1 List to Map

@Getter
@Setter
public class User {
    private Long id;
    private String name;

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

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}

package com.mervyn.filterdemo;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Test {
    public static void main(String[] args) {
        List<User> names = new ArrayList<>();
        names.add(new User(1L,"郭雨桐"));
        names.add(new User(2L,"尚书雨"));
        names.add(new User(3L,"李雪晴"));
        names.add(new User(4L,"刘小晓"));
        Map<Long, User> userMap = names.stream().collect(Collectors.toMap(User::getId, user -> user));
    }
}

2 Map to List

package com.mervyn.filterdemo;

import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Test {
    public static void main(String[] args) {
        Map<Long, String> userMap = new HashMap<>();
        userMap.put(1L, "郭雨桐");
        userMap.put(2L, "尚书雨");
        userMap.put(3L, "李雪晴");
        userMap.put(4L, "刘小晓");
        // 将Map的value值转成list
        List<String> usernames = userMap.values().stream().collect(Collectors.toList());
        // 将Map的key值转成list
        List<Long> userIds = userMap.keySet().stream().collect(Collectors.toList());
    }
}