List转Map的三种方法

for循环

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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 + '\'' +
                    '}';
        }
    }
}

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使用guava

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

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使用JDK1.8

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

看来还是使用JDK 1.8方便一些。另外，转换成map的时候，可能出现key一样的情况，如果不指定一个覆盖规则，上面的代码是会报错的。转成map的时候，最好使用下面的方式：

Map<Long, User> maps = userList.stream().collect(Collectors.toMap(User::getId, Function.identity(), (key1, key2) -> key2));

有时候，希望得到的map的值不是对象，而是对象的某个属性，那么可以用下面的方式：

Map<Long, String> maps = userList.stream().collect(Collectors.toMap(User::getId, User::getAge, (key1, key2) -> key2));

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