HashMap的方法containsKey行为不符合预期 [英] HashMap way of doing containsKey not behaving as expected

问题描述

今天，当我不得不使用 contains（）查找类 Coord 时，我正在做一些寻路。在另一个 Coord 的 keySet（）中。
我发现，当我习​​惯使用预制方法 containsKey（）时，它根本无法按我的意愿工作。我进行了测试，以了解正在发生的情况，这里是：

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.

此外，这是 Coord class：

Also, here is the Coord class :

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

HashMap 通过哈希码查找对象。合同的一部分是，键类必须覆盖 hashCode（）和 equals（）。各个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 查找它，请覆盖 Coord中的 hashCode 。

To find it using get, override hashCode in Coord.

每次重写<$ c $时，最好重写 hashCode c>等于，反之亦然。

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

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