如何为两个对象使用Collections方法(removeAll()和retainAll()).(对象是父子关系)

Question

我期望得到以下但实际上没有.我想知道如何显示两个集合之间的差异.(对象是父子关系)在这种情况下,我可以使用像removeAll()这样的标准方法,还是可以推荐另一种方法,比如使用apache-commons.谢谢.

CONSTRAINT
------------------------------
1.Item.class is unmodifiable(eg. I can not add equals method)
2.If id is same between two objects, they are assumed as same things.
------------------------------

EXPECTED
------------------------------
removed object are:
2
same object are:
1
3
add object are:
4
------------------------------

ACTUAL
------------------------------
removed object are:
1
2
3
same object are:
add object are:
1
3
4
------------------------------

package com.javastudy;

import java.util.ArrayList;
import java.util.List;

public class CollectionCompareToObjects {

    public static void main(String[] args) {

        List<Item> before = new ArrayList<Item>();
        List<ItemEx> after = new ArrayList<ItemEx>();

        before.add(new Item(1L));
        before.add(new Item(2L)); // delete
        before.add(new Item(3L));

        after.add(new ItemEx(1L));
        after.add(new ItemEx(3L));
        after.add(new ItemEx(4L)); // added

        List<Item> removed = new ArrayList<Item>(before);
        removed.removeAll(after);

        System.out.println("removed objects are:");
        for(Item item : removed){
            System.out.println(item.getId());
        }

        List<Item> same = new ArrayList<Item>(before);
        same.retainAll(after);

        System.out.println("same objects are:");
        for(Item item : same){
            System.out.println(item.getId());
        }

        List<Item> added = new ArrayList<Item>(after);
        added.removeAll(before);

        System.out.println("add objects are:");
        for(Item item : added){
            System.out.println(item.getId());
        }

    }

}


package com.javastudy;

public class Item {

    private Long id;

    public Item(Long id) {
        this.id = id;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

}

package com.javastudy;

public class ItemEx extends Item {

    private String name;

    public ItemEx(Long id) {
        super(id);
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

Answer 1

Java集合依赖于equals和hashCode方法(后者由HashMaps,HashSets和其他人使用).

如果您希望能够使用Java集合的数据结构功能(例如removeAll,retainAll等等),则需要为对象提供正确的equals和hashCode.

如果你不能修改Item类,你可以用你自己的实现编写一个包装类equals:

public class ItemWrapper {
    private final Item item;
    public ItemWrapper(Item item) {
        this.item = item;
    }

    public Item getItem() {
        return item;
    }

    @Override
    public boolean equals(Object obj) {
        return obj instanceof ItemWrapper && item.getId().equals(((ItemWrapper) obj).item.getId());
    }

    @Override
    public int hashCode() {
        return item.getId().hashCode();
    }
}

ItemWrapper为每个原始文件创建一个新的Item,将ItemWrappers 存储在Java集合中,并使用所需的方法(removeAll/ retainAll).然后迭代生成的集合并Item通过调用每个方法来检索ItemWrappers getItem().

你的另一个选择是子类ArrayList,但它似乎是一个更复杂的解决方案.

另一个选择是不要将Java集合用于删除/保留逻辑,而是自己实现它们.