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：

 1 package cn.itcast.day_15;
 2 
 3 import java.util.ArrayList;
 4 import java.util.List;
 5 
 6 public class DemoList {
 7     public static void main(String[] args) {
 8 
 9         List<String> list = new ArrayList<>();
10         list.add("aa");
11         list.add("bb");
12         list.add("abb");
13         list.add("ccc");
14         list.add("add");
15         for (String s : list){
16             System.out.println(s);
17         }
18 
19         System.out.println("以a开头的字符串：");
20         ArrayList<String> listA = new ArrayList<>();
21         for (String s : list){
22             if (s.startsWith("a")){
23                 listA.add(s);
24             }
25         }
26         for (String s : listA){
27             System.out.println(s);
28         }
29 
30         ArrayList<String> listB = new ArrayList<>();
31         for (String s : listA){
32             if (s.length() == 3){
33                 listB.add(s);
34             }
35         }
36         System.out.println("以a开头且长度为3的字符串");
37         for (String s : listB){
38             System.out.println(s);
39         }
40     }
41 }

（2）使用Stream流

 1 package cn.itcast.day_15;
 2 
 3 import java.util.ArrayList;
 4 import java.util.List;
 5 
 6 public class Streamtest {
 7     public static void main(String[] args) {
 8         List<String> list = new ArrayList<>();
 9         list.add("aa");
10         list.add("bb");
11         list.add("abb");
12         list.add("ccc");
13         list.add("add");
14 
15         list.stream()
16                 .filter((String name)->{
17                     return name.startsWith("a");
18                 }).filter((String name)->{
19                     return  name.length() == 3;
20         }).forEach((String name)->{
21             System.out.println(name);
22         });
23     }
24 }

2. Stream流中的foreach方法:

例2：打印数组中的字符串

 1 package cn.itcast.day_15;
 2 
 3 import java.util.stream.Stream;
 4 
 5 public class StreamTest01 {
 6     public static void main(String[] args) {
 7         Stream<String> stream = Stream.of("张三", "李四", "王五");
 8         stream.forEach((String name)->{
 9             System.out.println(name);
10         });
11     }
12 }

例3：filter和foreach方法结合

 1 package cn.itcast.day_15;
 2 
 3 import java.util.stream.Stream;
 4 
 5 public class StreamTest02 {
 6     public static void main(String[] args) {
 7 
 8         Stream<String> stream = Stream.of("aa", "bbb", "cccc", "afdd");
 9         stream.filter((String name)->{
10             return name.startsWith("a");
11         }).forEach((String name)->{
12             System.out.println(name);
13         });
14     }
15 }

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

 1 package cn.itcast.day_15;
 2 
 3 import java.util.stream.Stream;
 4 
 5 public class StreamTest03 {
 6     public static void main(String[] args) {
 7         //获取一个stream类型的stream流
 8         Stream<String> s = Stream.of("1","2","3","4");
 9         //将String类型的整数映射为Integer类型的整数
10         Stream<Integer> is = s.map((String s1) -> {
11             return Integer.parseInt(s1);
12         });
13 
14         //遍历Stream流中的数据
15         is.forEach((Integer integer)->{
16             System.out.println(integer);
17         });
18     }
19 }

4. Stream流中的count方法

例4：统计字符串中数字个数

 1 package cn.itcast.day_15;
 2 
 3 import java.util.ArrayList;
 4 import java.util.stream.Stream;
 5 
 6 public class StreamTest04 {
 7     public static void main(String[] args) {
 8         ArrayList<Integer> list = new ArrayList<>();
 9         list.add(1);
10         list.add(2);
11         list.add(3);
12         list.add(4);
13         list.add(5);
14         list.add(6);
15         Stream<Integer> stream = list.stream();
16         long count = stream.count();
17         System.out.println(count);
18     }
19 }

5. Stream流中的limit方法

例5：截取字符串中前三个元素

 1 package cn.itcast.day_15;
 2 
 3 import java.util.stream.Stream;
 4 
 5 public class Streamtest05 {
 6     public static void main(String[] args) {
 7         String[] arr = {"aaa","bbb","cc","dfg","ad"};
 8         Stream<String> stream = Stream.of(arr);
 9         Stream<String> limit = stream.limit(3);
10         limit.forEach((String name)->{
11             System.out.println(name);
12         });
13     }
14 }

6. Stream流中的skip方法

例6：跳过前3个字符

 1 package cn.itcast.day_15;
 2 
 3 import java.util.stream.Stream;
 4 
 5 public class DemoTest06 {
 6     public static void main(String[] args) {
 7         String[] arr = {"aaa","bbb","cc","dfg","ad"};
 8         Stream<String> stream = Stream.of(arr);
 9         Stream<String> stream1 = stream.skip(3);
10         stream1.forEach((String name)->{
11             System.out.println(name);
12         });
13     }
14 }

7. Stream流中的concat方法

例7：将两个流对象合成一个流

 1 package cn.itcast.day_15;
 2 
 3 import java.util.stream.Stream;
 4 
 5 public class StreamTest06 {
 6     public static void main(String[] args) {
 7         String[] arr1 = {"aaa","bbb","cc","dfg","ad"};
 8         String[] arr2 = {"bbb","ccc","ddd","asdf"};
 9         Stream<String> stream1 = Stream.of(arr1);
10         Stream<String> stream2 = Stream.of(arr2);
11         Stream<String> concat = Stream.concat(stream1, stream2);
12         concat.forEach((String name)->{
13             System.out.println(name);
14         });
15     }
16 }