Stream
JDK开始新增的一套API（java.util.stream），操作集合或者数组的数据
API（Application Programming Interface，应用程序编程接口）
Stream流大量结合了Lambda的语言风格来编程，提供了一种更加强大，更加简单的
package com.Stream;import java.util.ArrayList;
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);//[张三丰, 张无忌, 周芷若, 赵敏, 张强]//赵找出姓张，且是三个字的名字，存入到一个新集合中去List<String> list = new ArrayList<>();names.forEach(s -> {if(s.startsWith("张")&&s.length()==3){list.add(s);}});System.out.println(list);//开始使用stream流，List<String> list2 = names.stream().filter(s -> s.startsWith("张")).filter(a -> a.length() == 3).collect(Collectors.toList());System.out.println(list2);//filter(s -> s.startsWith("张")).filter(a -> a.length() == 3)可以改成filter(s -> s.startsWith("张")&&s.length() == 3)}
}
Stream使用步骤（流水线）
数据源（集合/数组/...）----->过滤----->排序----->去重----->获取结果（alt+Enter）
package com.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();//names.stream();alt+Enter//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","玛德","德玛西亚");Stream<String> stream1 = set.stream();stream1.filter(x->x.contains("德")).forEach(System.out::println);//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",172.3);map.put("马尔扎哈",172.3);map.put("卡尔扎巴",172.3);//stream()方法是Collections提供的，Map不属于Collections方法，可以先对map集合使用keySet方法，转换为一个set类（Collections类）//map.keySet().stream().forEach(System.out::println);只能处理键属于Set<String> keys = map.keySet();keys.stream().forEach(System.out::println);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(e -> e.getKey().equals("巴")).forEach(System.out::println);//4.如何获取的Stream流String[] names2 = {"张翠山","东方不败","唐大山","孤独求败"};Stream<String> s1 = Arrays.stream(names2);//Arrays.stream(names2)或者names.stream()+alt+EnterStream<String> s2 = Stream.of(names2);//Stream.of(names2)}
}
package com.Stream;import com.CollectionandMap.Student;import java.util.ArrayList;
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,88.5,100.0,60.0,99.0,9.5,99.6,25.0);//需求1：找出成绩大于60的数据，并升序，再输出scores.stream().filter(s->s>=60).sorted().forEach(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",26,172.5);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()>=18 &&s.getAge()<=30).sorted((o1, o2) -> o2.getAge()- o1.getAge())//Lambda表达式调用Comparator比较器方法.forEach(System.out::println);System.out.println("============================");//需求3：取出身高最高的前三名学生，并输出students.stream().sorted((o1, o2) -> Double.compare(o2.getHeight(),o1.getHeight())).limit(3).forEach(System.out::println);//limit返回长度为？的stream流（从前到后数）,取前几个数据System.out.println("============================");//需求4：取出身高倒数的两名学生，并输出students.stream().sorted((o1,o2) ->Double.compare(o2.getHeight(),o1.getHeight())).skip(students.size()-2).forEach(System.out::println);//skip是跳过，跳过前面几个的意思System.out.println("============================");//需求5：找出身高超过168的学生叫什么名字，并取出重复名字再输出students.stream().filter(s -> s.getHeight()>=168).map(Student::getName)//map映射方法，本质是将流中的 Student 对象映射为其 name 属性值.distinct() .forEach(System.out::println);//distinct()去重复//(Student::getName)对Student类调用它的getName方法System.out.println("============================");students.stream().filter(s -> s.getHeight()>=168)//这个需要重写Student类中的equals和hashCode方法.distinct() .forEach(System.out::println);//distinct()去重复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);}
}
package com.Stream;import com.CollectionandMap.Student;import java.util.*;
import java.util.stream.Collectors;public class StreamTest4 {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//students.stream().sorted((o1, o2) -> Double.compare(o2.getHeight(),o1.getHeight())).limit(1).forEach(System.out::println);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);//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//Stream流只能收集一次，collect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集合中返回Map<String, Double> map = students.stream().filter(a -> a.getHeight() >= 170).distinct() .collect(Collectors.toMap(a -> a.getName(), a -> a.getHeight()));System.out.println(map);//Object[] array = students.stream().filter(a -> a.getHeight() >= 170).toArray();//数组中的内容可以重复Student[] array = students.stream().filter(a -> a.getHeight() >= 170).toArray(len->new Student[len]);System.out.println(Arrays.toString(array));}
}