Write a Java program to swap two strings without using a third variable.

Q: Write a Java program to swap two strings without using a third variable.

Swap Two Strings Without Third Variable java public class StringSwapping { public static void main(String args) { String a = "Hello"; String b = "World"; System.out.println("before swapping: "); System.out.println("the value of a is: " + a); System.out.println("the value of b is: " + b); // Step 1: append a and b a = a + b; // a = "HelloWorld" // Step 2: store initial string a in String b b = a.substring(0, a.length() - b.length()); // b = "HelloWorld".substring(0, 10-5) = "HelloWorld".substring(0,5) = "Hello" // Step 3: store initial string b in String a a = a.substring(b.length()); // a = "H

Answer

Swap Two Strings Without Third Variable

Java

public class StringSwapping {
    public static void main(String[] args) {
        String a = "Hello";
        String b = "World";

        System.out.println("before swapping: ");
        System.out.println("the value of a is: " + a);
        System.out.println("the value of b is: " + b);

        // Step 1: append a and b
        a = a + b;   // a = "HelloWorld"

        // Step 2: store initial string a in String b
        b = a.substring(0, a.length() - b.length());
        // b = "HelloWorld".substring(0, 10-5) = "HelloWorld".substring(0,5) = "Hello"

        // Step 3: store initial string b in String a
        a = a.substring(b.length());
        // a = "HelloWorld".substring(5) = "World"

        System.out.println("the value of a and b after swapping: ");
        System.out.println("the value of a is: " + a);
        System.out.println("the value of b is: " + b);
    }
}

Output

CODE

before swapping:
the value of a is: Hello
the value of b is: World
the value of a and b after swapping:
the value of a is: World
the value of b is: Hello

Step-by-Step Trace

CODE

a = "Hello" (length=5)
b = "World" (length=5)

Step 1: a = a + b = "HelloWorld"  (length=10)

Step 2: b = a.substring(0, a.length()-b.length())
           = "HelloWorld".substring(0, 10-5)
           = "HelloWorld".substring(0, 5)
           = "Hello"

Step 3: a = a.substring(b.length())
           = "HelloWorld".substring(5)
           = "World"

Result: a="World", b="Hello"  ✓

Simpler: Using Temp Variable (Standard Practice)

Java

String a = "Hello";
String b = "World";
String temp = a;
a = b;
b = temp;
System.out.println("a=" + a + ", b=" + b);  // a=World, b=Hello

With StringBuilder

Java

StringBuilder a = new StringBuilder("Hello");
StringBuilder b = new StringBuilder("World");

a.append(b);                                     // a = "HelloWorld"
b = new StringBuilder(a.substring(0, a.length() - b.length()));  // b = "Hello"
a = new StringBuilder(a.substring(b.length()));  // a = "World"

Important Note

The without-temp approach only works correctly when both strings have no overlapping characters that could confuse the substring extraction. For production code, always use a temp variable — it's clearer and safer.