java8 stream转map操作

package com.example.mass_study.test01.anything;

        import java.util.ArrayList;
        import java.util.List;
        import java.util.Map;
        import java.util.concurrent.ConcurrentHashMap;
        import java.util.function.Function;
        import java.util.function.Predicate;
        import java.util.stream.Collectors;

/**
 * @description:
 * @author: luguilin
 * @date: 2022-04-13 11:17
 **/


public class StreamToMapTest {
    public static void main(String[] args) {
        List<User> users = new ArrayList<>();
        User user1 = new User(1, "1");
        User user2 = new User(2, "2");
        User user3 = new User(3, "3");
        users.add(user1);
        users.add(user2);
        users.add(user3);
        // 将id属性读取出来组成list
        List<Integer> idList = users.stream().map(User::getId).collect(Collectors.toList());
        System.out.println(idList);

        // 将id属性读取出来组成list 并去重
        List<Integer> idList2 = users.stream().map(User::getId).distinct().collect(Collectors.toList());
        System.out.println(idList2);

        // 组装成map
        Map<Integer, String> mp1 = users.stream().collect(Collectors.toMap(User::getId, User::getName));
        System.out.println(mp1);

        User user3_1 = new User(3, "3-1");
        users.add(user3_1);

        // 组装成map
        // key 重复会报错
        // Map<Integer, String> mp2 = users.stream().collect(Collectors.toMap(User::getId, User::getName));
        // System.out.println(mp1);

        // 组装成map 值取前面的
        Map<Integer, String> mp3 = users.stream().collect(Collectors.toMap(User::getId, User::getName, (v1, v2) -> v1));
        System.out.println(mp3);

        // 组装成map 值取后面的
        Map<Integer, String> mp2 = users.stream().collect(Collectors.toMap(User::getId, User::getName, (v1, v2) -> v2));
        System.out.println(mp2);

        // 组装成map 值取累加
        Map<Integer, String> mp4 = users.stream().collect(Collectors.toMap(User::getId, User::getName, (v1, v2) -> v1+v2));
        System.out.println(mp4);

        // 根据id 组装list对象，对应id的对象成组
        Map<Integer, List<User>> mp5 = users.stream().collect(Collectors.groupingBy(User::getId));
        System.out.println(mp5);

        // 根据id 组装list对象，对应id的对象成组,如果确认对象只有一个，
        Map<Integer, User> mp8 = users.stream().collect(Collectors.toMap(User::getId, Function.identity(), (o1, o2) -> o2));
        System.out.println(mp8);

        // 根据id 组装list对象，对应id的对象的某个属性成组
        Map<Integer,List<String>> mp6 = users.stream().collect(Collectors.groupingBy(User::getId, Collectors.mapping(User::getName, Collectors.toList())));
        System.out.println(mp6);

        Map<Integer, List<String>> mp7 = users.stream().collect(Collectors.toMap(User::getId,
                p ->  {
                    List<String> getNameList = new ArrayList<>();
                    getNameList.add(p.getName());
                    return getNameList;
                },
                (List<String> value1, List<String> value2) -> {
                    value1.addAll(value2);
                    return value1;
                }
        ));
        System.out.println(mp7);

        User user4 = new User(4, null);
        users.add(user4);

        // 属性为空值，也可以输出
        Map<Integer, List<String>> mp8 = users.stream().collect(Collectors.toMap(User::getId,
                p ->  {
                    List<String> getNameList = new ArrayList<>();
                    getNameList.add(p.getName());
                    return getNameList;
                },
                (List<String> value1, List<String> value2) -> {
                    value1.addAll(value2);
                    return value1;
                }
        ));
        System.out.println(mp8);

        // 以对象为值
        Map<Integer, User> mp9 = users.stream()
                .filter(distinctByKey(User::getId))
                .collect(Collectors.toMap(User::getId, User -> User));
        System.out.println(mp9);
    }

    public static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
        Map<Object,Boolean> seen = new ConcurrentHashMap<>();
        return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
    }
}

class User{
    private Integer id;
    private String name;

    public User() {
    }

    public User(Integer id, String name) {
        this.id = id;
        this.name = name;
    }

    public Integer getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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