大佬们告诉我会输出什么？为什么

会输出什么呢？

public static void main(String[] args) {
		StringBuffer s1 = new StringBuffer("AA");
		StringBuffer s2 = new StringBuffer("BB");
		add(s1,s2);
		System.out.println(s1);
		System.out.println(s2);
	}
	public static void add(StringBuffer s1,StringBuffer s2){
		s1.append(s2);
		s2=s1;
	}