Java8中Stream详解

Java8提供了Stream（流）处理集合的关键抽象概念，它可以对集合进行的操作，可以执行非常复杂的查找、过滤和映射数据等操作。Stream API 借助于同样新出现的Lambda表达式，极大的提高编程效率和程序可读性。

1.Java Stream vs Collection

我们列出流相比于Collection的不同的特征：

不是数据结构
需要用lambdas表达式
不能用下标访问
可以容易的被聚合为数组或者列表
懒访问支持
可并行

英文表述如下：

Not a data structure
Designed for lambdas
Do not support indexed access
Can easily be aggregated as arrays or lists
Lazy access supported
Parallelizable

2.创建流

2.1 Stream.of()

Stream<Integer> stream = Stream.of(1,2,3,4,5,6,7,8,9);
stream.forEach(p -> System.out.println(p));

2.2. Stream.of(array)

 Stream<Integer> stream = Stream.of( new Integer[]{1,2,3,4,5,6,7,8,9} );
 stream.forEach(p -> System.out.println(p));

2.3. List.stream()

         List<Integer> list = new ArrayList<Integer>();
         for(int i = 1; i< 10; i++){
             list.add(i);
         }
         Stream<Integer> stream = list.stream();
         stream.forEach(p -> System.out.println(p));

2.4. Stream.generate() or Stream.iterate()

         Stream<Integer> randomNumbers = Stream
             .generate(() -> (new Random()).nextInt(100));

         randomNumbers.limit(20)
              .forEach(System.out::println);

2.5. Stream of String chars or tokens

        IntStream stream = "12345_abcdefg".chars();
        stream.forEach(p -> System.out.println(p));
        
    //OR
        
        Stream<String> stream = Stream.of("A$B$C".split("\\$"));
        stream.forEach(p -> System.out.println(p));

3. Stream Collectors

在执行过中间操作之后，我们会用stream的Collector方法收集处理的元素到集合中。

3.1 收集流元素到列表(List)

         List<Integer> list = new ArrayList<Integer>();
         for(int i = 1; i< 10; i++){
             list.add(i);
         }
         Stream<Integer> stream = list.stream();
         List<Integer> evenNumbersList = stream.filter(i -> i%2 == 0)
                                                .collect(Collectors.toList());
         System.out.print(evenNumbersList);

3.2 收集流元素到数组(Array)

Stream<Integer> stream = list.stream();
         Integer[] evenNumbersArr = stream.filter(i -> i%2 == 0).toArray(Integer[]::new);

你也可以收集流到Set, Map 等中。

4.流操作

先准备一个列表数据

List<String> memberNames = new ArrayList<>();
memberNames.add("Amitabh");
memberNames.add("Shekhar");
memberNames.add("Aman");
memberNames.add("Rahul");
memberNames.add("Shahrukh");
memberNames.add("Salman");
memberNames.add("Yana");
memberNames.add("Lokesh");

4.1 中间操作

中间操作返回一个流，所以我们可以在一行关联多个方法。

4.1.1. Stream.filter()

memberNames.stream().filter((s) -> s.startsWith("A"))
                    .forEach(System.out::println);

输出；

Amitabh
Aman

4.1.2. Stream.map()

map中间操作通过给出函数转换流中的每个元素为另一个对象

 memberNames.stream().filter((s) -> s.startsWith("A"))
                     .map(String::toUpperCase)
                     .forEach(System.out::println);

4.2. 终端操作

4.2.1. Stream.forEach()

memberNames.forEach(System.out::println);

4.2.2. Stream.collect()

List<String> memNamesInUppercase = memberNames.stream().sorted()
                            .map(String::toUpperCase)
                            .collect(Collectors.toList());
        
System.out.print(memNamesInUppercase);

4.2.3. Stream.match()

所有的matching操作都是终端从挨揍，并且返回boolean 结果

boolean matchedResult = memberNames.stream()
        .anyMatch((s) -> s.startsWith("A"));
System.out.println(matchedResult);

matchedResult = memberNames.stream()
        .allMatch((s) -> s.startsWith("A"));
System.out.println(matchedResult);

matchedResult = memberNames.stream()
        .noneMatch((s) -> s.startsWith("A"));
System.out.println(matchedResult);

4.2.4. Stream.count()

long totalMatched = memberNames.stream()
    .filter((s) -> s.startsWith("A"))
    .count();

4.2.5. Stream.reduce()

reduce()方法根据给出的函数执行流上元素的减少操作，结果是Optional接收这个reduced value。

we are reducing all the strings by concatenating them using a separator #.

Optional<String> reduced = memberNames.stream()
        .reduce((s1,s2) -> s1 + "#" + s2);

reduced.ifPresent(System.out::println);

输出：

Amitabh#Shekhar#Aman#Rahul#Shahrukh#Salman#Yana#Lokesh

5. Stream 流中的短路操作

5.1. Stream.anyMatch()

boolean matched = memberNames.stream()
        .anyMatch((s) -> s.startsWith("A"));

5.2. Stream.findFirst()

String firstMatchedName = memberNames.stream()
            .filter((s) -> s.startsWith("L"))
            .findFirst()
                        .get();

6.Java Steam中的并发操作

List<Integer> list = new ArrayList<Integer>();
         for(int i = 1; i< 10; i++){
             list.add(i);
         }
         
        //Here creating a parallel stream
         Stream<Integer> stream = list.parallelStream();    

         Integer[] evenNumbersArr = stream.filter(i -> i%2 == 0).toArray(Integer[]::new);
         System.out.print(evenNumbersArr);

参考网址：

https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html

https://blog.csdn.net/pan_junbiao/article/details/105913518

https://howtodoinjava.com/java8/java-streams-by-examples/