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.

Examples:

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 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.

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());

Output:

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

References:
http://docs.oracle.com/

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