String 不可变 笔试题

public class Test {  
    public static void main(String[] args){  
        String str = "aa";  
        A a = new A();  
        System.out.println("str="+str);  
        System.out.println("a.str="+a.str);  
    }  
    public void transform(String str,A a){  
        str = "aaaaa" ;  
        a.change("bbbbb");  
    }  
}  
class A{  
    public String str = "bb";  
    public void change(String str){  
        this.str = str;  
    }  
}

答案
str=aa
a.str=bbbbb