JDK8新特性：Stream流

1.获取Stream流

2.Stream流常见的中间方法

3.Stream流常见的终结方法

1、 Stream 是什么？有什么作用？结合了什么技术？

●也叫 Stream 流，是Jdk8开始新增的一套 API ( java . util . stream .*)，可以用于操作集合或者数组的数据。.

优势： Stream 流大量的结合了 Lambda 的语法风格来编程，提供了一种更加强大更加简单的方式操作集合或者数组中的数据，代码更简洁，可读性更好。

2、说说 Stream 流处理数据的步骤是什么？

java 复制代码

package com.xinbao.d8_stream;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;

public class StreamTest1 {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        Collections.addAll(names,"张三丰", "张无忌", "周芷若", "赵敏", "张强");
        System.out.println(names);
        //找出姓张且是三个字的名字，存到新集合中去
        List<String> list = new ArrayList<>();
        for (String name : names) {
            if (name.startsWith("张") && name.length() == 3){
                list.add(name);
            }
        }
        System.out.println(list);

        //使用Stream流解决问题
        List<String> list2 = names.stream().filter(s -> s.startsWith("张"))
                .filter(a -> a.length() == 3).collect(Collectors.toList());
        System.out.println(list2);
    }
}

java 复制代码

E:\JVsoft\Java\jdk-17\bin\java.exe -javaagent:E:\JVsoft\IntelliJIDEA2021.1.1\lib\idea_rt.jar=4519:E:\JVsoft\IntelliJIDEA2021.1.1\bin -Dfile.encoding=UTF-8 -classpath E:\JVsoft\code\out\production\collection-map-stream-app com.xinbao.d8_stream.StreamTest1
[张三丰, 张无忌, 周芷若, 赵敏, 张强]
[张三丰, 张无忌]
[张三丰, 张无忌]

1.获取Stream流

java 复制代码

package com.xinbao.d8_stream;

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

public class StreamTest2 {
    public static void main(String[] args) {
        //1.如何获取List集合的Stream流
        List<String> names = new ArrayList<>();
        Collections.addAll(names,"张三丰", "张无忌", "周芷若", "赵敏", "张强");
        Stream<String> stream = names.stream();

        //2.如何获取Set集合的Stream流
        Set<String> set = new HashSet<>();
        Collections.addAll(set,"刘德华", "张曼玉", "张国荣", "刘嘉玲", "王祖贤");
        Stream<String> stream1 = set.stream();//alt + enter
        stream1.filter(s -> s.contains("张")).forEach(s -> System.out.println(s));

        //3.如何获取Map集合的Stream流
        Map<String,Double> map = new HashMap<>();
        map.put("古力娜扎", 167.5);
        map.put("迪丽热巴", 164.5);
        map.put("马尔扎哈", 187.7);
        map.put("哈尼克孜", 163.8);

        Set<String> keys = map.keySet();
        Stream<String> ks = keys.stream();

        Collection<Double> values = map.values();
        Stream<Double> vs = values.stream();

        Set<Map.Entry<String, Double>> entries = map.entrySet();
        Stream<Map.Entry<String, Double>> kvs = entries.stream();
        kvs.filter(s -> s.getKey().contains("哈")).
                forEach(s -> System.out.println(s.getKey() + "-->" + s.getValue()));

        //4.如何获取数组的Stream流
        String[] names2 = {"刘德华", "张曼玉", "张国荣", "刘嘉玲", "王祖贤"};
        Stream<String> stream2 = Arrays.stream(names2);
        Stream<String> names21 = Stream.of(names2);
    }
}

java 复制代码

E:\JVsoft\Java\jdk-17\bin\java.exe -javaagent:E:\JVsoft\IntelliJIDEA2021.1.1\lib\idea_rt.jar=39123:E:\JVsoft\IntelliJIDEA2021.1.1\bin -Dfile.encoding=UTF-8 -classpath E:\JVsoft\code\out\production\collection-map-stream-app com.xinbao.d8_stream.StreamTest2
张国荣
张曼玉
马尔扎哈-->187.7
哈尼克孜-->163.8

2.Stream流常见的中间方法

java 复制代码

package com.xinbao.d6_map_impl;

import java.util.Objects;


public class Student implements Comparable<Student>{
    private String name;
    private int age;
    private double height;

    //this o
    @Override
//    public int compareTo(Student o) {
//        return this.age - o.age;//年龄升序
//    }
    public int compareTo(Student o) {
        return o.age - this.age;
    }

    public Student() {
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return age == student.age && Double.compare(student.height, height) == 0 && Objects.equals(name, student.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age, height);
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", height=" + height +
                '}';
    }

    public Student(String name, int age, double height) {
        this.name = name;
        this.age = age;
        this.height = height;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public double getHeight() {
        return height;
    }

    public void setHeight(double height) {
        this.height = height;
    }
}

java 复制代码

package com.xinbao.d8_stream;

import com.xinbao.d6_map_impl.Student;

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

public class StreamTest3 {
    public static void main(String[] args) {
        List<Double> scores = new ArrayList<>();
        Collections.addAll(scores,88.5,22.5,88.7,56.9,24.8,46.7,96.3);
        //需求1：找出成绩大于60的数据，排序输出
        scores.stream().filter(s -> s>60).sorted().forEach(s -> System.out.println(s));

        List<Student> students = new ArrayList<>();
        students.add(new Student("蜘蛛精", 25, 168.5));
        students.add(new Student("蜘蛛精", 25, 168.5));
        students.add(new Student("至尊宝", 23, 163.5));
        students.add(new Student("牛魔王", 28, 183.5));
        System.out.println(students);

        //需求2：找出年龄大于23小于30的学生，升序输出
        students.stream().filter(s -> s.getAge() > 23 && s.getAge() < 30)
                .sorted((o1,o2) -> o1.getAge() - o2.getAge()).forEach(System.out::println);

        //需求3：取出身高最高的前三名学生，并输出
        System.out.println("-------------------------------------------------------------");
        students.stream().sorted((o1, o2) -> Double.compare(o2.getHeight(),o1.getHeight()))
                .limit(3).forEach(System.out::println);

        //需求4：取出身高倒数的两名学生输出
        System.out.println("-------------------------------------------------------------");
        students.stream().sorted((o1,o2) -> Double.compare(o2.getHeight(),o1.getHeight()))
                .skip(students.size() - 2).forEach(System.out::println);

        //需求5：找出身高超过168的学生名字，去重再输出
        System.out.println("--------------------------------------------------------------");
        students.stream().filter(s -> s.getHeight()>168).map(Student::getName)//.map(s->s.getName) 映射
                .distinct().forEach(System.out::println);

        //distinct去重复，自定义类型的对象（希望内容一样就认为重复，重写hashCode,equals）
        students.stream().filter(s -> s.getHeight()>168).distinct().forEach(System.out::println);

        Stream<String> ls = Stream.of("李四", "王五", "刘六六");
        Stream<String> zs = Stream.of("张三");
        Stream<String> concat = Stream.concat(ls, zs);
        concat.forEach(System.out::println);
    }
}

java 复制代码

E:\JVsoft\Java\jdk-17\bin\java.exe -javaagent:E:\JVsoft\IntelliJIDEA2021.1.1\lib\idea_rt.jar=26802:E:\JVsoft\IntelliJIDEA2021.1.1\bin -Dfile.encoding=UTF-8 -classpath E:\JVsoft\code\out\production\collection-map-stream-app com.xinbao.d8_stream.StreamTest3
88.5
88.7
96.3
[Student{name='蜘蛛精', age=25, height=168.5}, Student{name='蜘蛛精', age=25, height=168.5}, Student{name='至尊宝', age=23, height=163.5}, Student{name='牛魔王', age=28, height=183.5}]
Student{name='蜘蛛精', age=25, height=168.5}
Student{name='蜘蛛精', age=25, height=168.5}
Student{name='牛魔王', age=28, height=183.5}
-------------------------------------------------------------
Student{name='牛魔王', age=28, height=183.5}
Student{name='蜘蛛精', age=25, height=168.5}
Student{name='蜘蛛精', age=25, height=168.5}
-------------------------------------------------------------
Student{name='蜘蛛精', age=25, height=168.5}
Student{name='至尊宝', age=23, height=163.5}
--------------------------------------------------------------
蜘蛛精
牛魔王
Student{name='蜘蛛精', age=25, height=168.5}
Student{name='牛魔王', age=28, height=183.5}
李四
王五
刘六六
张三

3.Stream流常见的终结方法

java 复制代码

package com.xinbao.d8_stream;

import com.xinbao.d6_map_impl.Student;

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

public class StreamTest4 {
    public static void main(String[] args) {
        List<Student> students = new ArrayList<>();
        Student s1 = new Student("蜘蛛精", 25, 168.5);
        Student s2 = new Student("蜘蛛精", 25, 168.5);
        Student s3 = new Student("至尊宝", 23, 163.5);
        Student s4 = new Student("牛魔王", 28, 183.5);
        Student s5 = new Student("狐狸精", 23, 161.5);
        Student s6 = new Student("白骨精", 25, 161.5);
        Collections.addAll(students, s1, s2, s3, s4, s5, s6);

        //需求1：计算出高于168的人数
        long count = students.stream().filter(s -> s.getHeight() > 168).count();
        System.out.println(count);

        //需求2：找出最高学生对象并输出
        Student max = students.stream().max((o1, o2) -> Double.compare(o1.getHeight(), o2.getHeight())).get();
        System.out.println(max);

        Optional<Student> max1 = students.stream().max((o1, o2) -> Double.compare(o1.getHeight(), o2.getHeight()));
        System.out.println(max1);

        //需求3：找出最矮学生对象并输出
        Student min = students.stream().min((o1, o2) -> Double.compare(o1.getHeight(), o2.getHeight())).get();
        System.out.println(min);

        //需求4：找出高于165的学生对象，放到新集合中返回
        List<Student> collect = students.stream().filter(s -> s.getHeight() > 165).collect(Collectors.toList());
        System.out.println(collect);

        Set<Student> collect2 = students.stream().filter(s -> s.getHeight() > 165).collect(Collectors.toSet());
        System.out.println(collect2);//去重

        //流只能收集一次（类比迭代器）
//        Stream<Student> str = students.stream().filter(s -> s.getHeight() > 165);
//        List<Student> list = str.collect(Collectors.toList());
//        System.out.println(list);
//        Set<Student> set = str.collect(Collectors.toSet());//报错，流已经终结
//        System.out.println(set);

        //需求五：找出高于165的学生对象，并把对象的名字和身高，存入到一个Map集合返回
        Map<String,Double> map =
                students.stream().filter(s -> s.getHeight()>165)//不会自动去重，需要.distinct()去重
                        .distinct().collect(Collectors.toMap(a -> a.getName(), a -> a.getHeight()));
        System.out.println(map);

        Object[] objects = students.stream().filter(s -> s.getHeight() > 165).toArray();
        System.out.println(Arrays.toString(objects));
        Student[] students1 = students.stream().filter(s -> s.getHeight() > 165).toArray(len -> new Student[len]);
        System.out.println(Arrays.toString(students1));
    }
}

java 复制代码

E:\JVsoft\Java\jdk-17\bin\java.exe -javaagent:E:\JVsoft\IntelliJIDEA2021.1.1\lib\idea_rt.jar=27234:E:\JVsoft\IntelliJIDEA2021.1.1\bin -Dfile.encoding=UTF-8 -classpath E:\JVsoft\code\out\production\collection-map-stream-app com.xinbao.d8_stream.StreamTest4
3
Student{name='牛魔王', age=28, height=183.5}
Optional[Student{name='牛魔王', age=28, height=183.5}]
Student{name='狐狸精', age=23, height=161.5}
[Student{name='蜘蛛精', age=25, height=168.5}, Student{name='蜘蛛精', age=25, height=168.5}, Student{name='牛魔王', age=28, height=183.5}]
[Student{name='牛魔王', age=28, height=183.5}, Student{name='蜘蛛精', age=25, height=168.5}]
{蜘蛛精=168.5, 牛魔王=183.5}
[Student{name='蜘蛛精', age=25, height=168.5}, Student{name='蜘蛛精', age=25, height=168.5}, Student{name='牛魔王', age=28, height=183.5}]
[Student{name='蜘蛛精', age=25, height=168.5}, Student{name='蜘蛛精', age=25, height=168.5}, Student{name='牛魔王', age=28, height=183.5}]

进程已结束，退出代码为 0