Java8 快速实现List转map 、分组、过滤等操作

** Java 8 是 Java 编程语言的一个重要版本，它引入了许多新特性和改进，使得 Java 开发变得更加高效和现代。**

下面我们就来使用Java8 快速实现List转map 、分组、过滤等操作。

定义1个用户对象

java 复制代码

public class User {
    
    private Integer id;
    
    private String name;
    
    private Integer age;
    
    private String phone;
    
    public User(Integer id, String name, Integer age, String phone) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.phone = phone;
    }
     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 Integer getAge() {
        return age;
    }

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

    public String getPhone() {
        return phone;
    }

    public void setPhone(String phone) {
        this.phone = phone;
    }
}

添加测试数据：

java 复制代码

List<User> userList = new ArrayList<>();

User user1 = new User(1, "张三", 18, "12345678901");

User user2 = new User(2, "李四", 18, "12345678901");

User user3 = new User(3, "王五", 18, "12345678901");

User user4 = new User(4, "李白", 18, "12345678901");

userList.add(user1);
userList.add(user2);
userList.add(user3);
userList.add(user4);

1、分组

List里面的对象元素，以某个属性来分组，例如，以id分组，将id相同的放在一起：

java 复制代码

Map<Integer, List<User>> groupBy = userList.stream().collect(Collectors.groupingBy(User::getId));

结果：
groupBy:{1=[User{id=1, name='张三', age=18, phone='12345678901'}], 2=[User{id=2, name='李四', age=18, phone='12345678901'}], 3=[User{id=3, name='王五', age=18, phone='12345678901'}], 4=[User{id=4, name='李白', age=18, phone='12345678901'}]}

2、List转Map

java 复制代码

Map<Integer, User> UserMap = userList.stream().collect(Collectors.toMap(User::getId, a -> a,(k1,k2)->k1));

结果：
userMap:{1=User{id=1, name='张三', age=18, phone='12345678901'}, 2=User{id=2, name='李四', age=18, phone='12345678901'}, 3=User{id=3, name='王五', age=18, phone='12345678901'}, 4=User{id=4, name='李白', age=18, phone='12345678901'}}

3、过滤Filter

java 复制代码

List<User> filterList = userList.stream().filter(a -> a.getName().equals("张三")).collect(Collectors.toList());

结果：
filterList:[User{id=1, name='张三', age=18, phone='12345678901'}]

4、求和

java 复制代码

Integer total= userList.stream().map(User::getAge).reduce(0, Integer::sum);

结果：
total:72

5、查找流中最大最小值

java 复制代码

Optional<Dish> maxDish = Dish.menu.stream().
      collect(Collectors.maxBy(Comparator.comparing(Dish::getCalories)));
maxDish.ifPresent(System.out::println);
 
Optional<Dish> minDish = Dish.menu.stream().
      collect(Collectors.minBy(Comparator.comparing(Dish::getCalories)));
minDish.ifPresent(System.out::println);

6、去重

java 复制代码

List<Integer> unique = userList.stream().map(User::getAge).distinct().collect(Collectors.toList());

结果：
unique:[18]