HashSet contains（）方法 [英] HashSet contains() method

问题描述

我执行下面的代码，发现输出是 false 。

I executed below code and found the output was false.

import java.util.Set;
import java.util.HashSet;

public class Name {
    private String first, last;

    public Name(String first, String last) {
        this.first = first;
        this.last = last;
    }

    public boolean equals(Object o) {
        if (!(o instanceof Name))
            return false;
        Name n = (Name) o;
        return n.first.equals(first) && n.last.equals(last);
    }

    public static void main(String[] args) {
        Set<Name> s = new HashSet<Name>();
        s.add(new Name("Donald", "Duck"));
        System.out.println(s.contains(new Name("Donald", "Duck")));
    }
}

我想知道它的行为， false 。

推荐答案

hashCode（）方法以及 equals（）。这两种方法都用于 HashSet 的正确功能，因此必须在用户定义的类中覆盖 c> class 作为键，否则 hashCode（） > Object 类被使用，没有两个不同的对象可以被视为与它们的 hashCode c $ c>总是不同的，并且在 contains（）的情况下总是返回 false 。

You need to override hashCode() method also along with equals(). Both the methods are used for proper functionality of HashSet, so must be overriden in a user-defined class if you are making that class as a key, else hashCode() of Object class is getting used and no two different objects can be considered as same as their hashCode() would always be different, and will for sure return falsealways in the case of contains().

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