Stream流

Stream流

关注做什么，而不是怎么做。

例1：对集合中的数据进行筛选：

1. Stream流中的filter方法：

（1）传统方式：

package cn.itcast.day_15;

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

public class DemoList {
    public static void main(String[] args) {

        List<String> list = new ArrayList<>();
        list.add("aa");
        list.add("bb");
        list.add("abb");
        list.add("ccc");
        list.add("add");
        for (String s : list){
            System.out.println(s);
        }

        System.out.println("以a开头的字符串：");
        ArrayList<String> listA = new ArrayList<>();
        for (String s : list){
            if (s.startsWith("a")){
                listA.add(s);
            }
        }
        for (String s : listA){
            System.out.println(s);
        }

        ArrayList<String> listB = new ArrayList<>();
        for (String s : listA){
            if (s.length() == 3){
                listB.add(s);
            }
        }
        System.out.println("以a开头且长度为3的字符串");
        for (String s : listB){
            System.out.println(s);
        }
    }
}

（2）使用Stream流

package cn.itcast.day_15;

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

public class Streamtest {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("aa");
        list.add("bb");
        list.add("abb");
        list.add("ccc");
        list.add("add");

        list.stream()
                .filter((String name)->{
                    return name.startsWith("a");
                }).filter((String name)->{
                    return  name.length() == 3;
        }).forEach((String name)->{
            System.out.println(name);
        });
    }
}

2. Stream流中的foreach方法:

例2：打印数组中的字符串

package cn.itcast.day_15;

import java.util.stream.Stream;

public class StreamTest01 {
    public static void main(String[] args) {
        Stream<String> stream = Stream.of("张三", "李四", "王五");
        stream.forEach((String name)->{
            System.out.println(name);
        });
    }
}

例3：filter和foreach方法结合

package cn.itcast.day_15;

import java.util.stream.Stream;

public class StreamTest02 {
    public static void main(String[] args) {

        Stream<String> stream = Stream.of("aa", "bbb", "cccc", "afdd");
        stream.filter((String name)->{
            return name.startsWith("a");
        }).forEach((String name)->{
            System.out.println(name);
        });
    }
}

Stream流属于管道流，只能被消费一次。

当第一个stream流消费完，数据就会转到下一个stream流。

3. 如果要将流中的元素映射到另一个流中，可以使用map方法。

例3：将String类型的字符串映射为Integer类型

package cn.itcast.day_15;

import java.util.stream.Stream;

public class StreamTest03 {
    public static void main(String[] args) {
        //获取一个stream类型的stream流
        Stream<String> s = Stream.of("1","2","3","4");
        //将String类型的整数映射为Integer类型的整数
        Stream<Integer> is = s.map((String s1) -> {
            return Integer.parseInt(s1);
        });

        //遍历Stream流中的数据
        is.forEach((Integer integer)->{
            System.out.println(integer);
        });
    }
}

4. Stream流中的count方法

例4：统计字符串中数字个数

package cn.itcast.day_15;

import java.util.ArrayList;
import java.util.stream.Stream;

public class StreamTest04 {
    public static void main(String[] args) {
        ArrayList<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(2);
        list.add(3);
        list.add(4);
        list.add(5);
        list.add(6);
        Stream<Integer> stream = list.stream();
        long count = stream.count();
        System.out.println(count);
    }
}

 5. Stream流中的limit方法

例5：截取字符串中前三个元素

package cn.itcast.day_15;

import java.util.stream.Stream;

public class Streamtest05 {
    public static void main(String[] args) {
        String[] arr = {"aaa","bbb","cc","dfg","ad"};
        Stream<String> stream = Stream.of(arr);
        Stream<String> limit = stream.limit(3);
        limit.forEach((String name)->{
            System.out.println(name);
        });
    }
}

6. Stream流中的skip方法

例6：跳过前3个字符

package cn.itcast.day_15;

import java.util.stream.Stream;

public class DemoTest06 {
    public static void main(String[] args) {
        String[] arr = {"aaa","bbb","cc","dfg","ad"};
        Stream<String> stream = Stream.of(arr);
        Stream<String> stream1 = stream.skip(3);
        stream1.forEach((String name)->{
            System.out.println(name);
        });
    }
}

7. Stream流中的concat方法

例7：将两个流对象合成一个流

package cn.itcast.day_15;

import java.util.stream.Stream;

public class StreamTest06 {
    public static void main(String[] args) {
        String[] arr1 = {"aaa","bbb","cc","dfg","ad"};
        String[] arr2 = {"bbb","ccc","ddd","asdf"};
        Stream<String> stream1 = Stream.of(arr1);
        Stream<String> stream2 = Stream.of(arr2);
        Stream<String> concat = Stream.concat(stream1, stream2);
        concat.forEach((String name)->{
            System.out.println(name);
        });
    }
}