如何使 HashMap 以数组为键工作? [英] How to make HashMap work with Arrays as key?

问题描述

我使用布尔数组作为 HashMap 的键.但问题是当不同的数组作为键传递时，HashMap 无法获取键，尽管元素相同.(因为它们是不同的对象).

I am using boolean arrays as keys for a HashMap. But the problem is HashMap fails to get the keys when a different array is passed as key, although the elements are same. (As they are different objects).

如何让它以数组为键?这是代码:

How can I make it work with arrays as keys ? Here is the code :

public class main {
public static HashMap<boolean[], Integer> h;


public static void main(String[] args){
    boolean[] a = {false, false};

    h = new HashMap<boolean[], Integer>();
    h.put(a, 1);


    if(h.containsKey(a)) System.out.println("Found a");

    boolean[] t = {false, false};

    if(h.containsKey(t)) System.out.println("Found t");
    else System.out.println("Couldn't find t");

}

}

数组a 和t 都包含相同的元素，但HashMap 不为t 返回任何内容.

Both the arrays a and t contain the same elements, but HashMap doesn't return anything for t.

我如何使它工作?

推荐答案

你不能这样做.t 和 a 都会有不同的 hashCode() 值，因为 java.lang.Array.hashCode()> 方法继承自 Object，它使用引用来计算哈希码(默认实现).因此，数组的哈希码是依赖于引用的，这意味着您将获得 t 和 a 的不同哈希码值.此外，equals 不适用于这两个数组，因为这也是基于引用的.

You cannot do it this way. Both t and a will have different hashCode() values because the the java.lang.Array.hashCode() method is inherited from Object, which uses the reference to compute the hash-code (default implementation). Hence the hash code for arrays is reference-dependent, which means that you will get a different hash-code value for t and a. Furthermore, equals will not work for the two arrays because that is also based on the reference.

您可以这样做的唯一方法是创建一个自定义类，将 boolean 数组保留为内部成员.然后，您需要覆盖 equals 和 hashCode，以确保包含具有相同值的数组的实例相等并且也具有相同的哈希码.

The only way you can do this is to create a custom class that keeps the boolean array as an internal member. Then you need to override equals and hashCode in such a way that ensures that instances that contain arrays with identical values are equal and also have the same hash-code.

更简单的选择可能是使用 List 作为键.根据文档，<List 的 code>hashCode() 实现定义为:

An easier option might be to use List<Boolean> as the key. Per the documentation the hashCode() implementation for List is defined as:

int hashCode = 1;
Iterator<E> i = list.iterator();
while (i.hasNext()) {
    E obj = i.next();
    hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode());
}

如您所见，这取决于列表中的值而不是引用，因此这应该适合您.

As you can see, it depends on the values inside your list and not the reference, and so this should work for you.

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