問題描述

今天我正在做一些尋路，當我不得不使用 contains() 來查找一個類 Coord 是否在另一個 keySet() 坐標.我發現當我使用預制方法 containsKey() 時，它根本無法按我的意愿工作.我做了一個測試以了解發生了什么，結果如下:

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);

令人驚訝的是，這是我發現的:

And surprisingly, here is what I found :

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

我只是想知道發生了什么以及為什么.

I just wanted to know what is happening and why.

另外，這里是 Coord 類:

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;
    }
}

推薦答案

HashMaps 通過哈希碼查找對象.合同的一部分是密鑰類必須覆蓋 hashCode() 以及 equals().單獨對象的默認哈希碼不相等，因此 get 找不到該對象.相反，當您遍歷所有條目時，不使用哈希碼，因此僅調用 equals 并找到對象.

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，請覆蓋 Coord 中的 hashCode.

To find it using get, override hashCode in Coord.

無論何時重寫 equals，重寫 hashCode 總是好的，反之亦然.

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

這篇關于HashMap 的 containsKey 方式未按預期運行的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網！