public class Foo{
public static void main(String[] sgf){
  
  StringBuffer a = new StringBuffer("A");
  StringBuffer b = new StringBuffer("B");
  
  operate(a,b);
  
  System.out.println(a+","+b);//这里要求不能有变动
}

static void operate(StringBuffer x,StringBuffer y){
  
    x.append(y);
    y=x;
}
}