java8 stream 用法

 1 　　private List<User> users = Arrays.asList(
 2             new User("1001", "John", 16),
 3             new User("1002", "Carly", 17),
 4             new User("1003", "Tom", 11),
 5             new User("1004", "Mike", 11));
 6 
 7 
 8     private void lambdaFunctions() {
 9 
10         // 转为某一属性的集合
11         List<String> names = users.stream().map(User::getName).collect(Collectors.toList());
12 
13         // 转为以某一属性为key,另一属性为value的map
14         Map<String, Integer> idAgeMap = users.stream().collect(Collectors.toMap(User::getId, User::getAge));
15 
16         // 转为以某一属性为key,对象本身为value的map
17         Map<String, User> idUserMap = users.stream().collect(Collectors.toMap(User::getId, User -> User));
18 
19         // 转为以某一属性为key,对象本身的集合为value
20         Map<String, List<User>> idUsersMap = users.stream().collect(Collectors.groupingBy(User::getId, Collectors.toList()));
21 
22         // 过滤
23         List<User> users1 = users.stream().filter(user -> user.getAge() >= 16).collect(Collectors.toList());
24 
25         //过滤
26         List<User> users2 = users.stream().filter(user -> {
27             return user.getAge() < 16;
28         }).collect(Collectors.toList());
29 
30 
31         // A对象集合转为B对象集合
32         List<Person> persons = users.stream().map(user -> {
33             return new Person(user.getId(), user.getName(), user.getAge());
34         }).collect(Collectors.toList());
35 
36 
37     }