List转Map的三种方法

1、使用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 + '\'' +
                    '}';
        }
    }
}

2、使用guava

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

3、使用Java8新特性Stream的Collectors类

（1）基本用法

//声明一个List集合
List<Person> list = new ArrayList();  
        list.add(new Person("1001", "小A"));  
        list.add(new Person("1002", "小B"));  
        list.add(new Person("1003", "小C"));
        System.out.println(list);
//将list转换map
Map<String, String> map = list.stream().collect(Collectors.toMap(Person::getId,Person::getName));

Map<String, ParamCalcrate> paramCalcrateMap = paramCalcrates.stream().collect(Collectors.toMap(paramCalcrate -> paramCalcrate.getTpId(), Function.identity()));

（2）转换过程中的两个问题

a、key重复

重复时用后面的value 覆盖前面的value

Map<String, String> map = list.stream().collect(Collectors.toMap(Person::getId, Person::getName,(key1 , key2)-> key2 ));

重复时将前面的value 和后面的value拼接起来

Map<String, String> map = list.stream().collect(Collectors.toMap(Person::getId, Person::getName,(key1 , key2)-> key1+","+key2 ));

重复时将重复key的数据组成集合

Map<String, List<String>> map = list.stream().collect(Collectors.toMap(Person::getId,
	    		p ->  {
	    		 	List<String> getNameList = new ArrayList<>();
	    		 		getNameList.add(p.getName());
	    		 		return getNameList;
	    		 	},
    		     	(List<String> value1, List<String> value2) -> {
    		     		value1.addAll(value2);
    		     		return value1;
    		     	}
	    		 ));
       

b、valve为null

在转换流中加上判空，即便value为空,依旧输出

Map<String, List<String>> map = list.stream().collect(Collectors.toMap(Person::getId,
	    		p ->  {
	    		 	List<String> getNameList = new ArrayList<>();
	    		 		getNameList.add(p.getName());
	    		 		return getNameList;
	    		 	},
    		     	(List<String> value1, List<String> value2) -> {
    		     		value1.addAll(value2);
    		     		return value1;
    		     	}
	    		 ))