如何用两个键（Key-Pair，Value）创建一个HashMap？ [英] How to create a HashMap with two keys (Key-Pair, Value)?

问题描述

我有一个整数的二维数组。我希望他们被放入一个HashMap。但我想从基于数组索引的HashMap中访问元素。例如：

对于A [2] [5]， map.get（2,5），它返回与该键相关联的值。但是，我如何使用一对密钥创建一个hashMap？或者一般来说，我可以通过使用get（key1，key1，key2，...，keyN），Value）来获取多个键： Map< ，key2，... keyN）。

编辑：发布问题3年后，我想添加更多内容

对于 NxN矩阵，我遇到了另一种方式。

可以表示数组索引 i 和 j 作为一个单一的键以下面的方式：

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

指数可以从键以这种方式：

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

解决方案

有几种选择：
$ b

2维

地图地图

  Map< Integer，Map< Integer，V>> map = // ... 
 // ... 
 
 map.get（2）.get（5）; 
  

包装密钥对象

  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））返回false; 
 Key key =（Key）o; 
 return x == key.x&& y == key.y; 
} 
 
 @Override 
 public int hashCode（）{
 int result = x; 
结果= 31 *结果+ y; 
返回结果; 
 
 
 
 
 
实现 equals（）和 hashCode（）在这里至关重要。然后你只需使用： 
 
 
  Map< Key，V> map = // ... 
  
和：
 
 
  map.get（new Key（2，5））; 
  
 
 
 
   Table  来自Guava 
 
 
 
 表< Integer，Integer，V> table = HashBasedTable.create（）; 
 // ... 
 
 table.get（2，5）; 
  
 表使用地图            code> class是缩放到n维的唯一方法。您可能还会考虑： 
 
 
  Map< List< Integer>，V> map = // ... 
  
但从性能角度来看这很糟糕，以及可读性和正确性没有简单的方法来执行列表大小）。
 
 
也许看看你有元组的Scala和 case 类（用一行代替整个 Key 类）。
 
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:

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).

EDIT :  3 years after posting the question, I want to add a bit more to it

I came across another way for NxN matrix. 

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); 
And the indices can be retrevied from the key in this way:
int i = key / N;
int j = key % N;

解决方案
There are several options:

2 dimensions

Map of maps

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

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


Wrapper key object

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;
    }

}
Implementing equals() and hashCode() is crucial here. Then you simply use:
Map<Key, V> map = //...
and:
map.get(new Key(2, 5));


Table from Guava

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

table.get(2, 5);
Table uses map of maps underneath.

N dimensions

Notice that special Key class is the only approach that scales to n-dimensions. You might also consider:
Map<List<Integer>, V> map = //...
but that's terrible from performance perspective, as well as readability and correctness (no easy way to enforce list size).

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

                        
这篇关于如何用两个键（Key-Pair，Value）创建一个HashMap？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！