Java进阶13讲__第九讲

Stream流 

1. 案例初体验

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

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

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;@Overridepublic 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

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

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));}
}