Here is an example that prompts the user to enter two string and then swaps both the string without using third variable.

import java.util.*;

class SwapTwoStringsWithoutUsingThirdVariable 
{
    public static void main(String[] args) 
    {
        Scanner input=new Scanner(System.in);
        System.out.print("Enter string1: ");
        String st1=input.next();

        System.out.print("Enter string2: ");
        String st2=input.next();
        System.out.println();
        System.out.println("Before swapping:");
        System.out.println("st1="+st1+" and st2="+st2);
        st1=st1+st2;
        st2 = st1.substring(0,(st1.length()-st2.length()));
        st1 = st1.substring(st2.length(),(st1.length()));         

        System.out.println();
        System.out.println("After swapping:");
        System.out.println("st1="+st1+" and st2="+st2);
    }
}