// List轉(zhuǎn)Map
Map<Long, User> userMap = userList.stream().collect(Collectors.toMap(User::getId, Function.identity()));

// 如果list中g(shù)etId有重復驱显，需添加重復key策略，否則轉(zhuǎn)換的時候會報重復key的異常瞳抓，如下埃疫，u1和u2的位置決定當出現(xiàn)重復key時使用前者還是后者
userMap = userList.stream().collect(Collectors.toMap(User::getId, Function.identity(), (u1, u2) -> u2));

// List轉(zhuǎn)某個字段的List
List<Long> userIdList = userList.stream().map(User::getId).collect(Collectors.toList());

// List根據(jù)某個字段分組
Map<Long, List<User>> userListMap = userList.stream().collect(Collectors.groupingBy(User::getId));

// List根據(jù)對象中某個字段去重
List<User> distinctUserList = userList.stream()
                .collect(Collectors.collectingAndThen(
                        Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(User::getId))), ArrayList::new));

// 簡單類型list去重
List<Integer> newUserIdList = userIdList.stream().distinct().collect(Collectors.toList());

// String類型List以逗號隔開轉(zhuǎn)為字符串
String userNames = String.join(",", userNameList);

// Integer類型List以逗號隔開轉(zhuǎn)為字符串
String userIds = userIdList.stream().map(String::valueOf).collect(Collectors.joining(","));

// List根據(jù)條件過濾元素
List<Long> userFilterIdList = userIdList.stream().filter(id -> !userListA.contains(id)).collect(Collectors.toList());

// List根據(jù)某個屬性匯總
int sum = userList.stream().mapToInt(User::getAge).sum();
BigDecimal bigDecimalSum = userList.stream().map(User::getValue).reduce(BigDecimal.ZERO, BigDecimal::add);

// List根據(jù)某個屬性排序，默認正序
userList.sort(Comparator.comparing(User::getAge));

// List根據(jù)某個屬性排序孩哑，倒序
userList.sort(( User user1, User user2) -> user2.getAge() - user1.getAge());

// 并行執(zhí)行
userList = userIdList.parallelStream().map(user -> userService.selectList(userList))
                .flatMap(Collection::stream).collect(Collectors.toList());

// 排序
userList = userList.stream()
        .sorted(Comparator.comparing(User::getAge).reversed()
            .thenComparing(
                (User o1, User o2) -> o2.getSort() - o1.getSort())
            .thenComparing(User::getName))
        .collect(Collectors.toList());