I have a program as follows:
public class Test{
    public static void main(String[] main)
    {
        StringBuffer test3 = new StringBuffer("A");
            StringBuffer test4 = new StringBuffer("B");
            operate(test3,test4);
            System.out.println(test3 + "," + test4);
    }

    public static void operate(StringBuffer a, StringBuffer b)
    {
        a.append(b);
        b = a;
       
        System.out.println("b:" + b);
    }
}

why the output is "AB,B" not "AB,AB"?
thanks