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Java Program to Swap Two Numbers Using a Temporary Variable

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Swapping two numbers means exchanging their values: if a is 10 and b is 20, after the swap a is 20 and b is 10.

You cannot simply write a = b; b = a;. The first line overwrites the old value of a, so both variables end up as 20. A temporary variable keeps a copy of one value while the other is moved.

1. How the Swap Works

StepStatementabtemp
Start1020-
1temp = a;102010
2a = b;202010
3b = temp;201010

2. Swap Two Numbers Using a Temporary Variable

SwapWithTemp.java 
public class SwapWithTemp {
    public static void main(String[] args) {
        int a = 10;
        int b = 20;

        System.out.println("Before swapping: a = " + a + ", b = " + b);

        int temp = a;   // save a
        a = b;          // copy b into a
        b = temp;       // copy the saved value into b

        System.out.println("After swapping:  a = " + a + ", b = " + b);
    }
}
Output
Before swapping: a = 10, b = 20
After swapping:  a = 20, b = 10

3. Swap Two Numbers with User Input

The same three lines work for any type. This version reads two double values, so decimal numbers can be swapped too.

SwapWithTempInput.java 
import java.util.Scanner;

public class SwapWithTempInput {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter first number: ");
        double first = scanner.nextDouble();
        System.out.print("Enter second number: ");
        double second = scanner.nextDouble();
        scanner.close();

        double temp = first;
        first = second;
        second = temp;

        System.out.println("After swapping: first = " + first + ", second = " + second);
    }
}
Output (with sample input)
Enter first number: 1500
Enter second number: 30.5
After swapping: first = 30.5, second = 1500.0

4. Summary

  • Swapping exchanges the values of two variables.
  • temp = a; a = b; b = temp; is the standard, always-safe way to swap.
  • Writing a = b; b = a; loses the original value of a.
  • The temporary-variable method works for every type: numbers, strings and objects.

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