stream学习

Stream流

定义

Steam流，用于操作集合或者数组中的数据，大量结合了Lamda表达式的语法风格，代码简洁。

重点： 流只能收集一次

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获取Stream流

Stream流要与数据源建立连接。

1.list

​ 直接调用steam()即可

// list
List<String> names = new ArrayList<>();
Collections.addAll(names, "品贵","红旗", "周芷若","张三丰");
Stream<String> stream = names.stream();
stream.filter(s -> s.contains("品")).forEach(s -> System.out.println(s));

2.set

​ 直接调用stream()即可

//set
Set<String> set = new HashSet<>();
Collections.addAll(set, "品贵","红旗", "三个人", "周芷若","张三丰","张三丰");
Stream<String> stream1 = set.stream();
stream1.filter(s -> s.contains("三")).forEach(s -> System.out.println(s));

3.map

​ 需要先将map转换为entrySet(),再通过entryset()进行获取stream()

//map
       Map<String, Double> map = new HashMap<>(){{
               put("古力娜扎", 172.3);
               put("迪丽热巴", 175.3);
               put("古力娜扎", 171.4);
               put("稀里哗啦", 189.3);
               put("巴啦啦小魔仙", 160.1);
       }};
//        map.stream()
       Set<Map.Entry<String, Double>> entries = map.entrySet();
       Stream<Map.Entry<String, Double>> kvs = entries.stream();
       kvs.filter(e -> e.getKey().contains("巴")).forEach(System.out::println);
// 简便写法
map.entrySet().stream().filter(e -> e.getKey().contains("巴")).forEach(System.out::println);

4.数组

​ 使用Arrays.stream()

​ 使用Stream.of()

//数组
String[] names2 = {"张翠山", "东方不败", "大唐不夜城", "回民街"};
Stream<String> s1 = Arrays.stream(names2);
Stream<String> s2 = Stream.of(names2);

Steam流的常见中间方法

中间方法指的是调用完成后会返回新的Stream流，可以继续使用，支持链式编程。

package stream;


import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.stream.Stream;

public class Test2 {
    public static void main(String[] args) {
        List<Double> scores = new ArrayList<>();
        // 找出大于60分并升序排序后输出
        Collections.addAll(scores, 88.5, 100.0, 60.6, 99.0, 9.5, 99.6, 25.9);
        scores.stream().filter(s -> s >= 60).sorted().forEach(System.out::println);

        System.out.println("-------------------------------------------");

        List<Student> students = new ArrayList<>();
        Student s1 = new Student("蜘蛛精", 26, 172.5);
        Student s2 = new Student("蜘蛛精", 26, 174.5);
        Student s3 = new Student("紫霞", 216, 122.5);
        Student s4 = new Student("白晶晶", 25, 175.5);
        Student s5 = new Student("牛魔王", 13, 166.5);
        Collections.addAll(students, s1, s2, s3, s4, s5);
        // 找出年龄大于等于23 年龄小于等于30 并且按照年龄输出
        students.stream().filter(s -> s.getAge() >= 23 && s.getAge() <= 30).sorted((o1, o2) -> o2.getAge() - o1.getAge()).forEach(System.out::println);
        System.out.println("-------------------------------------------");
        // 取出身高前3名的学生 并输出
        students.stream().sorted((o1, o2) -> Double.compare(o2.getHeight(), o1.getHeight())).limit(3).forEach(System.out::println);

        System.out.println("-------------------------------------------");
        // 取出身高倒数2名学生，并输出
        students.stream().sorted((o1, o2) -> Double.compare(o1.getHeight(), o2.getHeight())).limit(2).forEach(System.out::println);
        System.out.println("-------------------------------------------");
        // 找出身高超过168的学生叫什么名字，要求去除重复的名字，再输出
        students.stream().filter(s -> s.getHeight() > 168).map(Student::getName).distinct().forEach(System.out::println);
        System.out.println("-------------------------------------------");
        // 找出身高超过168的学生叫什么名字，要求去除重复的内容，再输出
        // 首先需要重写equals和hashCode函数 让内容相同的对象设置为一样的
        students.stream().filter(s -> s.getHeight() > 168).distinct().forEach(System.out::println);
        System.out.println("-------------------------------------------");
        Stream<String> st1 = Stream.of("张三", "李四");
        Stream<String> st2 = Stream.of("张三2", "李四2", "王五");
        Stream.concat(st1, st2).forEach(System.out::println);


    }
}

Stream流常见的终结方法

package stream;

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

public class Test3 {
    public static void main(String[] args) {
        List<Student> students = new ArrayList<>();
        Student s1 = new Student("蜘蛛精", 26, 172.5);
        Student s2 = new Student("蜘蛛精", 26, 172.5);
        Student s3 = new Student("紫霞", 23, 167.6);
        Student s4 = new Student("白晶晶", 25, 169.0);
        Student s5 = new Student("牛魔王", 35, 183.8);
        Student s6 = new Student("牛夫人", 34, 168.5);
        Collections.addAll(students, s1, s2, s3, s4, s5, s6);

        // 需求1：计算身高超过168的人数
        System.out.println(students.stream().filter(s -> s.getHeight() >= 168).count());
        //需求2：找出身高最高的学生对象并输出
        System.out.println(students.stream().max((o1, o2) -> Double.compare(o1.getHeight(), o2.getHeight())).get());
        //需求3：找出身高最矮的学生对象并输出
        System.out.println(students.stream().min(((o1, o2) -> Double.compare(o1.getHeight(), o2.getHeight()))).get());
        //需求4：找出身高超过170的学生对象，放到一个新集合
        List<Student> student1 = students.stream().filter(s -> s.getHeight() >= 170).collect(Collectors.toList());
        System.out.println(student1);

        Set<Student> student2 = students.stream().filter(s -> s.getHeight() >= 170).collect(Collectors.toSet());
        System.out.println(student2);

        //需求5：找出身高超过170的学生对象并把学生的名字和身高，存入到一个Map集合中
        Map<String, Double> map = students.stream().filter(s -> s.getHeight() > 170).distinct()
                .collect(Collectors.toMap(Student::getName, Student::getHeight));

        map.forEach((k, v) -> {
            System.out.println(k + "-----" + v);
        });
        // 收集到数组中
        Object[] array = students.stream().filter(s -> s.getHeight() > 170).toArray();
        System.out.println(Arrays.toString(array));
    }
}