Java静态方法参数

Question

为什么以下代码返回 100 100 1 1 1 而不是 100 1 1 1 1？

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() + " ");
}
}

为什么它似乎将 Hotel 按值传递给 doStuff() ？

Answer 1

它完全按照您的指示去做:-)

Hotel h1 = new Hotel(100);
System.out.print(h1.getRoomNr() + " "); // 100
Hotel h2 = doStuff(h1);
System.out.print(h1.getRoomNr() + " "); // 100 - h1 is not changed, h2 is a distinct new object
System.out.print(h2.getRoomNr() + " "); // 1
h1 = doStuff(h2);
System.out.print(h1.getRoomNr() + " "); // 1 - h1 is now changed, h2 not
System.out.print(h2.getRoomNr() + " "); // 1

正如其他人指出的(并且解释得非常清楚 in this article )，Java 按值传递。在这种情况下，它将引用 h1 的副本传递给 doStuff。副本被新引用覆盖(然后返回并分配给 h2)，但 h1 的原始值不受影响:它仍然引用第一个酒店房间号为 100 的对象。