怎么理解字符串对象是不可改变的对象？字符串引用变量的值可以改变？

        String s1 = "What is ";
        System.out.println("s1:"+s1);
        String s2 = s1;
        s1 = s1+"Java!";
        System.out.println("s2:"+s2);//s2 引用了s1,s1对象始终没有改变
        System.out.println("s1:"+s1);