Java HashMap containsKey始终为false [英] Java HashMap containsKey always false

问题描述

我有一个有趣的情况，我将 Coordinate 存储到 HashMap< Coordinate，GUIGameField> 中。

现在，奇怪的是，我有一段代码，应该防范，不应该使用两次坐标。但是，如果我调试此代码：

$ $ p $ $ code if（mapForLevel.containsKey（coord））{
throw new IllegalStateException（这个坐标已经被使用了！）;
} else {
... do stuff ...
}

... containsKey 总是返回 false ，尽管我将一个散列码为9731的坐标存储到map和当前coord也有hashcode 9731。

之后， mapForLevel.entrySet（）看起来像：

 （java.util.HashMap $ EntrySet）[（270,90）=gui.GUIGameField@29e357，（270， 90）= gui.GUIGameField@ca470] 
  

我可能做错了什么？我用完了想法。感谢您的帮助！

  public class Coordinate {
 int xCoord; 
 int yCoord; 
 
公共坐标（int x，int y）{
 ...存储参数属性... 
} 
 
 ... getters& amp ; setters ... 
 
 @Override 
 public int hashCode（）{
 int hash = 1; 
 hash = hash * 41 + this.xCoord; 
 hash = hash * 31 + this.yCoord; 
返回散列; 
 
 
 
 $ div $解析方案

你除了 hashCode 之外，还应该覆盖 equals 以使其正常工作。



编辑：我错误地指出你应该在 equals 中使用 hashCode code> - 这是不正确的。虽然 hashCode 必须为两个相等的对象返回相同的结果，但仍可能为不同的对象返回相同的结果。


I have the funny situation, that I store a Coordinate into a HashMap<Coordinate, GUIGameField>.

Now, the strange thing about it is, that I have a fragment of code, which should guard, that no coordinate should be used twice. But if I debug this code:

if (mapForLevel.containsKey(coord)) {
    throw new IllegalStateException("This coordinate is already used!");
} else {
    ...do stuff...
}

... the containsKey always returns false, although I stored a coordinate with a hashcode of 9731 into the map and the current coord also has the hashcode 9731.

After that, the mapForLevel.entrySet() looks like:

(java.util.HashMap$EntrySet) [(270,90)=gui.GUIGameField@29e357, (270,90)=gui.GUIGameField@ca470]

What could I have possibly done wrong? I ran out of ideas. Thanks for any help!

public class Coordinate {
    int xCoord;
    int yCoord;

    public Coordinate(int x, int y) {
        ...store params in attributes...
    }

    ...getters & setters...

    @Override
    public int hashCode() {
        int hash = 1;
        hash = hash * 41 + this.xCoord;
        hash = hash * 31 + this.yCoord;
        return hash;
    }
}

解决方案

You should override equals in addition to hashCode for it to work correctly.

EDIT : I have wrongly stated that you should use hashCode in your equals - this was not correct. While hashCode must return the same result for two equal objects, it still may return the same result for different objects.

这篇关于Java HashMap containsKey始终为false的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！