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