Java对象集合按某个字段转为map

在开发中，很多场景下，聪数据源获取到了对象的集合数据后，需要对这部分的数据进行转化，其中按照某个字段转化为 <字段,对象> 或者 <字段,对象集合>的需求非常多，下面来看一个具体的案例吧

对象类

class User {
    private String userId;
    private String userName;
    private int age;

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

    public User() {
    }

    public User(String userId, String userName, int age) {
        this.userId = userId;
        this.userName = userName;
        this.age = age;
    }

    public String getUserId() {
        return userId;
    }

    public void setUserId(String userId) {
        this.userId = userId;
    }

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public int getAge() {
        return age;
    }

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

1、对象列表转为

传统实现

public class Utils {

    public static void main(String[] args) {

        User user1 = new User("001","小张",25);
        User user2 = new User("002","小王",15);
        User user3 = new User("003","小黄",19);

        List userList = new ArrayList<>();
        userList.add(user1);
        userList.add(user2);
        userList.add(user3);

        Map userMap = new HashMap<>();
        for(User user : userList){
            userMap.put(user.getUserId(),user);
        }
        System.out.println(userMap);


    }

通过打印输出的控制台结果，可以实现 结果的获取，这是使用传统的方式实现，对大多数同学来说，这种实现也是一种常规比较容易的方式

Java8实现

public static void main(String[] args) {

        User user1 = new User("001","小张",25);
        User user2 = new User("002","小王",15);
        User user3 = new User("003","小黄",19);

        List userList = new ArrayList<>();
        userList.add(user1);
        userList.add(user2);
        userList.add(user3);


        Map stringUserMap = toMap(userList, User::getUserId);
        System.out.println(stringUserMap);

    }

    
    public static  Map toMap(Collection list, Function function) {
        return list.stream().collect(Collectors.toMap(function, a -> a, (k1, K2) -> k1));
    }

Java8提供了很多函数式接口可供开发中调用，比如上面的这个需求就可以直接调用相关的函数进行操作，可以得到同样的输出结果

上面的实现方式，适合那些对象中某个字段是唯一的场景

2、对象列表按照字段进行分组

public static void main(String[] args) {

        User user1 = new User("001","小张",25);
        User user2 = new User("002","小王",15);
        User user3 = new User("003","小黄",19);

        List userList = new ArrayList<>();
        userList.add(user1);
        userList.add(user2);
        userList.add(user3);


        System.out.println("==========");
        Map> integerListMap = groupBy(userList, User::getAge);
        System.out.println(integerListMap);

    }


    public static  Map> groupBy(Collection list, Function function) {
        return list.stream().collect(groupingBy(function));
    }

直接使用Java8提供的groupby函数即可，运行上面的代码，观察效果