Java:在集合中添加/删除短元素和整数元素时的不同输出 [英] Java: Different outputs when add/remove short and integer elements in a Set

问题描述

我不确定如何提出这个问题.但是，这两行代码有什么不同呢?

I wasn't sure on how to ask this question. But, what is the different between these 2 lines of code?

Set<Integer> a = new HashSet<Integer>();
for (int i = 0; i < 100; i++) {
    a.add(i);
    a.remove(i - 1);
}

System.out.println(a.size());

我预计 99 是输出

输出为 1

Set<Short> a = new HashSet<Short>();
for (Short i = 0; i < 100; i++) {
    a.add(i);
    a.remove(i - 1);
}

System.out.println(a.size());

我预计 99 是输出

输出为 100

推荐答案

表达式 i - 1 的类型是 int 因为整数算术表达式中的所有操作数都是扩大到至少 int.Set 有 add(Short) 和 remove(Object) 所以 remove 不需要强制转换/自动装箱代码>调用.因此，您试图从一组 Short 中删除 Integer.

The type of the expression i - 1 is int because all operands in an integer arithmetic expression are widened to at least int. Set<Short> has add(Short) and remove(Object) so there's no casting/autoboxing needed on the remove call. Therefore you are trying to remove Integers from a set of Shorts.

请注意，由于这个原因，声明 Set 几乎没有意义:

Note that for this reason it almost never makes sense to declare a Set<Number>:

final Set<Number> ns = new HashSet<>();
final short s = 1;
ns.add(s);
ns.add(s+0);
ns.add(s+0L);
System.out.println(ns); // prints [1, 1, 1]

作为奖励回合，如果您将 set 实现更改为 TreeSet，魔法就会消失，它会抛出 ClassCastException，从而放弃这个技巧.

As a bonus round, if you change the set implementation to TreeSet, the magic disappears and it throws a ClassCastException, giving away the trick.

在本质上，这个问题与相等是对称关系这一事实有关，它不能区分右手边和左手边.这些语义是 Java 的单一调度方法无法实现的.

Deep down, this issue has to do with the fact that equality is a symmetric relation, which must not distinguish the right hand side from the left hand side. These semantics are impossible to achieve with Java's single-dispatch methods.

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