StringBuffer

public class Foo
{
      public static void main(String[] args)
      {
            StringBuffer a = new StringBuffer("A");
            StringBuffer b = new StringBuffer("B");
            operate(a,b);
            System.out.println(a+" , "+b);
      }
      
      static void operate(StringBuffer x, StringBuffer y)
      {
            x.append(y);
            y = x;
      }
}

The code compiles and prints "AB , B".