Stream流

Stream流概述

Stream流能够简化数组和集合操作

举一个案例：体验一下Stream流

package com.key.d17_packeg_01.Steam;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class test {
    /**
     * 体验Steam流编程，简化集合和数组
     * @param args
     */
    public static void main(String[] args) {
        List<String> namelist = new ArrayList<>();
        Collections.addAll(namelist,"张无忌","张三丰","周芷若","灭绝师太","张强","谢逊");
        System.out.println(namelist);
        //1、从集合中找出姓张的，放到新的集合中
        List<String> zhanglist = new ArrayList<>();
        for (String name : namelist) {
            //System.out.println(name);
            if (name.startsWith("张")){
                zhanglist.add(name);
            }
        }
        System.out.println(zhanglist);
        //2、从集合中找出名字为3个字的，放到新集合中
        List<String> countlist = new ArrayList<>();
        for (String countname : namelist) {
            if (countname.length() == 3){
                countlist.add(countname);
            }
        }
        System.out.println(countlist);
        /**
         * 使用Steam流实现上面的代码
         */
        /*
        namelist.stream().filter(s -> s.startsWith("张")).filter(s -> s.length() == 3);
        Steam流可以支持链式编程，可以通过filter拼接过滤条件
        filter()就是过滤函数，在filter()中加上条件即可
         */
        namelist.stream().filter(s -> s.startsWith("张") && s.length() == 3).forEach(s -> System.out.println(s));
    }
}

Stream流的核心思想

Stream流的三类方法

Stream流的获取

package com.key.d17_packeg_01.Steam;
import java.util.*;
import java.util.stream.Stream;
public class GetSteam {
    public static void main(String[] args) {
        /*
        Collection集合获取Steam流
         */
        Collection<String> list = new ArrayList<>();
        Stream<String> s = list.stream();
        /*
        Map集合获取Steam流
         */
        Map<String,Integer> list1 = new HashMap<>();
        //Map集合调用Stream流时，需要将键和值分开取Stream
        //取键流
        Stream<String> key = list1.keySet().stream();
        //取值流
        Stream<Integer> value = list1.values().stream();
        //取键值对流
        Stream<Map.Entry<String,Integer>> mapStream = list1.entrySet().stream();
        /*
        数组取Stream流
         */
        String[] arr = {"张三","李四","王五"};
        Stream<String> name = Arrays.stream(arr);
        Stream<String> name1 = Stream.of(arr);
    }
}

Steam流的常用API

package com.key.d17_packeg_01.Steam;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.stream.Stream;
public class StreamAPI {
    public static void main(String[] args) {
        List<String> name = new ArrayList<>();
        Collections.addAll(name,"张艺兴","周琼洁","宋轶","郭麒麟","于谦");
        System.out.println(name);
        //取前两个名字为三个字的明星
        name.stream().filter(s -> s.length() == 3).limit(2).forEach(s -> System.out.println(s));
        //另一种写法，替换s -> System.out.println(s)；前提：入参一样，这里都传入了s
        // name.stream().filter(s -> s.length() ==  3).limit(2).forEach(System.out::println);
        //跳过前两个名字为三个数字的明星，输出剩下的名字为三个字的明星
        System.out.println("----------");
        name.stream().filter(s -> s.length() == 3).skip(2).forEach(s -> System.out.println(s));
        System.out.println("----------");
        /*
        map()加工方法：给每个元素前加上固定值
        这里map用了重写方法
         */
        name.stream().map(s -> "唱歌的" + s).forEach(s -> System.out.println(s));
        System.out.println("----------");
        //合并流
        Stream<String> s1 = name.stream().filter(s -> s.startsWith("周"));
        Stream<String> s2 = Stream.of("java1","java2");
        Stream<String> s3 = Stream.concat(s1,s2);   //合并s1和s2两个Stream流
        s3.forEach(s -> System.out.println(s));
    }
}

Stream流的综合应用案例

收集Stream流

package com.key.d17_packeg_01.Steam;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class StreamDemo {
    public static void main(String[] args) {
        List<String> namelist = new ArrayList<>();
        Collections.addAll(namelist,"张艺兴","周琼洁","宋轶","郭麒麟","于谦","周冬雨","张鹤伦");
        Stream<String> zhouname = namelist.stream().filter(s -> s.startsWith("周"));   //提取namelist中姓周的明星
        //把姓周的明星收集到集合中去
        List<String> zhoulist = zhouname.collect(Collectors.toList());
        System.out.println(zhoulist);
    }
}