如何定义我自己的元素类以与 Set 一起使用

问题描述

我有以下代码:

public class MyElement {
    String name;
    String type;

    MyElement(String name, String type) {
        this.name = name;
        this.type = type;
    }
}

public class Test {

    public static void main(String[] args) {
        Set<MyElement> set = new HashSet<MyElement>();
        set.add(new MyElement("foo", "bar"));
        set.add(new MyElement("foo", "bar"));
        set.add(new MyElement("foo", "bar"));
        System.out.println(set.size());
        System.out.println(set.contains(new MyElement("foo", "bar")));
    }
}

执行时返回:

3

false

我本来希望结果为 1 且为真.为什么我的元素不被认为是相同的，我该如何纠正这个问题?谢谢，韦恩.

I would have expected the result to be 1 and true. Why are my elements not being recognised as being the same and how do I rectify this? Thanks, Wayne.

推荐答案

您需要根据通用合约在 MyElement 上实现 equals(Object o) 和 hashCode().如果没有 Set.contains() 将使用比较对象的内存地址的默认实现.由于您在 contains 调用中创建了 MyElement 的新实例，因此它返回为 false.

You need to implement equals(Object o) and hashCode() on MyElement per the general contract. Absent that Set.contains() will use the default implementation which compares the memory address of the objects. Since you're creating a new instance of MyElement in the contains call it comes back as false.

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