Java 8 stream 经典示例

 1 package org.study2.java8.stream;
 2 
 3 import org.junit.Test;
 4 
 5 import java.util.*;
 6 import java.util.stream.Collectors;
 7 import java.util.stream.Stream;
 8 
 9 /**
10  * Java 8 – Streams
11  * url:http://www.studytrails.com/java/java8/java8_streams/
12  * url:https://www.cnblogs.com/andywithu/p/7404101.html
13  */
14 public class StreamDemo {
15     @Test
16     public void test() {
17         List<String> genre = new ArrayList<String>(Arrays.asList("rock", "pop", "jazz", "reggae"));
18         long a = genre.stream().filter(s -> s.startsWith("r")).count();
19         System.out.println(a);
20 
21         boolean b = genre.stream().allMatch(s -> !s.isEmpty());
22         System.out.println(b);
23 
24         boolean b2 = genre.stream().anyMatch(s -> s.indexOf("r") == 0);
25         System.out.println(b2);
26 
27         System.out.println(genre.stream().peek(s -> System.out.println(s)).anyMatch(s -> s.indexOf("r") == 0));
28         System.out.println(genre.stream().peek(s -> System.out.println(s)).count());
29 
30     }
31 
32     //   现在有一个List<User> 的集合，如何把这个list转换成Map<Integer, User> 其中，key是user id，value是User对象
33     @Test
34     public void test1() {
35         List<User> users = Arrays.asList(new User(1, "Tomcat"), new User(2, "Apache"), new User(3, "Nginx"));
36         Map<Integer, User> map = users.stream().collect(Collectors.toMap(obj -> obj.getId(), obj -> obj));
37         System.out.println(map);
38 
39         // 或者使用方法的引用
40         Map<Integer, User> map2 = users.stream().collect(Collectors.toMap(User::getId, obj -> obj));
41         System.out.println(map2);
42     }
43 
44     //    把list中的每个map中的id取出来，转换成list<String>
45     @Test
46     public void test2() {
47         List<Map<String, String>> list = new ArrayList<>();
48 
49         Map<String, String> map1 = new HashMap<>();
50         map1.put("id", "101");
51         map1.put("name", "Tomcat");
52 
53         Map<String, String> map2 = new HashMap<>();
54         map2.put("id", "102");
55         map2.put("name", "Apache");
56 
57         Map<String, String> map3 = new HashMap<>();
58         map3.put("id", "103");
59         map3.put("name", "Nginx");
60 
61         list.add(map1);
62         list.add(map2);
63         list.add(map3);
64 
65         List<String> ids = list.stream().map(entity -> entity.get("id")).collect(Collectors.toList());
66         System.out.println(ids);
67 
68         //   或者
69         List<Object> ids2 = Arrays.asList(list.stream().map(entity -> entity.get("id")).toArray());
70     }
71 
72     //    把url的请求参数（如：type=1&from=APP&source=homePage）转换成Map<String, String>
73     @Test
74     public void test3() {
75         String queryString = "type=1&from=APP&source=homePage";
76         Map<String, String> map = Stream.of(queryString.split("&")).map(obj -> obj.split("=")).collect(Collectors.toMap(entry -> entry[0], entry -> entry[1]));
77         System.out.println(map);
78     }
79 
80     //    把List<String>转换成List<Integer>
81     @Test
82     public void test4() {
83         List<String> strs = Arrays.asList("1", "2", "3");
84         List<Integer> ints = strs.stream().map(obj -> Integer.valueOf(obj)).collect(Collectors.toList());
85     }
86 
87 
88 }

 1 package org.study2.java8.stream;
 2 
 3 
 4 public class User {
 5 
 6     private int id;
 7     private String name;
 8 
 9     public User(int id, String name) {
10         this.id = id;
11         this.name = name;
12     }
13 
14     public int getId() {
15         return id;
16     }
17 
18     public String toString() {
19         return "User [>;
20     }
21 
22 }