我今天想分享的是 收集 这part的玩法:
OK,开始结合代码示例一起玩下:
<!--lombok依赖引入,代码简洁一点--> <dependency> <groupId>org.projectlombok</groupId> <artifactId>lombok</artifactId> <version>1.18.20</version> <scope>compile</scope> </dependency>准备一个UserDTO.java
/** * @Author:堆代码 duidaima.com * @Date: 2023-3-13 01:25 * @Description: */ @Data public class UserDTO { /** * 姓名 */ private String name; /** * 年龄 */ private Integer age; /** * 性别 */ private String sex; /** * 是否有方向 */ private Boolean hasOrientation; }准备一个模拟获取List的函数:
private static List<UserDTO> getUserList() { UserDTO userDTO = new UserDTO(); userDTO.setName("小冬"); userDTO.setAge(18); userDTO.setSex("男"); userDTO.setHasOrientation(false); UserDTO userDTO2 = new UserDTO(); userDTO2.setName("小秋"); userDTO2.setAge(30); userDTO2.setSex("男"); userDTO2.setHasOrientation(true); UserDTO userDTO3 = new UserDTO(); userDTO3.setName("春"); userDTO3.setAge(18); userDTO3.setSex("女"); userDTO3.setHasOrientation(true); List<UserDTO> userList = new ArrayList<>(); userList.add(userDTO); userList.add(userDTO2); userList.add(userDTO3); return userList; }
List<UserDTO> userList = getUserList(); Stream<UserDTO> usersStream = userList.stream(); HashSet<UserDTO> usersHashSet = usersStream.collect(Collectors.toCollection(HashSet::new));转成 SetusersSet :
List<UserDTO> userList = getUserList(); Stream<UserDTO> usersStream = userList.stream(); Set<UserDTO> usersSet = usersStream.collect(Collectors.toSet());转成 ArrayList:
List<UserDTO> userList = getUserList(); Stream<UserDTO> usersStream = userList.stream(); ArrayList<UserDTO> usersArrayList = usersStream.collect(Collectors.toCollection(ArrayList::new));转成 Object[] objects :
List<UserDTO> userList = getUserList(); Stream<UserDTO> usersStream = userList.stream(); Object[] objects = usersStream.toArray();转成 UserDTO[] users :
List<UserDTO> userList = getUserList(); Stream<UserDTO> usersStream = userList.stream(); UserDTO[] users = usersStream.toArray(UserDTO[]::new); for (UserDTO user : users) { System.out.println(user.toString()); }
// 堆代码 duidaima.com List<UserDTO> userList = getUserList(); Stream<UserDTO> usersStream = userList.stream(); Optional<UserDTO> maxUserOptional = usersStream.max((s1, s2) -> s1.getAge() - s2.getAge()); if (maxUserOptional.isPresent()) { UserDTO masUser = maxUserOptional.get(); System.out.println(masUser.toString()); } List<UserDTO> userList = getUserList(); Stream<UserDTO> usersStream = userList.stream(); Optional<UserDTO> maxUserOptionalNew = usersStream.max(Comparator.comparingInt(UserDTO::getAge)); if (maxUserOptionalNew.isPresent()) { UserDTO masUser = maxUserOptionalNew.get(); System.out.println(masUser.toString()); }找出年龄最小:
Optional<UserDTO> minUserOptional = usersStream.min(Comparator.comparingInt(UserDTO::getAge)); if (minUserOptional.isPresent()) { UserDTO minUser = minUserOptional.get(); System.out.println(minUser.toString()); } Optional<UserDTO> min = usersStream.collect(Collectors.minBy((s1, s2) -> s1.getAge() - s2.getAge()));求平均值:
List<UserDTO> userList = getUserList(); Stream<UserDTO> usersStream = userList.stream(); Double avgScore = usersStream.collect(Collectors.averagingInt(UserDTO::getAge));求和:
Integer reduceAgeSum = usersStream.map(UserDTO::getAge).reduce(0, Integer::sum); int ageSumNew = usersStream.mapToInt(UserDTO::getAge).sum();统计数量:
long countNew = usersStream.count();按照具体年龄分组:
//按照具体年龄分组 Map<Integer, List<UserDTO>> ageGroupMap = usersStream.collect(Collectors.groupingBy((UserDTO::getAge)));分组过程加写判断逻辑:
//按照性别 分为"男"一组 "女"一组 Map<Integer, List<UserDTO>> groupMap = usersStream.collect(Collectors.groupingBy(s -> { if (s.getSex().equals("男")) { return 1; } else { return 0; } }));多级复杂分组:
//多级分组 // 1.先根据年龄分组 // 2.然后再根据性别分组 Map<Integer, Map<String, Map<Integer, List<UserDTO>>>> moreGroupMap = usersStream.collect(Collectors.groupingBy( //1.KEY(Integer) VALUE (Map<String, Map<Integer, List<UserDTO>>) UserDTO::getAge, Collectors.groupingBy( //2.KEY(String) VALUE (Map<Integer, List<UserDTO>>) UserDTO::getSex, Collectors.groupingBy((userDTO) -> { if (userDTO.getSex().equals("男")) { return 1; } else { return 0; } }))));