根据对象 id 更新列表中的对象答案

【问题标题】：Update object in list based on object id根据对象 id 更新列表中的对象
【发布时间】：2021-10-20 13:18:53
【问题描述】：

我想知道是否可以通过 id 直接更新 List 中的对象。这是我的对象结构：

public class MyObject {
    public UUID uuid;
    public String string1;
    public int int1;
    ...
}

在另一个对象中，我有一个 MyObject 列表，我想通过对象 ID 更新其中一个。除了以下代码，还有其他解决方案吗？

List<MyObject> list = new ArrayList<>();
// Add many object into my list
MyObject newObject = new MyObject(...);

list.removeIf(o -> o.getUuid().equals(newObject.getUuid()));
list.add(newObject);

【问题讨论】：

您必须移除对象并完全替换它吗？是你想要的吗？还是您只想更新对象的一个字段？我的意思是，如果您有办法获取要更新的对象的索引，或者直接获取对象，为什么不直接更新对象中的字段呢？（除非它们是最终的或其他）
如果你想有其他替代方法，答案是很多方法。例如，您可以按索引查找。
是的，我想替换整个对象
好吧，那么 tbh，如果你想替换整个项目，我看不出有什么更好的......除非你愿意切换到另一个数据结构：那么也许你可以使用HashMap，因为您已经使用唯一 id 作为键，您可以选择：HashMap 并使用 put() 或 replace()，具体取决于您想要的内容编辑：我可以回答, 如果这是你想要的

标签： java list arraylist

【解决方案1】：

紧凑，但老套...

List<MyObject> list2 =
list.stream().map(item->item.getUuid().equals(newObject.getUuid()) ? newObject : item)
.collect(Collectors.toList();

【讨论】：

【解决方案2】：

如果您真的想坚持使用List，您可以遍历列表并每次手动检查 UUID。

List<MyObject> list = new ArrayList<>();
for (int i = 0; i < list.size(), i++) {
    if (list.get(i).getUuid().equals(newObject.getUuid())) {
        list.set(i, newObject());
        // Probably want to stop the loop here.
    }
}

将UUID 用作Map 的键使其更简单。

List<MyObject> list = new ArrayList<>();
Map<UUID, MyObject> map = list.stream().collect(Collecotrs.toMap(o -> o.getUuid(), o -> o));
map.put(newObject.getUuid(), newObject);
list = new ArrayList<>(map.values());

【讨论】：

这里唯一要检查的是是否需要 put() 或 replace()，因为如果对象不存在，put() 会添加该对象，但这是由 OP 决定的。很好的答案，比我的评论写得更好！

【解决方案3】：

我认为最简单的方法是实现 hashcode/equals，使其以 UUID 为目标。调用 add() 和 remove() 时，将在列表中添加和删除与 UUID 匹配的对象。

这是一个例子：

import java.util.*;

public class ListExample {
    public static void main(String[] args) {
        System.out.println("Implementation using a LinkedList.");
        List<MyObject> list = new ArrayList<>();
        list.add(new MyObject(UUID.fromString("299971c6-31b8-11ec-8d3d-0242ac130003"), "A", 0));
        list.add(new MyObject(UUID.fromString("31e77904-31b8-11ec-8d3d-0242ac130003"), "B", 1));
        list.add(new MyObject(UUID.fromString("368b0214-31b8-11ec-8d3d-0242ac130003"), "C", 2));
        System.out.println(list);
        System.out.println("Remove an entry.");
        list.remove(new MyObject(UUID.fromString("368b0214-31b8-11ec-8d3d-0242ac130003")));
        System.out.println(list);
        list.add(new MyObject(UUID.fromString("9b21edea-31b9-11ec-8d3d-0242ac130003"), "D", 3));
        System.out.println(list);
        System.out.println("Adding a duplicate (creates a new entry).");
        list.add(new MyObject(UUID.fromString("299971c6-31b8-11ec-8d3d-0242ac130003"), "A", 0));
        System.out.println(list);
        System.out.println("Remove duplicate (removes the first entry but not the last).");
        list.remove(new MyObject(UUID.fromString("299971c6-31b8-11ec-8d3d-0242ac130003"), "A", 0));
        System.out.println(list);
    }

    public static class MyObject {
        public UUID uuid;
        public String string1;
        public int int1;

        public MyObject(UUID uuid) {
            this.uuid = uuid;
        }

        public MyObject(UUID uuid, String string1, int int1) {
            this.uuid = uuid;
            this.string1 = string1;
            this.int1 = int1;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            MyObject myObject = (MyObject) o;
            return Objects.equals(uuid, myObject.uuid);
        }

        @Override
        public int hashCode() {
            return Objects.hash(uuid);
        }

        @Override
        public String toString() {
            return "MyObject{" +
                "uuid=" + uuid +
                ", string1='" + string1 + '\'' +
                ", int1=" + int1 +
                '}';
        }
    }

}

输出：

Implementation using a LinkedList.
[MyObject{uuid=299971c6-31b8-11ec-8d3d-0242ac130003, string1='A', int1=0}, MyObject{uuid=31e77904-31b8-11ec-8d3d-0242ac130003, string1='B', int1=1}, MyObject{uuid=368b0214-31b8-11ec-8d3d-0242ac130003, string1='C', int1=2}]
Remove an entry.
[MyObject{uuid=299971c6-31b8-11ec-8d3d-0242ac130003, string1='A', int1=0}, MyObject{uuid=31e77904-31b8-11ec-8d3d-0242ac130003, string1='B', int1=1}]
[MyObject{uuid=299971c6-31b8-11ec-8d3d-0242ac130003, string1='A', int1=0}, MyObject{uuid=31e77904-31b8-11ec-8d3d-0242ac130003, string1='B', int1=1}, MyObject{uuid=9b21edea-31b9-11ec-8d3d-0242ac130003, string1='D', int1=3}]
Adding a duplicate (creates a new entry).
[MyObject{uuid=299971c6-31b8-11ec-8d3d-0242ac130003, string1='A', int1=0}, MyObject{uuid=31e77904-31b8-11ec-8d3d-0242ac130003, string1='B', int1=1}, MyObject{uuid=9b21edea-31b9-11ec-8d3d-0242ac130003, string1='D', int1=3}, MyObject{uuid=299971c6-31b8-11ec-8d3d-0242ac130003, string1='A', int1=0}]
Remove duplicate (removes the first entry but not the last).
[MyObject{uuid=31e77904-31b8-11ec-8d3d-0242ac130003, string1='B', int1=1}, MyObject{uuid=9b21edea-31b9-11ec-8d3d-0242ac130003, string1='D', int1=3}, MyObject{uuid=299971c6-31b8-11ec-8d3d-0242ac130003, string1='A', int1=0}]

这里的主要问题是可以创建重复项（因为我们使用列表）

如果您需要一种保留顺序并排除重复项的方法，请使用 LinkedHashSet

import java.util.*;

public class SetExample {
    public static void main(String[] args) {
        System.out.println("Implementation using a LinkedHashSet");
        Set<MyObject> set = new LinkedHashSet<>();
        set.add(new MyObject(UUID.fromString("299971c6-31b8-11ec-8d3d-0242ac130003"), "A", 0));
        set.add(new MyObject(UUID.fromString("31e77904-31b8-11ec-8d3d-0242ac130003"), "B", 1));
        set.add(new MyObject(UUID.fromString("368b0214-31b8-11ec-8d3d-0242ac130003"), "C", 2));
        System.out.println(set);
        System.out.println("Remove an entry.");
        set.remove(new MyObject(UUID.fromString("368b0214-31b8-11ec-8d3d-0242ac130003")));
        System.out.println(set);
        set.add(new MyObject(UUID.fromString("9b21edea-31b9-11ec-8d3d-0242ac130003"), "D", 3));
        System.out.println(set);
        System.out.println("Adding a duplicate (does not create a new entry).");
        set.add(new MyObject(UUID.fromString("299971c6-31b8-11ec-8d3d-0242ac130003"), "A", 0));
        System.out.println(set);
    }

    public static class MyObject {
        public UUID uuid;
        public String string1;
        public int int1;

        public MyObject(UUID uuid) {
            this.uuid = uuid;
        }

        public MyObject(UUID uuid, String string1, int int1) {
            this.uuid = uuid;
            this.string1 = string1;
            this.int1 = int1;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            MyObject myObject = (MyObject) o;
            return Objects.equals(uuid, myObject.uuid);
        }

        @Override
        public int hashCode() {
            return Objects.hash(uuid);
        }

        @Override
        public String toString() {
            return "MyObject{" +
                "uuid=" + uuid +
                ", string1='" + string1 + '\'' +
                ", int1=" + int1 +
                '}';
        }
    }

输出：

Implementation using a LinkedHashSet
[MyObject{uuid=299971c6-31b8-11ec-8d3d-0242ac130003, string1='A', int1=0}, MyObject{uuid=31e77904-31b8-11ec-8d3d-0242ac130003, string1='B', int1=1}, MyObject{uuid=368b0214-31b8-11ec-8d3d-0242ac130003, string1='C', int1=2}]
Remove an entry.
[MyObject{uuid=299971c6-31b8-11ec-8d3d-0242ac130003, string1='A', int1=0}, MyObject{uuid=31e77904-31b8-11ec-8d3d-0242ac130003, string1='B', int1=1}]
[MyObject{uuid=299971c6-31b8-11ec-8d3d-0242ac130003, string1='A', int1=0}, MyObject{uuid=31e77904-31b8-11ec-8d3d-0242ac130003, string1='B', int1=1}, MyObject{uuid=9b21edea-31b9-11ec-8d3d-0242ac130003, string1='D', int1=3}]
Adding a duplicate (does not create a new entry).
[MyObject{uuid=299971c6-31b8-11ec-8d3d-0242ac130003, string1='A', int1=0}, MyObject{uuid=31e77904-31b8-11ec-8d3d-0242ac130003, string1='B', int1=1}, MyObject{uuid=9b21edea-31b9-11ec-8d3d-0242ac130003, string1='D', int1=3}]

【讨论】：

【解决方案4】：

使用这样的东西：

    List<MyObject> list = new ArrayList<MyObject>();
    
    int id = 4; //the UUID of the Object you are searching for
    
    MyObject found = list.stream()
            .filter(MyObject -> (MyObject.getUUID()==id))
            .findAny()
            .orElse(null);
    
    int i = list.indexOf(found);
    int newUUID = 3;
    list.get(i).setUUID(newUUID);

【讨论】：