Collectors
1.获取最小值minBy
Optional<Integer> min = list.stream().collect(Collectors.minBy((o1, o2) -> o1 - o2));
if (min.isPresent()) {
System.out.println(min.get());
}
或者直接使用stream内置的min方法
list.stream().min((o1, o2) ->{return o1 - o2;}).get()
2.求和 summingInt summingLong summingDouble
Integer sum = list.stream().collect(Collectors.summingInt(Integer::intValue));
3.分组
Map<String, List<Fruit>> groupMap = fruitList.stream().collect(Collectors.groupingBy(Fruit::getName));
Map<String, List<Integer>> groupby = list.stream().collect(Collectors.groupingBy(item -> {
if (item >= 5) {
return "datuwu";
} else {
return "xiaoxuwu";
}
}));
List<Integer> datuwu = groupby.get("datuwu");
List<Integer> xiaoxuwu = groupby.get("xiaoxuwu");
datuwu.forEach(System.out::println);
System.out.println();
xiaoxuwu.forEach(System.out::println);
4.toMap
初始化list对象集合
User user = new User("linqin", 12);
User user1 = new User("linqin1", 13);
User user2 = new User("linqin2", 14);
User user3 = new User("linqin3", 12);
List<User> list = new ArrayList<>();
list.add(user);
list.add(user1);
list.add(user2);
list.add(user3);
tomap 要注意重复数据的处理。否者重复key会报错。（ps：注意 toMap 的 value 不能为空，需要提前处理）
Map<Integer, String> collect = list.stream().collect(Collectors.toMap(User::getAge, User::getName, (o1, o2) -> o2));
Map<Integer, User> collect = list.stream().filter(i -> i.getAge() != null).collect(Collectors.toMap(User::getAge, a-> a, (o1, o2) -> o2));
[12=linqin3, 13=linqin1, 14=linqin2]
stream的map方法，区别：只能取出一个key或者value的值。
List<Integer> collect1 = list.stream().map(User::getAge).collect(Collectors.toList());
collect1.forEach(System.out::println);
12 13 14 12
5.toList
List<String> collect = list.stream().collect(Collectors.toList());
6.toSet去重
Set<String> collect1 = list.stream().collect(Collectors.toSet());
7.自定义集合collection
List<String> linkedListResult = list.stream().collect(Collectors.toCollection(LinkedList::new));
8.joining
String join = list.stream().collect(Collectors.joining(","));
9.partitioningBy分组
Map<Boolean, List<String>> twoList = list.stream().collect(Collectors.partitioningBy(item -> item.contains("++")));
List<String> containP = twoList.get(true);
List<String> noContainP = twoList.get(false);
containP.forEach(System.out::println);
System.out.println("-----------++---");
noContainP.forEach(System.out::println);
System.out.println("-------------++-");
10.counting
Long num = list.stream().collect(Collectors.counting());
和stream的count()一样效果
list.stream().count()
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