为什么我的 HashMap 允许重复键?

问题描述

嘿，我正在使用 HashMap 来跟踪公告板上的服务和服务请求.但是，我必须有哈希码和等于错误，因为我得到了重复的键.谁能告诉这可能是为什么?

Hey I'm using a HashMap to keep track of services and service-requests on a BulletinBoard. However, I must have the hashcode and equals wrong because I'm getting duplicate keys. Can anyone tell why this might be?

keySet的内容:

Services: [1, 1, 6, 6, 3]
Requests: [8, 7, 6, 5, 8, 4, 5, 6, 2]

以下是相关代码:

private static final HashMap<Advert, Integer> services = new HashMap<>();
...

public class Advert {

private int id;
private BoardPoster poster;

public Advert(BoardPoster poster) {
    this.poster = poster;
}

public BoardPoster getPoster() {
    return poster;
}

public void spawn() {
    id = RANDOM.nextInt(ADVERT_RANGE);
}

public int getID() {
    return id;
}

@Override
public String toString() {
    return Integer.toString(id);
}

@Override
public boolean equals(Object o) {
    if (o != null && o instanceof Advert) {
        return ((Advert) o).id == id;
    }
    return false;
}

@Override
public int hashCode() {
    return 67 * 5 + this.id;
}
}

推荐答案

最可能的原因是您用作键的对象是可变的.因此，如果您执行以下操作:

The likeliest reason is that the objects you use as keys are mutable. So if you do something like:

map.put(anAdvert, 1);
anAdvert.spawn(); //modifies id, which affects hashcode and equals

地图的行为会出乎意料.

The behaviour of the map will be unexpected.

cf 地图的javadoc

注意:如果将可变对象用作映射键，则必须非常小心.如果对象的值以影响等于比较的方式更改，而对象是映射中的键，则不会指定映射的行为.

Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map.

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