of holding one number temporarily so you don't lose it when you overwrite its variable. Use a temporary container to store one number, then assign the second number to the first variable, and finally put the stored number into the second variable.

Algorithm

1

Get the first number and store it in variable a

2

Get the second number and store it in variable b

3

Create a temporary variable temp and assign it the value of a

4

Assign the value of b to a

5

Assign the value of temp to b

6

Print the swapped values of a and b

Code

java

public class SwapNumbers {
    public static void main(String[] args) {
        int a = 5;
        int b = 10;
        int temp;
        temp = a;
        a = b;
        b = temp;
        System.out.println("After swap: a = " + a + ", b = " + b);
    }
}

Output

After swap: a = 10, b = 5

Dry Run

Let's trace swapping a=5 and b=10 through the code

1

Initial values

a = 5, b = 10

2

Store a in temp

temp = 5

3

Assign b to a

a = 10

4

Assign temp to b

b = 5

5

Print swapped values

a = 10, b = 5

Step	a	b	temp
Initial	5	10	undefined
Store a in temp	5	10	5
Assign b to a	10	10	5
Assign temp to b	10	5	5
Print result	10	5	5

Why This Works

Step 1: Use a temporary variable

We use temp to hold the value of a so it is not lost when a is overwritten.

Step 2: Assign second number to first

We assign the value of b to a, so now a has the second number.

Step 3: Assign temp to second number

We assign the stored value in temp to b, completing the swap.

Alternative Approaches

Swap without temporary variable using addition and subtraction

java

public class SwapNumbers {
    public static void main(String[] args) {
        int a = 5, b = 10;
        a = a + b;
        b = a - b;
        a = a - b;
        System.out.println("After swap: a = " + a + ", b = " + b);
    }
}

This method avoids extra memory but can cause overflow with large numbers.

Swap using XOR bitwise operator

java

public class SwapNumbers {
    public static void main(String[] args) {
        int a = 5, b = 10;
        a = a ^ b;
        b = a ^ b;
        a = a ^ b;
        System.out.println("After swap: a = " + a + ", b = " + b);
    }
}

This method uses bitwise operations and no extra memory but is less readable.

Complexity: O(1) time, O(1) space

Time Complexity

Swapping two numbers takes a fixed number of steps regardless of input size, so it is O(1).

Space Complexity

Only a few variables are used, so space complexity is O(1).

Which Approach is Fastest?

All methods run in constant time; using a temporary variable is simplest and safest, while arithmetic or XOR methods avoid extra variables but can be less clear or risky.

Approach	Time	Space	Best For
Temporary variable	O(1)	O(1)	Clarity and safety
Addition and subtraction	O(1)	O(1)	Avoiding extra variable but risk overflow
XOR bitwise	O(1)	O(1)	Low-level operations, no extra memory