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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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测试数据pareto( "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
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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
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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
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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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原文链接：http://HdhCmsTestcnblogs测试数据/kaituorensheng/p/9852462.html
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