java8中Stream的使用示例教程

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　　

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

以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，如果有疑问大家可以留言交流，谢谢大家对的支持。