java - scjp 传递对象

Question

public class Hotel {
    private int roomNr;

    public Hotel(int roomNr) {
        this.roomNr = roomNr;
    }

    public int getRoomNr() {
        return this.roomNr;
    }

    static Hotel doStuff(Hotel hotel) {
        hotel = new Hotel(1);
        return hotel;
    }

    public static void main(String args[]) {
        Hotel h1 = new Hotel(100);
        System.out.print(h1.getRoomNr() + " ");
        Hotel h2 = doStuff(h1);
        System.out.print(h1.getRoomNr() + " ");
        System.out.print(h2.getRoomNr() + " ");
        h1 = doStuff(h2);
        System.out.print(h1.getRoomNr() + " ");
        System.out.print(h2.getRoomNr() + " ");
    }
}

为什么调用 doStuff(h1) 后 h1 没有改变？据我了解，应该传递对对象的引用，并且在方法中应该将其替换为新对象。

Answer 1

在这一部分

static Hotel doStuff(Hotel hotel) {
    hotel = new Hotel(1);
    return hotel;
}

变量hotel是一个新的局部变量，接收引用值。这个新的局部变量在第一行加载了对新 Hotel 实例的新引用，并返回了这个新引用。

外部局部变量h1不会改变。

<小时/>

main:h1 = 0x0000100                     (the old Hotel's address)
       |
     copying
       |
        ------->  doStuff:hotel = 0x0000100  (the method call)
                  doStuff:hotel = 0x0000200  (the new Hotel's address)
                              |
                          copying
                              |
main:h2 = 0x0000200 <---------