StringBuffer疑问

为何结果为AB、B？

public static void main(String[] args) {

StringBuffer a=new StringBuffer("A");
StringBuffer b=new StringBuffer("B");
warp(a,b);
System.out.println(a+","+b);
}
public static void warp(StringBuffer x,StringBuffer y) {
x.append(y);
y=x;
}