函数式编程-收集器-优快云博客

本文链接：https://blog.youkuaiyun.com/James_JR10/article/details/113846676

本文展示了Java集合框架中Stream API的转换与收集操作，包括toList、toSet和toMap的使用。通过实例测试，演示了如何将用户列表转换为DTO列表、收集用户名到Set以及按年龄排序的TreeSet，以及如何处理重复元素和创建排序的Map。这些例子深入解释了Java 8及更高版本中流处理的实用技巧。

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集合对象收集器

1、toList

 @Test
    public void givenUsers_withMap_thenTransformUsingStream() {
        List<UserDTO> userDTOList = userList.stream().map(user -> UserDTO.builder()
                .username(user.getUsername())
                .enabled(user.isEnabled() ? "可用" : "不可用").build()).collect(Collectors.toList());
        assertEquals(3, userDTOList.size());
    }

2、toSet

 @Test
    public void givenUsers_withToSet_thenSuccess() {
       Set<String> usernames = userList.stream()
               .map(User::getName)
               .collect(toSet());
       assertEquals(2, usernames.size());
    }
    
    @Test
    public void givenUsers_withToCollection_thenSuccess() {
        Comparator<User> byAge = Comparator.comparing(User::getAge);
        TreeSet<User> users = userList.stream()
                .collect(toCollection(() -> new TreeSet<>(byAge)));
        assertEquals(30, users.stream().map(User::getAge).findFirst().orElse(-1));
    }

3、toMap

 @Test
    public void givenUsers_withToMap_thenSuccess() {
        Map<String, User> userMap = userList.stream()
                .collect(toMap(
                        User::getUsername,
                        user -> user
                ));
        assertTrue(userMap.containsKey("lisi"));
        Map<String, User> duplicateMap = Stream.concat(userList.stream(), userList.stream())
                .peek(user -> log.debug("username, {}", user.getUsername()))
                .collect(toMap(
                        User::getUsername,
                        user -> user,
                        (existing, replace) -> existing
                ));
        assertEquals(3, duplicateMap.keySet().size());
        TreeMap<String, User> sortedMap = Stream.concat(userList.stream(), userList.stream())
                .peek(user -> log.debug("username, {}", user.getUsername()))
                .collect(toMap(
                        User::getUsername,
                        user -> user,
                        (existing, replace) -> existing,
                        TreeMap::new
                ));
        assertEquals("lisi", sortedMap.keySet().stream().findFirst().get());
    }