Three ways to convert List to Map

for loop

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import ;
import ;

import ;
import ;
import ;
import ;

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

        User user2 = new User();
        (2L);
        ("13");

        (user1);
        (user2);

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

        (maps);

    }

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

        public Long getId() {
            return id;
        }

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

        public String getAge() {
            return age;
        }

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

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

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Using 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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Using JDK1.8

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Map<Long, User> maps = ().collect((User::getId,()));

It seems that using JDK 1.8 is more convenient. Also, convert tomapWhen it may appearkeyIn the same case, if you do not specify a coverage rule, the above code will report an error. Convert tomapWhen it comes to this, it is best to use the following method:

Map<Long, User> maps = ().collect((User::getId, (), (key1, key2) -> key2));

Sometimes, I hope to getmapThe value of the object is not an object, but a property of the object. You can use the following method:

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

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