带有“包含”的问题哈希集方法（Java） [英] Issue with "contains" hashset method (Java)

问题描述

以下代码没有给我期望的结果：

The following code is not giving me the result I'm expecting:

public static void main (String[] args) {

    Set<Pair> objPair = new LinkedHashSet<Pair>();
    objPair.add(new Pair(1, 0));

    System.out.println("Does the pair (1, 0) exists already? "+objPair.contains(new Pair(1, 0)));

}

private static class Pair {

    private int source;
    private int target;

    public Pair(int source, int target) {
      this.source = source;
      this.target = target;
    }        
}

结果将是：

Does the pair (1, 0) exists already? false

我不明白为什么它不起作用。
还是我使用的 contains方法错误（或由于错误的原因）。

I can't understand why it's not working. Or maybe I'm using the "contains" method wrong (or for the wrong reasons).

还有另一个问题，如果我两次添加相同的值，即使是集合，它也会被接受

There is also another issue, if I add the same value twice, it will be accepted, even being a set

objPair.add(new Pair(1, 0));
objPair.add(new Pair(1, 0));

它不接受/识别我创建的Pair对吗？

It won't accept/recognize the class Pair I've created?

谢谢。

推荐答案

如果没有您自己的hashCode（）实现，则 Java仅当两个 Pair 对象完全相同时才认为它们相等和 new 创建一个新对象。在您的情况下，如果 Pair 对象的 source 和<$ c具有相同的值，则它们应被视为相等。 $ c> target －为此，您需要告诉Java如何测试 Pair 对象是否相等。 （并且要使哈希图按您期望的方式工作，还需要生成与与equals 一致的哈希码-粗略地说，这意味着等于对象必须生成相同的hashCode，并且不相等的对象应生成不同的哈希码。

Without your own hashCode() implementation, Java considers two Pair objects equal only if they are the exact same object and new, by definition, always creates a 'new' object. In your case, you want Pair objects to be consider equal if they have the same values for source and target -- to do this, you need to tell Java how it should test Pair objects for equality. (and to make hash maps work the way you expect, you also need to generate a hash code that is consistent with equals -- loosely speaking, that means equal objects must generate the same hashCode, and unequal objects should generate different hash codes.

大多数IDE会为您生成不错的hashcode（）和equals（）方法我的生成的是：

Most IDEs will generate decent hashcode() and equals() methods for you. Mine generated this:

  @Override
   public int hashCode() {
      int hash = 3;
      hash = 47 * hash + this.source;
      hash = 47 * hash + this.target;
      return hash;
   }

   @Override
   public boolean equals(Object obj) {
      if (obj == null) {
         return false;
      }
      if (getClass() != obj.getClass()) {
         return false;
      }
      final Pair other = (Pair) obj;
      if (this.source != other.source) {
         return false;
      }
      if (this.target != other.target) {
         return false;
      }
      return true;
   }

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