Java 8 stream 经典示例

 package org.study2.java8.stream;

 import org.junit.Test;

 import java.util.*;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;

 /**
  * Java 8 – Streams
  * url:http://www.studytrails.com/java/java8/java8_streams/
  * url:https://www.cnblogs.com/andywithu/p/7404101.html
  */
 public class StreamDemo {
     @Test
     public void test() {
         List<String> genre = new ArrayList<String>(Arrays.asList("rock", "pop", "jazz", "reggae"));
         long a = genre.stream().filter(s -> s.startsWith("r")).count();
         System.out.println(a);

         boolean b = genre.stream().allMatch(s -> !s.isEmpty());
         System.out.println(b);

         boolean b2 = genre.stream().anyMatch(s -> s.indexOf("r") == 0);
         System.out.println(b2);

         System.out.println(genre.stream().peek(s -> System.out.println(s)).anyMatch(s -> s.indexOf("r") == 0));
         System.out.println(genre.stream().peek(s -> System.out.println(s)).count());

     }

     //   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
     @Test
     public void test1() {
         List<User> users = Arrays.asList(new User(1, "Tomcat"), new User(2, "Apache"), new User(3, "Nginx"));
         Map<Integer, User> map = users.stream().collect(Collectors.toMap(obj -> obj.getId(), obj -> obj));
         System.out.println(map);

         // 或者使用方法的引用
         Map<Integer, User> map2 = users.stream().collect(Collectors.toMap(User::getId, obj -> obj));
         System.out.println(map2);
     }

     //    把list中的每个map中的id取出来，转换成list<String>
     @Test
     public void test2() {
         List<Map<String, String>> list = new ArrayList<>();

         Map<String, String> map1 = new HashMap<>();
         map1.put("id", "101");
         map1.put("name", "Tomcat");

         Map<String, String> map2 = new HashMap<>();
         map2.put("id", "102");
         map2.put("name", "Apache");

         Map<String, String> map3 = new HashMap<>();
         map3.put("id", "103");
         map3.put("name", "Nginx");

         list.add(map1);
         list.add(map2);
         list.add(map3);

         List<String> ids = list.stream().map(entity -> entity.get("id")).collect(Collectors.toList());
         System.out.println(ids);

         //   或者
         List<Object> ids2 = Arrays.asList(list.stream().map(entity -> entity.get("id")).toArray());
     }

     //    把url的请求参数（如：type=1&from=APP&source=homePage）转换成Map<String, String>
     @Test
     public void test3() {
         String queryString = "type=1&from=APP&source=homePage";
         Map<String, String> map = Stream.of(queryString.split("&")).map(obj -> obj.split("=")).collect(Collectors.toMap(entry -> entry[0], entry -> entry[1]));
         System.out.println(map);
     }

     //    把List<String>转换成List<Integer>
     @Test
     public void test4() {
         List<String> strs = Arrays.asList("1", "2", "3");
         List<Integer> ints = strs.stream().map(obj -> Integer.valueOf(obj)).collect(Collectors.toList());
     }

 }

User.Class

 package org.study2.java8.stream;

 public class User {

     private int id;
     private String name;

     public User(int id, String name) {
         this.id = id;
         this.name = name;
     }

     public int getId() {
         return id;
     }

     public String toString() {
         return "User [id=" + id + ", name=" + name + "]";
     }

 }

详细讲解参考：

https://www.cnblogs.com/andywithu/p/7404101.html