前言

Java8中提供了Stream对集合操作作出了极大的简化，学习了Stream之后，我们以后不用使用for循环就能对集合作出很好的操作。

本文将给大家详细介绍关于java8 Stream使用的相关内容，下面话不多说了，来一起看看详细的介绍吧

1. 原理

Stream 不是集合元素，它不是数据结构并不保存数据，它是有关算法和计算的，它更像一个高级版本的 Iterator。

原始版本的 Iterator，用户只能显式地一个一个遍历元素并对其执行某些操作；

高级版本的 Stream，用户只要给出需要对其包含的元素执行什么操作，比如:

• 所有元素求和
• 过滤掉长度大于 10 的字符串
• 获取每个字符串的首字母

Stream 就如同一个迭代器（Iterator），单向，不可往复，数据只能遍历一次，遍历过一次后即用尽了，就好比流水从面前流过，一去不复返。

而和迭代器又不同的是，Stream 可以并行化操作

Stream 的另外一大特点是，数据源本身可以是无限的

2.使用步骤

获取一个数据源（source）→ 数据转换→执行操作获取想要的结果

每次转换原有 Stream 对象不改变，返回一个新的 Stream对象（可以有多次转换），这就允许对其操作可以像链条一样排列，变成一个管道，如下图所示。

3. Stream的构造

public void test4() { Stream stream = Stream.of("a", "b", "c", 23); stream.forEach(key -> System.out.println(key)); String[] array = new String[]{"abc", "efg"}; stream = Stream.of(array); stream = Arrays.stream(array); stream.forEach(key -> System.out.println(key)); List<String> list = Arrays.asList(array); stream = list.stream(); //IntStream、LongStream、DoubleStream IntStream stream2 = IntStream.of(1, 2, 3, 3); DoubleStream stream4 = DoubleStream.of(1, 2, 3, 3.4); stream2.forEach(key -> System.out.println(key)); stream4.forEach(key -> System.out.println(key)); }

结果

a
b
c
23
abc
efg
1
2
3
3
1.0
2.0
3.0

4. Stream的转换

public void test6() { Stream stream = Stream.of("abc", "def"); String[] array = (String[])stream.toArray(String[]::new); System.out.println(array.length); List<String> list = (List<String>)Stream.of("1", "2", "3").collect(Collectors.toList()); String str = Stream.of("abc", "mn").collect(Collectors.joining()).toString(); System.out.println(array); System.out.println(list); System.out.println(str); }

结果

2

[Ljava.lang.String;@17f052a3
[1, 2, 3]
abcmn

5.一个 Stream 只可以使用一次

public void test6_5() { Stream stream = Stream.of(1, 2, 3, 2); System.out.println("count:" + stream.count()); System.out.println("count:" + stream.count());}

输出

Exception in thread "main" java.lang.IllegalStateException: stream has already been operated upon or closed
at java.util.stream.AbstractPipeline.<init>(AbstractPipeline.java:203)
at java.util.stream.LongPipeline.<init>(LongPipeline.java:91)
at java.util.stream.LongPipeline$StatelessOp.<init>(LongPipeline.java:572)
at java.util.stream.ReferencePipeline$5.<init>(ReferencePipeline.java:221)
at java.util.stream.ReferencePipeline.mapToLong(ReferencePipeline.java:220)
at java.util.stream.ReferencePipeline.count(ReferencePipeline.java:526)
at streamTest.StreamTest.test6_5(StreamTest.java:68)
at streamTest.StreamTest.main(StreamTest.java:181)
count:4

6.转换大写

public void test7() { List<String> list = Arrays.asList("a", "MnM"); List<String> result = list.stream(). map(String::toUpperCase). collect(Collectors.toList()); System.out.println(list); System.out.println(result); }

输出

[a, MnM]
[A, MNM]

7.平方

public void test8() { List<Integer> list2 = Arrays.asList(1, 2, 4); List<Integer> list3 = list2.stream(). map(key -> key * key). collect(Collectors.toList()); System.out.println(list2); System.out.println(list3); }

输出

[1, 2, 4]
[1, 4, 16]

8.找偶数

public void test8_5() { List<Integer> list2 = Arrays.asList(1, 2, 4); List<Integer> list3 = list2.stream(). filter(key -> key % 2 == 0). collect(Collectors.toList()); System.out.println(list2); System.out.println(list3); }

输出

[1, 2, 4]
[2, 4]

9. 区间值

public void test5() { System.out.println("\n"); IntStream.range(1, 3).forEach(System.out::println); System.out.println("\n"); IntStream.rangeClosed(1, 3).forEach(System.out::println); }

结果

1
2


1
2
3

10.并发

public void test5_pa() { IntStream.rangeClosed(1, 10).parallel().forEach(System.out::println); }

输出

3
7
1
5
2
8
10
6
9
4　　

是否并发思考

11. 新的Stream继续操作

public void test6_6() { Stream.of("one", "two", "three", "four") .filter(e -> e.length() > 3) .peek(e -> System.out.println("Filtered value: " + e)) .map(String::toUpperCase) .peek(e -> System.out.println("Mapped value: " + e)) .collect(Collectors.toList()); }

结果

Filtered value: three
Mapped value: THREE
Filtered value: four
Mapped value: FOUR

12. Optional

public static void print(String text) { System.out.println("<<<<<<"); System.out.println(Optional.ofNullable(text)); List<String> obj = new ArrayList<>(); Optional.ofNullable(text).ifPresent(System.out::println); System.out.println(">>>>>>>>>>>>\n"); } public static int getLength(String text) { return Optional.ofNullable(text).map(String::length).orElse(-1); } public void test14() { String strA = " abcd ", strB = null; print(strA); print(""); print(strB); System.out.println(getLength(strA)); System.out.println(getLength("")); System.out.println(getLength(strB)); }

结果

<<<<<<
Optional[ abcd ]
abcd
>>>>>>>>>>>>

<<<<<<
Optional[]

>>>>>>>>>>>>

<<<<<<
Optional.empty
>>>>>>>>>>>>

6
0
-1

13. 字符串拼接、最值、求和、过滤

public void test15() { String concat = Stream.of("A", "B", "C").reduce("", String::concat); System.out.println("concat:" + concat); double minValue = Stream.of(-1.5, 1.0, -3.0, -2.0).reduce(Double.MAX_VALUE, Double::min); System.out.println("min:" + minValue); int sumValue = Stream.of(1, 2, 3, 4).reduce(0, Integer::sum); System.out.println("sum1:" + sumValue); int sumValue2 = Stream.of(1, 2, 3, 4).reduce(Integer::sum).get(); System.out.println("sum2:" + sumValue2); concat = Stream.of("a", "B", "c", "D", "e", "F").filter(x -> x.compareTo("Z") > 0).reduce("", String::concat); System.out.println("concat:" + concat); }

结果

concat:ABC
min:-3.0
sum1:10
sum2:10
concat:ace

14. limit, skip

public void test16() { List<Person> persons = new ArrayList<>(); IntStream.range(1, 1000).forEach(key->persons.add(new Person(key, "jihite:" + key))); List<String> personList = persons.stream().map(Person::getName).limit(10).skip(3).collect(Collectors.toList()); System.out.println(personList); }

输出

[jihite:4, jihite:5, jihite:6, jihite:7, jihite:8, jihite:9, jihite:10]

15.找出最长一行的长度

public void test19() throws IOException { String path = "**/Person.java"; BufferedReader br = new BufferedReader(new FileReader(path)); int longest = br.lines() .mapToInt(String::length) .max() .getAsInt(); br.close(); System.out.println(longest); }

输出

16.找出全文的单词，转小写，并排序

public void test20() throws IOException { String path = "**/Person.java"; BufferedReader br = new BufferedReader(new FileReader(path)); List<String> words = br.lines() .flatMap(line->Stream.of(line.split(" "))) .filter(word->word.length()>0) .map(String::toLowerCase) .distinct() .sorted() .collect(Collectors.toList()); br.close(); System.out.println(words); words.forEach(key-> System.out.println(key)); }

输出

*
*/
/**
//
2018/10/24
21:40
=
@author:
@date:
@description:
class
getname()
int
name)

参考

Java 8 中的 Streams API 详解

总结

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