英文:
Java HashMap<int[], Integer>
问题
我想将一个数组映射到一个 ArrayList，使得如果两个数组相同，它们映射到相同的东西。
这段代码输出 null：
HashMap<int[], Integer> map = new HashMap<>();
map.put(new int[1], 0);
System.out.println(map.get(new int[1]));
而我希望输出是 0。有没有一种简单的方法可以做到这一点？我应该使用 TreeMap 吗？在 TreeMap 中如何实现这个目标？
英文:
I want to map an array to an ArrayList such that two arrays mapped to the same thing if they are identical.
This outputs null:
HashMap<int[], Integer> map = new HashMap<>();
map.put(new int[1], 0);
System.out.println(map.get(new int[1]));
And I want the output to be 0. Is there an easy way to do this? Should I use TreeMap? How do I do this in TreeMap?
答案1
得分: 3
使用TreeMap可以工作，如果您传入一个可以用于数组的Comparator实例。幸运的是，标准库已经有一个您可以使用的方法 Arrays.compare：
TreeMap<int[], Integer> map = new TreeMap<>(Arrays::compare);
map.put(new int[1], 1);
System.out.println(map.get(new int[1])); // 输出: 1
话虽如此，将可变对象（如数组）用作映射键不是一个很好的主意。如果您意外更改用作键的数组，映射将无法按预期工作。
HashMap不起作用的原因是因为数组不基于数组内容实现hashCode或equals方法，而是基于对象标识。
Arrays.compare 在 Java 9 中添加。对于 Java 8，您需要自己编写Comparator。以下是一个选项：
Comparator<int[]> arrayComparator = (a, b) -> {
for (int i = 0; i < Math.min(a.length, b.length); i++) {
int compare = Integer.compare(a[i], b[i]);
if (compare != 0) return compare;
}
return Integer.compare(a.length, b.length);
};
英文:
Using a TreeMap works, if you pass in a Comparator instance that can be used with arrays. Luckily the standard library already has a method you can use Arrays.compare:
TreeMap<int[], Integer> map = new TreeMap<>(Arrays::compare);
map.put(new int[1], 1);
System.out.println(map.get(new int[1])); // output: 1
That said, using a mutable object such as an array as the map key is not a very good idea. If you accidentally change an array used as a key, the map will stop working as expected.
The reason why HashMap will not work is because arrays don't implement a hashCode or equals method based on the array contents. They are based on the object identity instead.
Arrays.compare was added in Java 9. For Java 8 you'll need to write the Comparator yourself. Here is one option:
Comparator<int[]> arrayComparator = (a, b) -> {
for (int i = 0; i < Math.min(a.length, b.length); i++) {
int compare = Integer.compare(a[i], b[i]);
if (compare != 0) return compare;
}
return Integer.compare(a.length, b.length);
};
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