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;
}
}
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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