请问以下输出是什么？，请写明原因。
public class Test {
public static void main(String[] args) throws Exception {
StringBuilder a= new StringBuilder("A");
StringBuilder b= new StringBuilder("B");
exchange(a,b);
System.out.println(a + "." + b);
}
public static void exchange(StringBuilder x, StringBuilder y) {
x.append(y);
y=x;
}
}
