List转Map的三种方法

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原文链接: blog.csdn.net

for循环


import com.google.common.base.Function;
import com.google.common.collect.Maps;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class ListToMap {
    public static void main(String[] args) {
        List<User> userList = new ArrayList<>();
        User user1 = new User();
        user1.setId(1L);
        user1.setAge("12");

        User user2 = new User();
        user2.setId(2L);
        user2.setAge("13");

        userList.add(user1);
        userList.add(user2);

        Map<Long, User> maps = new HashMap<>();
        for (User user : userList) {
            maps.put(user.getId(), user);
        }

        System.out.println(maps);

    }

    public static class User {
        private Long id;
        private String age;

        public Long getId() {
            return id;
        }

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

        public String getAge() {
            return age;
        }

        public void setAge(String age) {
            this.age = age;
        }

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


使用guava


 Map<Long, User> maps = Maps.uniqueIndex(userList, new Function<User, Long>() {
            @Override
            public Long apply(User user) {
                return user.getId();
            }
   });

使用JDK1.8


Map<Long, User> maps = userList.stream().collect(Collectors.toMap(User::getId, user -> user));

看来还是使用JDK 1.8方便一些。