Java进阶13讲__第九讲

Stream流

1. 案例初体验

java 复制代码

package cn.hdc.oop9.stream.using;

import java.util.LinkedList;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class t1 {
    public static void main(String[] args) {
        LinkedList<String> list = new LinkedList<>();
        LinkedList<String> l1 = new LinkedList<>();
        list.add("wsn");
        list.add("wzn");
        list.add("wyr");
        list.add("jsz");
        list.add("zqy");
//        需求：把w开头的且长度为3的元素存储到一个新的集合
//        使用Stream前
        list.forEach(item -> {
            if (item.length() == 3 && item.charAt(0) == 'w') {
                l1.add(item);
            }
        });
        System.out.println(l1);
//        使用Stream后
        List<String> l = list.stream()
                .filter(item -> item.startsWith("w"))
                .filter(item -> item.length() == 3)
                .collect(Collectors.toList());
        System.out.println(l);
    }
}

2. 什么是Stream?

3. 使用步骤

3.1 获取Stream流

3.1.1 代码试用

java 复制代码

package cn.hdc.oop9.stream.api;

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

public class t1 {
    public static void main(String[] args) {
        //1、如何获取List集台的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> s1 = names.stream();
        //2、如何获取Set集合的stream流?
        Set<String> set = new HashSet<>();
        Collections.addAll(set, "刘德华", "张受玉", "蜘蛛精", "马德", "德玛西亚");
        Stream<String> s2 = set.stream();
        s2.filter(item -> item.contains("德")).forEach(item -> System.out.println(item));
        //3、如何获取Map集合的stream流?
        Map<String, Double> map = new HashMap<>();
        map.put("古力娜扎", 172.3);
        map.put("迪丽热巴", 168.3);
        map.put("马尔扎哈", 166.3);
        map.put("卡尔扎巴", 168.3);
        Stream<String> s3 = map.keySet().stream();
        Stream<Double> s4 = map.values().stream();
        Stream<Map.Entry<String, Double>> s5 = map.entrySet().stream();
        s5.filter(item -> item.getKey().contains("巴")).forEach(item -> System.out.println(item));
        //4.如何获取数组的Stream流
        String[] names2 = {"张翠山", "东方不败", "唐大山", "独孤求败"};
        Arrays.stream(names2).filter(item -> item.contains("山")).forEach(item -> System.out.println(item));
        Stream.of(names2).filter(item -> item.length() > 3).forEach(item -> System.out.println(item));
    }
}

3.2 Stream中间方法

3.2.1 什么是中间方法

3.2.2 常见中间方法

3.2.3 代码试用

java 复制代码

package cn.hdc.oop9.stream.api;

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

public class t2 {
    public static void main(String[] args) {
        List<Double> scores = new ArrayList<>();
        Collections.addAll(scores, 88.5, 100.0, 60.0, 99.0, 9.5, 99.6, 25.0);
        //需求1:找出成绩大于等于60分的数据，并升序后，再输出。
        scores.stream().filter(item -> item >= 60).sorted().forEach(item -> System.out.println(item));
        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, 183.3);
        Student s6 = new Student("牛夫人", 34, 168.5);
        Collections.addAll(students, s1, s2, s3, s4, s5, s6);
        //需求2:找出年龄大于等于23,且年龄小于等于30岁的学生，并按照年龄降序输出
        students.stream().filter(item -> item.getAge() >= 23 && item.getAge() <= 30).sorted((o1, o2) -> Integer.compare(o2.getAge(), o1.getAge())).forEach(item -> System.out.println(item));
        System.out.println();
        //需求3:取出身高最高的前3名学生，并输出。
        students.stream().sorted((o1, o2) -> Double.compare(o2.getHeight(), o1.getHeight())).limit(3).forEach(item -> System.out.println(item));
        System.out.println();
        //需求4:取出身高倒数的2名学生，并输出。
        students.stream().sorted((o1, o2) -> Double.compare(o1.getHeight(), o2.getHeight())).limit(2).forEach(item -> System.out.println(item));
        System.out.println();
        //需求5: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(item -> item.getHeight() > 168).map(item -> item.getName()).distinct().forEach(item -> System.out.println(item));
        System.out.println();
        //需求6:流合并
        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);
    }
}

class Student {
    private String name;
    private int age;
    private double height;

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

    public Student() {
    }

    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;
    }
}

3.3 Stream终结方法

3.3.1 什么是终结方法

3.3.2 常见的终结方法

3.3.3 代码试用

java 复制代码

package cn.hdc.oop9.stream.api;

import java.util.*;

public class t3 {
    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, 183.3);
        Student s6 = new Student("牛夫人", 34, 168.5);
        Collections.addAll(students, s1, s2, s3, s4, s5, s6);
        // 需求1:请计算出身高超过168的学生有几人。
        long count = students.stream().filter(item -> item.getHeight() > 168).count();
        System.out.println(count);
        System.out.println();
        // 需求2:请找出身高最高的学生对象，并输出。
        Student max = students.stream().max((o1, o2) -> Double.compare(o1.getHeight(), o2.getHeight())).get();
        System.out.println(max);
        // 需求3:请找出身高最矮的学生对象，并输出。
        Student min = students.stream().min(Comparator.comparingDouble(Student::getHeight)).get();
        System.out.println(min);
    }
}

3.3.4 收集Stream流

3.3.5 代码试用

java 复制代码

package cn.hdc.oop9.stream.api;

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

public class t3 {
    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, 183.3);
        Student s6 = new Student("牛夫人", 34, 168.5);
        Collections.addAll(students, s1, s2, s3, s4, s5, s6);
        // 需求4:请找出身高超过170的学生对象，并放到一个新集合中去返回。
        List<Student> l1 = students.stream().filter(item -> item.getHeight() > 170).collect(Collectors.toList());
        Set<Student> l2 = students.stream().filter(item -> item.getHeight() > 170).collect(Collectors.toSet());
        System.out.println(l1);
        System.out.println(l2);
        // 需求5:请找出身高超过170的学生对象，并把学生对象的名字和身高，存入到一个Map集合返回
        Map<String, Double> map = students.stream().filter(item -> item.getHeight() > 170).distinct().collect(Collectors.toMap(item -> item.getName(), item -> item.getHeight()));
        System.out.println(map);
        // 需求6:请找出身高超过170的学生对象，并把学生对象存入到一个数组返回
        Object[] arr = students.stream().filter(item -> item.getHeight() > 170).toArray();
        Student[] arr1 = students.stream().filter(item -> item.getHeight() > 170).toArray(len -> new Student[len]);
        System.out.println(Arrays.toString(arr));
        System.out.println(Arrays.toString(arr1));
    }
}