java设计把两个字符串的值交换 而不使用中间变量

public  class  Test {
    public  static  void  main(String[] args) {
        String s1 =  "aaa" ;
        String s2 =  "cccx" ;

        s1 = s1 + s2;
        s2 = s1.substring( 0 , s1.length()-s2.length());
        s1 = s1.substring(s2.length());

        System.out.println(s1+ " - " +s2);
    }
}