Swap two Strings without using third user defined variable in Java

Given two string variables a and b, swap these variables without using temporary or third variable in Java. Use of library methods is allowed.


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

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

The idea is to string concatenation and substring() method to perform this operation. The substring() method comes in two forms as listed below:

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


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());
// 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 intial 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


Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

This article is attributed to GeeksforGeeks.org

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