Java Program to Swap two Strings Without Using any Third Variable

Given two string variables, a and b, your task is to write a Java Program to swap these variables without using any temporary or third variable. Use of library methods is allowed.

Examples: 

Input: a = "Hello"
       b = "World"

Output:
Strings before swap: a = Hello and b = World
Strings after swap: a = World and b = Hello

Method 1 : In order to swap two string variables without using any temporary or third variable, the idea is to use string concatenation and substring() methods to perform this operation. The substring() method comes in two forms, as listed below: 

• substring(int beginindex): This function will return the substring of the calling string starting from the index passed as an argument to this function till the last character in the calling string. 
• substring(int beginindex, int endindex): This function will return the Substring of the calling string starting from the beginindex(inclusive) and ending at the endindex(exclusive) passed as an argument to this function. 
   

Algorithm: 

1) Append second string to first string and 
   store in first string:
   a = a + b

2) call the method substring(int beginindex, int endindex)
   by passing beginindex as 0 and endindex as,
      a.length() - b.length():
   b = substring(0,a.length()-b.length());

3) call the method substring(int beginindex) by passing 
   b.length() as argument to store the value of initial 
   b string in a
   a = substring(b.length());

Below is the implementation of the above approach:

// Java program to swap two strings without using a temporary
// variable.
import java.util.*;

class Swap
{    
    public static void main(String args[])
    {
        // Declare two strings
        String a = "Hello";
        String b = "World";
        
        // Print String before swapping
        System.out.println("Strings before swap: a = " + 
                                       a + " and b = "+b);
        
        // append 2nd string to 1st
        a = a + b;
        
        // store initial string a in string b
        b = a.substring(0,a.length()-b.length());
        
        // store initial string b in string a
        a = a.substring(b.length());
        
        // print String after swapping
        System.out.println("Strings after swap: a = " + 
                                     a + " and b = " + b);        
    }    
}

Strings before swap: a = Hello and b = World
Strings after swap: a = World and b = Hello

Time Complexity: O(n+m) where n and m are lengths of given strings.
Auxiliary Space: O(n+m)

Method 2 : Using Xor operator

Algorithm :

1. Convert the input strings to char arrays.
2. The XOR swap algorithm uses the XOR operator to swap the values of two variables without using a temporary variable.
3. The program applies this algorithm to each pair of characters in the input string using loop.
4. After performing the xor operations convert back the char arrays into strings.

Below is the implementation of the above approach.

/*package whatever //do not write package name here */

import java.io.*;
public class Main
{
    public static void main(String[] args) {
        String a = "Hello";
        String b = "World";
        
        System.out.println("Before swap:");
        System.out.println("a = " + a);
        System.out.println("b = " + b);
        
        // Convert strings to char arrays
        char[] charArrayA = a.toCharArray();
        char[] charArrayB = b.toCharArray();
        
        // XOR swap algorithm to swap characters without temporary variable
        for(int i = 0; i < charArrayA.length && i < charArrayB.length; i++) {
            charArrayA[i] ^= charArrayB[i];
            charArrayB[i] ^= charArrayA[i];
            charArrayA[i] ^= charArrayB[i];
        }
        
        // Convert char arrays back to strings
        a = new String(charArrayA);
        b = new String(charArrayB);
        
        System.out.println("After swap:");
        System.out.println("a = " + a);
        System.out.println("b = " + b);
        
    }
}

Before swap:
a = Hello
b = World
After swap:
a = World
b = Hello

Time Complexity : O( n+m ) 

Auxiliary Space : O( n+m )

where n and m are lengths of the string of a and b respectively.