Lambda表达式

去重

• 两个list对象根据条件去重

List<User> userList 和 List<Person> personList
去掉 userList 中的 User 的 id 不等于 personList 中的 Person 的 id 的对象
List<User> userList = userList.stream()
    .filter(user -> !personList.stream().map(person - >person.getId()).collect(Collectors.toList())
    .contains(user.getId()))
    .collect(Collectors.toList());
    
list.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(()
                -> new TreeSet<>(Comparator.comparing(o -> o.getId()))), ArrayList::new));  

• 简单List集合求交、去重、差集

// 交集 （parallelStream() 使用并行流遍历，多线程; stream() ）
List<String> intersection = list1.stream().filter(item -> list2.contains(item)).collect(toList());
// 差集 (list1 - list2)
List<String> reduce1 = list1.stream().filter(item -> !list2.contains(item)).collect(toList());
// 差集 (list2 - list1)
List<String> reduce2 = list2.stream().filter(item -> !list1.contains(item)).collect(toList());
// 去重
List<String> listAllDistinct = listAll.stream().distinct().collect(toList());

排序

List<Person> sortedList = list.stream()
    .sorted((o1,o2) -> o1.getAge() - o2.getAge())
    .collect(Collectors.toList());
List<Person> sortedList =  list.sort(Comparator.comparing(ClassImagePraise::getSort));    
List<Person> sortedList =  list.sort(Comparator.comparing(ClassImagePraise::getSort)).reversed());  
// 多条件组合排序
// 先根据msg(升序),再根据id(升序)
list.sort(Comparator.comparing(Message:: getMsg).thenComparing(Message::getId));
// 根据msg(升序),再根据id(降序)
list.sort(Comparator.comparing(Message:: getMsg).thenComparing(Comparator.comparing(Message::getId).reversed()));
// 先根据msg(降序),再根据id(降序)
list.sort(Comparator.comparing(Message:: getMsg).thenComparing(Message::getId).reversed());
// 先根据msg(降序),再根据id(升序)
list.sort(Comparator.comparing(Message:: getMsg).reversed().thenComparing(Message::getId));

过滤

List<Person> filterList = list.stream().filter(item -> item.getAge()>3).collect(Collectors.toList());

筛选、转换

List<String> mapList1 = list.stream().map(Person::getName).collect(Collectors.toList());
List<String> mapList2 = list.stream().map(item -> item.getName()).collect(Collectors.toList());
// 过滤与筛选结合
List<String> teacherIds = pList.stream()
    .filter(s -> s.getId().equals("333"))
    .map(p -> p.getUserId())
    .collect(Collectors.toList());
    
tests.stream().collect(Collectors.toMap(item -> item.getId(), item -> item))    

统计

getAverage统计 sum() 。mapToDouble() 转换成double。还有其他类型转换。可以自己研究。max(),min(),average()
double sum = list.stream().mapToDouble(Person::getAge).sum();

Optional<Message> maxMassage = list.stream().collect(Collectors.maxBy(Comparator.comparing(Message::getId)));
Long maxId = maxMassage.get().getId();

LongSummaryStatistics lss = list.stream().collect(Collectors.summarizingLong(Message::getId));
    //  求和 lss.getSum()
    //  求最大值 lss.getMax()
    // 求最小值 lss.getMin()
    // 求平均值 lss.getAverage()

分组

Map<Integer, List<Person>> map = list.stream().collect(Collectors.groupingBy(Person::getAge));
// 多重分组
Map<String, Map<Integer, List<Person>>> map2 = list.stream()
    .collect(Collectors.groupingBy(t->t.getName(),Collectors.groupingBy(t->t.getAge())));
// 分组并计算综合        Collectors.summarizingLong()
Map<String, Map<Integer, LongSummaryStatistics>> map3 = list.stream()
    .collect(Collectors.groupingBy(t->t.getName(),Collectors.groupingBy(t->t.getAge(),Collectors.summarizingLong(Person::getSize))));