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

Output:
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 function will return substring of the calling string ranging from the index passed as argument to the present function till the last character within the calling string.
• substring(int beginindex,int endindex) This function will return Substring of the calling string ranging from the beginindex(inclusive) and ending at the endindex(exclusive) passed as argument to the present 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 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);	
	}
}

Output: 

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