如何创建具有两个键(键对、值)的 HashMap? [英] How to create a HashMap with two keys (Key-Pair, Value)?

问题描述

我有一个二维整数数组.我希望将它们放入 HashMap 中.但我想根据数组索引访问 HashMap 中的元素.类似的东西:

I have a 2D array of Integers. I want them to be put into a HashMap. But I want to access the elements from the HashMap based on Array Index. Something like:

对于 A[2][5]，map.get(2,5) 返回与该键关联的值.但是如何使用一对键创建 hashMap 呢?或者一般来说，多个键: Map<((key1, key2,..,keyN), Value) 以我可以使用 get(key1,key2,...键N).

For A[2][5], map.get(2,5) which returns a value associated with that key. But how do I create a hashMap with a pair of keys? Or in general, multiple keys: Map<((key1, key2,..,keyN), Value) in a way that I can access the element with using get(key1,key2,...keyN).

发布问题 3 年后，我想添加更多内容

我遇到了 NxN 矩阵 的另一种方法.

I came across another way for NxN matrix.

数组索引，i 和 j 可以表示为单个 key 如下:

Array indices, i and j can be represented as a single key the following way:

int key = i * N + j;
//map.put(key, a[i][j]); // queue.add(key); 

并且可以通过这种方式从 key 检索索引:

And the indices can be retrevied from the key in this way:

int i = key / N;
int j = key % N;

推荐答案

有几个选项:

Map<Integer, Map<Integer, V>> map = //...
//...

map.get(2).get(5);

包装键对象

public class Key {

    private final int x;
    private final int y;

    public Key(int x, int y) {
        this.x = x;
        this.y = y;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Key)) return false;
        Key key = (Key) o;
        return x == key.x && y == key.y;
    }

    @Override
    public int hashCode() {
        int result = x;
        result = 31 * result + y;
        return result;
    }

}

实现 equals() 和 hashCode() 在这里至关重要.然后你只需使用:

Implementing equals() and hashCode() is crucial here. Then you simply use:

Map<Key, V> map = //...

和:

map.get(new Key(2, 5));

Table 来自 Guava

Table<Integer, Integer, V> table = HashBasedTable.create();
//...

table.get(2, 5);

Table 使用 map of maps 下面.

请注意，特殊的 Key 类是唯一可扩展到 n 维的方法.您还可以考虑:

Notice that special Key class is the only approach that scales to n-dimensions. You might also consider:

Map<List<Integer>, V> map = //...

但从性能角度以及可读性和正确性的角度来看，这很糟糕(没有简单的方法来强制执行列表大小).

but that's terrible from performance perspective, as well as readability and correctness (no easy way to enforce list size).

也许看看 Scala，那里有元组和 case 类(用单行替换整个 Key 类).

Maybe take a look at Scala where you have tuples and case classes (replacing whole Key class with one-liner).

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