一句话

List<User> --> Map<User对象中欲作为key的字段的类型, User>

List<User> userList = getUserList();
Map<Integer, User> idAndUserMap = userList.stream().collect(Collectors.toMap(User::getId, Function.identity()));

场景

1. MyBatis 查询返回List很容易，但如何返回Map？
2. Hibernate 查询返回List很容易，但如何返回Map？
3. 如何使用 Java Stream API 将 List 转换为 Map？

先说1，经查 Mybatis looks up how to return data of Map and List collection types
Mybatis中提供注解：org.apache.ibatis.annotations.MapKey

你需要做的，就是把本来的 DAO 层方法的返回类型从 List 修改为 Map，并且在其上加上注解 @MapKey("你想拿来当key的字段的字段名称")

public interface UserMapper {

    @Select("select id, name, address from users")
    List<User> selectUserList();

    @MapKey("id")
    @Select("select id, name, address from users")
    Map<Integer, User> getUserIdAndUserMap();
}

再说2，经历过1之后，我想 hibernate 是否也有原生支持的API？奈何，他也许有，但我至今没查到。

于是就找到了3，3的解决方式可以作为场景1和2的通解，且在其他更多场景也可自由应用。
所以详细记录3。

语法测试

关键语句截图

Source Code

List2MapDemo.java

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.function.Function;
import java.util.stream.Collectors;

public class List2MapDemo {
    public static void main(String[] args) {

        System.out.println("\r\nList<YourObjectType> :");
        List<User> userList = getUserList();
        userList.forEach(currentElement -> {
            System.out.printf("element: %s\r\n", currentElement);
        });

        System.out.println("\r\nMap<IdType, YourObjectType> :");
        Map<Integer, User> idAndUserMap = userList.stream().collect(Collectors.toMap(User::getId, Function.identity()));
        Set<Integer> idSet = idAndUserMap.keySet();
        idSet.forEach(currentId -> {
            System.out.printf("key: %s  value: %s\r\n", currentId, idAndUserMap.get(currentId));
        });

        System.out.println("\r\nMap<IdType, ValueType> :");
        Map<Integer, String> idAndNameMap = userList.stream().collect(Collectors.toMap(User::getId, User::getName));
        Set<Integer> idSet2 = idAndNameMap.keySet();
        idSet2.forEach(currentId -> {
            System.out.printf("key: %s  value: %s\r\n", currentId, idAndNameMap.get(currentId));
        });


    }
    static List<User> getUserList() {
        List<User> userList = new ArrayList<>(5);
        userList.add(new User(1, "one", "address-1"));
        userList.add(new User(2, "two", "address-2"));
        userList.add(new User(3, "three", "address-3"));
        userList.add(new User(4, "four", "address-4"));
        userList.add(new User(5, "five", "address-5"));
        return userList;
    }
}

class User {
    private Integer id;
    private String name;
    private String address;

    public User(){};
    public User(Integer id, String name, String address){
        super();
        this.id = id;
        this.name = name;
        this.address = address;
    };

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

    public Integer getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }
}

控制台输出

List<YourObjectType> :
element: User{id=1, name='one', address='address-1'}
element: User{id=2, name='two', address='address-2'}
element: User{id=3, name='three', address='address-3'}
element: User{id=4, name='four', address='address-4'}
element: User{id=5, name='five', address='address-5'}

Map<IdType, YourObjectType> :
key: 1  value: User{id=1, name='one', address='address-1'}
key: 2  value: User{id=2, name='two', address='address-2'}
key: 3  value: User{id=3, name='three', address='address-3'}
key: 4  value: User{id=4, name='four', address='address-4'}
key: 5  value: User{id=5, name='five', address='address-5'}

Map<IdType, ValueType> :
key: 1  value: one
key: 2  value: two
key: 3  value: three
key: 4  value: four
key: 5  value: five