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

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：

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：

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

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

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

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

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

List<Integer> unique = userList.stream().map(User::getAge).distinct().collect(Collectors.toList());结果：
unique:[18]