Java8中stream流的collectingAndThen方法应用实例详解

/**
 * @desc: 用户实体类
 * @author: shy
 * @date: 2020/12/28 12:38
 */
@Data
@AllArgsConstructor
public class User {
    //用户Id
    int userId;
    //用户姓名
    String userName;
    //用户工资
    Double salary;
}

//数据准备
User user1 = new User(1, "hangman", 100d);
User user2 = new User(1, "list", 200d);
User user3 = new User(2, "wing", 200d);
User user4 = new User(2, "zeal", 100d);
User user5 = new User(3, "tuba", 300d);
List<User> userList = new ArrayList<>();
userList.add(user1);
userList.add(user2);
userList.add(user3);
userList.add(user4);
userList.add(user5);

//对集合的结果进行去重
List<User> list = userList.stream()
    .collect(Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(User::getUserId))), ArrayList::new));
System.out.println(list);

//对集合的结果进行去重
Map<Integer, User> userMap = userList.stream()
                .collect(Collectors.collectingAndThen(
                        Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(User::getId))),
                        treeSet -> treeSet.stream().collect(Collectors.toMap(User::getId, user -> user, (u1, u2) -> u1))
                ));
        System.out.println(userMap);

//查找工资最高的员工的姓名
String userName = userList.stream()
	.collect(Collectors.collectingAndThen(Collectors.maxBy(Comparator.comparing(User::getSalary)),(Optional<User> user) -> user.map(User::getUserName).orElse(null)));
System.out.println(userName);

//计算用户工资的平均值
Double avgSalary = userList.stream()
	.collect(Collectors.collectingAndThen(Collectors.averagingDouble(User::getSalary), Double::doubleValue));
System.out.println(avgSalary);