Java开发从入门到精通（二十）：Java的面向对象编程OOP：Stream流

Java大数据开发和安全开发

（一）Java的新特性：Stream流
- [1.1 什么是Stream?](#1.1 什么是Stream?)
- [1.2 Stream流的使用步骤](#1.2 Stream流的使用步骤)
- [1.3 获取Stream流](#1.3 获取Stream流)
- [1.4 Stream流常见的中间方法](#1.4 Stream流常见的中间方法)
- [1.5 Stream流常见的终结方法](#1.5 Stream流常见的终结方法)

（一）Java的新特性：Stream流

1.1 什么是Stream?

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

Stream流的优势:

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。

体验Stream流
需求:
把下面集合中所有以"张"开头，且是3个字的元素存储到一个新的集合。

java 复制代码

import java.util.ArrayList;
import java.util.List;

public class StreamTest {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("张无忌");
        list.add("周芷若");
        list.add("赵敏");
        list.add("张强");
        list.add("张三丰");
    }
}

使用集合和数组的API

java 复制代码

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

public class StreamTest {
    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);
        // names=[张三丰，张无忌，周芷若，赵敏，张强】
        //          name
        // 找出姓张，且是3个字的名字，存入到一个新集合中去。
        List<String> list =new ArrayList<>();
        for(String name :names) {
            if (name.startsWith("张") && name.length() == 3) {
                list.add(name);
            }
        }
            System.out.println(list);
    }
}

开始使用Stream流来解决这个需求

java 复制代码

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

public class StreamTest {
    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);
        //开始使用Stream流来解决这个需求。
        List<String> list2 = names.stream().filter(s -> s.startsWith("张")).filter(a ->a.length()==3).collect(Collectors.toList());
        System.out.println(list2);
    }
}

1.2 Stream流的使用步骤

1.3 获取Stream流

获取集合的Stream流

获取数组的Stream流

各种集合和数组获取Stream流的代码示例

java 复制代码

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

public class StreamTest {
    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(); //获取stream流

        // 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","蜘蛛精","马德","德玛西亚");
        Stream<String>stream1=set.stream();  //获取stream流
        stream1.filter(s ->s.contains("德")).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",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);

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

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

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

        // 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"};
        Stream<String>sl=Arrays.stream(names2); //获取stream流
        Stream<String>s2=Stream.of(names2); //获取stream流

    }
}

1.4 Stream流常见的中间方法

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)。

常见的中间方法的代码示例

java 复制代码

package com.qianxin.jihekuangjia;

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

public class StreamTest {
    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(s-> s>=60).sorted().forEach(s ->System.out.println(s));

        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(s ->s.getAge()>= 23 && s.getAge()<= 30)
                .sorted((o1,o2)->o2.getAge()-o1.getAge())
                .forEach(s ->System.out.println(s));
        // 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(s ->System.out.println(s));//不同写法1

        students.stream().sorted((o1,o2)-> Double.compare(o2.getHeight(), o1.getHeight()))
                .limit(3).forEach(System.out::println);//不同写法2
        // 需求4:取出身高倒数的2名学生，并输出。
        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);

        // 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,equal
        students.stream().filter(s->s.getHeight()> 168).distinct().forEach(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<String> allSt=Stream.concat(st1,st2);
        allSt.forEach(System.out::println);
    }
}

java 复制代码

package com.qianxin.jihekuangjia;

import java.util.Objects;

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

    @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(height, student.height) == 0 && Objects.equals(name, student.name);
    }

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

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

1.5 Stream流常见的终结方法

终结方法指的是调用完成后，不会返回新Stream了，没法继续使用流了
常见的终结方法的代码示例

java 复制代码

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

public class StreamTest {
    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 size = students.stream().filter(s ->s.getHeight()> 168).count();
        System.out.println(size);

        // 需求2:请找出身高最高的学生对象，并输出。
        Student s = students.stream().max((o1, o2)-> Double.compare(o1.getHeight(), o2.getHeight())).get();
        System.out.println(s);

        // 需求3:请找出身高最矮的学生对象，并输出。
        Student ss = students.stream().min((o1,o2)-> Double.compare(o1.getHeight(), o2.getHeight())).get();
        System.out.println(ss);    
    }
}

收集Stream流的方法

收集Stream流:就是把Stream流操作后的结果转回到集合或者数组中去返回

Stream流:方便操作集合/数组的手段; 集合/数组:才是开发中的目的

java 复制代码

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

public class StreamTest {
    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，
        // 流只能收集一次。
        List<Student> students1 = students.stream().filter(a -> a.getHeight()> 170).collect(Collectors.toList());
        System.out.println(students1);

        Set<Student> students2 = students.stream().filter(a -> a.getHeight()> 170).collect(Collectors.toSet());
        System.out.println(students2);
        // 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(a->a.getHeight()>170)
                        .distinct().collect(Collectors.toMap(a ->a.getName(),a-> a.getHeight()));
                        
		Object[] arr = students.stream().filter(a -> a.getHeight()> 170).toArray();
        System.out.println(Arrays.toString(arr));

        Student[] arr1 =students.stream().filter(a ->a.getHeight()>170).toArray(len -> new Student[len]);
        System.out.println(Arrays.toString(arr1));
    }
}