如何在ArrayList中找到重复项？< Object>？ [英] How to find duplicates in an ArrayList<Object>?

问题描述

这是一个很常见的问题，但我找不到这个部分：

This is a pretty common question, but I could not find this part:

说我有这个数组列表：

List<MyDataClass> arrayList = new List<MyDataClass>;

MyDataClass{
   String name;
   String age;
}

现在，我需要找到基于 age 在 MyDataClass 中删除​​它们。如何使用像HashSet这样的这里？

Now, I need to find duplicates on the basis of age in MyDataClass and remove them. How is it possible using something like HashSet as described here?

我想，我们需要在MyDataClass中覆盖 equals 吗？

I guess, we will need to overwrite equals in MyDataClass?

1. 但是，如果我没有这样做的奢侈呢？


2. HashSet如何在内部找到并且不添加重复项？我看到它的实现这里是在OpenJDK ，但无法理解。

1. But, what if I do not have the luxury of doing that?
2. And How does HashSet actually internally find and does not add duplicates? I saw it's implementation here in OpenJDK but couldn't understand.

推荐答案

你覆盖两者 equals 和 hashCode （ HashSet 依赖于两者！）

I'd suggest that you override both equals and hashCode (HashSet relies on both!)

要删除重复项，您只需创建一个新的 HashSet ArrayList作为参数，然后清除ArrayList并放回存储在 HashSet 中的元素。

To remove the duplicates you could simply create a new HashSet with the ArrayList as argument, and then clear the ArrayList and put back the elements stored in the HashSet.

class MyDataClass {
    String name;
    String age;

    @Override
    public int hashCode() {
        return name.hashCode() ^ age.hashCode();
    }

    @Override
    public boolean equals(Object obj) {
        if (!(obj instanceof MyDataClass))
            return false;

        MyDataClass mdc = (MyDataClass) obj;
        return mdc.name.equals(name) && mdc.age.equals(age);
    }
}

然后执行

List<MyDataClass> arrayList = new ArrayList<MyDataClass>();

Set<MyDataClass> uniqueElements = new HashSet<MyDataClass>(arrayList);
arrayList.clear();
arrayList.addAll(uniqueElements);

但是，如果我没有

然后我建议你做一些装饰师类

Then I'd suggest you do some sort of decorator-class that does provide these methods.

class MyDataClassDecorator {

    MyDataClass mdc;

    public MyDataClassDecorator(MyDataClass mdc) {
        this.mdc = mdc;
    }

    @Override
    public int hashCode() {
        return mdc.name.hashCode() ^ mdc.age.hashCode();
    }

    @Override
    public boolean equals(Object obj) {
        if (!(obj instanceof MyDataClassDecorator))
            return false;

        MyDataClassDecorator mdcd = (MyDataClassDecorator) obj;
        return mdcd.mdc.name.equals(mdc.name) && mdcd.mdc.age.equals(mdc.age);
    }
}

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