如何为两个对象使用集合方法(removeAll() 和 retainAll()) [英] How to use Collections methods(removeAll() and retainAll()) for two objects

问题描述

我希望结果如下，但实际上没有.即使当我尝试使用 String 而不是 Item Object 时它也有效.我想知道为什么会这样，以及如何编码以获得预期的结果.谢谢.

I expected to result below but actually not. Even though it worked when I tried to use String instead of Item Object. I would like to know why the reasons about that and how to code in order to get expected result.Thanks.

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

<小时>

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

<小时>

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<Item> after = new ArrayList<Item>();

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

  after.add(new Item(1L));
  after.add(new Item(3L));
  after.add(new Item(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 {

 Long id;

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

 public Long getId() {
  return id;
 }

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

}

推荐答案

您尚未实现 equals()，因此您的所有项目都是不同的对象，有些项目恰好具有相同的字段.

You haven't implemented equals() so ALL your Items are different objects some happen to have a field which is the same.

你需要实现equals.

You need to implement equals.

我也建议你使用 long 而不是 Long，除非你想要 id = null.

I also suggest you use long instead of Long, unless you want id = null.

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