前言：流写法也是Java8的新特性，通常跟lambda表达式一起使用，效果非常好，可以参考上一篇文章对lambda表达式的常用场景学习。Java8新特性-Lambda表达式_junewgl的博客-CSDN博客

1.导包

使用Stream方法，通常导包为import java.util.stream.Stream;(可以使用import java.util.stream.*;代替)

2.常用的方法举例

2.1过滤filter()--常用

假如需要对list中的数据过滤，保留字符串不等于“abc”并且不能是空字符串，返回结果是Stream类型。

核心代码：

        List<String>list = Arrays.asList("abc", "def", "gkh", "abc", "");
        Stream<String>stringStream = list.stream().filter(s -> !s.equals("abc") && !s.isEmpty());
        stringStream.forEach(s -> System.out.println(s));

测试结果

如果要把返回的Strem类型改为List类型，使用collect(Collectors.toList());(需要导入import java.util.stream.Collectors;)

List<String>stringStream = list.stream().filter(s -> !s.equals("abc") && !s.isEmpty()).collect(Collectors.toList());

2.2计数count()

        // 计算流的个数
        long count = stringStream.count();
        System.out.println(count);

2.3Stream转化为数组toArray()

        List<String>list = Arrays.asList("abc", "def", "gkh", "abc", "");
        String[]stringArrays = stringStream.toArray(String[] :: new);
        for(String s : stringArrays)System.out.println(s);

2.4Stream转化为列表collect(Collectors.toList())

        List<String>list = Arrays.asList("abc", "def", "gkh", "abc", "");
        List<String>stringStream = list.stream().filter(s -> !s.equals("abc") && !s.isEmpty()).collect(Collectors.toList());

2.5Stream转化为Setcollect(Collectors.toSet())

        List<String>list = Arrays.asList("abc", "def", "gkh", "abc", "");
        Stream<String>stringStream = list.stream().filter(s -> !s.equals("abc") && !s.isEmpty());
        Set<String>set = stringStream.collect(Collectors.toSet());
        set.forEach(System.out::println);

2.6限制输出一个：limit(1)

        List<String>list = Arrays.asList("abc", "def", "gkh", "abc", "");
        Stream<String>stringStream = list.stream().filter(s -> !s.equals("abc") && !s.isEmpty());
        Stream<String>streams = stringStream.limit(1);
        streams.forEach(System.out::println);

2.7排序sorted()--常用

        List<String>list = Arrays.asList("abc", "def", "gkh", "abc", "","dee");
        Stream<String>stringStream = list.stream().filter(s -> !s.equals("abc") && !s.isEmpty());
        Stream<String>streams = stringStream.sorted();
        streams.forEach(System.out::println);

2.8去重distinct()--常用

        List<String>list = Arrays.asList("abc", "def", "gkh", "abc", "","dee");
        Stream<String>stringStream = list.stream().filter(s -> !s.isEmpty());
        Stream<String>streams = stringStream.distinct();
        streams.forEach(System.out::println);

2.9合并多个流方法，比如去重和排序

        List<String>list = Arrays.asList("abc", "def", "gkh", "abc", "","dee");
        Stream<String>stringStream = list.stream().filter(s -> !s.isEmpty());
        Stream<String>streams = stringStream.sorted().distinct();
        streams.forEach(System.out::println);

注意对比这里的结果和2.8结果的区别。

3.注意-一个流只能操作一次

参考JavaGuid相关内容，很强！！！

  //注意 一个Stream只能操作一次，不能断开，否则会报错。
    Stream stream = strings.stream();
    //第一次使用
    stream.limit(2);
    //第二次使用
    stream.forEach(System.out::println);

 第二次使用会报错

Exception in thread "main" java.lang.IllegalStateException: stream has already been operated upon or closed

 4.附上完整测试代码

import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.Stream;
/*
test for java
*/

public class Main{
    public static void main(String[]args){
        // Stream学习 
        List<String>list = Arrays.asList("abc", "def", "gkh", "abc", "","dee");
        Stream<String>stringStream = list.stream().filter(s -> !s.isEmpty());
        Stream<String>streams = stringStream.distinct();
        streams.forEach(System.out::println);

        // Set<String>set = stringStream.collect(Collectors.toSet());
        // set.forEach(System.out::println);
        // List<String>list = Arrays.asList("abc", "def", "gkh", "abc", "");
        // List<String>stringStream = list.stream().filter(s -> !s.equals("abc") && !s.isEmpty()).collect(Collectors.toList());
        // stringStream.forEach(s -> System.out.println(s));

        // List<String>list = Arrays.asList("abc", "def", "gkh", "abc", "");
        
        // String[]stringArrays = stringStream.toArray(String[] :: new);
        // for(String s : stringArrays)System.out.println(s);

        // Stream<String>stringStream = list.stream().filter(s -> !s.equals("abc") && !s.isEmpty());
        
        
        // 计算流的个数
        // long count = stringStream.count();
        // System.out.println(count);

        // String[]stringArrays = stringStream.toArray(String[] :: new);
        // for(String s : stringArrays)System.out.println(s);
        // stringArrays.forEach(s -> System.out.println(s));
    }
}