HashMap 的 containsKey 方式未按预期运行

问题描述

今天我正在做一些寻路，当我不得不使用 contains() 来查找一个类 Coord 是否在另一个 keySet() 坐标.我发现当我使用预制方法 containsKey() 时，它根本无法按我的意愿工作.我做了一个测试以了解发生了什么，结果如下:

Today I was doing some pathfinding when I had to use contains() to find if a class Coord was in an other keySet() of Coord. I found that when I used to premade method containsKey(), it was simply not working as I wanted. I made a test to find out what is happening and here it is:

HashMap<Coord, Coord> test = new HashMap<Coord, Coord>();
test.put(new Coord(3, 3), new Coord(0, 0));

System.out.println("HashMap test for containsKey : " + test.containsKey(new Coord(3, 3)));

boolean containsKey = false;
for(Coord a : test.keySet())
{
    if(a.equals(new Coord(3, 3)))
    {
        containsKey = true;
    }
}

System.out.println("My test for containsKey : "+containsKey);

令人惊讶的是，这是我发现的:

And surprisingly, here is what I found :

HashMap test for containsKey : false
My test for containsKey : true

我只是想知道发生了什么以及为什么.

I just wanted to know what is happening and why.

另外，这里是 Coord 类:

public class Coord
{
    public float x, y;

    public Coord(float a, float b)
    {
        this.x = a;
        this.y = b;
    }

    @Override
    public boolean equals(Object b)
    {
        if(b instanceof Coord)
        {
            Coord casted = (Coord) b;
            return casted.x == x && casted.y == y;
        }
        return false;
    }
}

推荐答案

HashMaps 通过哈希码查找对象.合同的一部分是密钥类必须覆盖 hashCode() 以及 equals().单独对象的默认哈希码不相等，因此 get 找不到该对象.相反，当您遍历所有条目时，不使用哈希码，因此仅调用 equals 并找到对象.

HashMaps find objects by their hash code. Part of the contract is that the key class must override hashCode() as well as equals(). The default hash codes for separate objects aren't equal, so the object isn't found by get. In contrast, when you loop over all entries, the hash code isn't used, so only equals is called, and the object is found.

要使用 get 找到它，请覆盖 Coord 中的 hashCode.

To find it using get, override hashCode in Coord.

无论何时重写 equals，重写 hashCode 总是好的，反之亦然.

It is always good to override hashCode whenever you override equals, and vice versa.

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