java8中Stream的使用示例教程

更新時(shí)間：2018年10月31日 14:43:01 作者：jihite

Stream是Java8的一大亮點(diǎn),是對容器對象功能的增強(qiáng)，下面這篇文章主要給大家介紹了關(guān)于java8中Stream使用的相關(guān)資料，文中通過示例代碼介紹的非常詳細(xì)，對大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價(jià)值，需要的朋友們下面隨著小編來一起學(xué)習(xí)學(xué)習(xí)吧

前言

Java8中提供了Stream對集合操作作出了極大的簡化，學(xué)習(xí)了Stream之后，我們以后不用使用for循環(huán)就能對集合作出很好的操作。

本文將給大家詳細(xì)介紹關(guān)于java8 Stream使用的相關(guān)內(nèi)容，下面話不多說了，來一起看看詳細(xì)的介紹吧

1. 原理

Stream 不是集合元素，它不是數(shù)據(jù)結(jié)構(gòu)并不保存數(shù)據(jù)，它是有關(guān)算法和計(jì)算的，它更像一個(gè)高級版本的 Iterator。

原始版本的 Iterator，用戶只能顯式地一個(gè)一個(gè)遍歷元素并對其執(zhí)行某些操作；

高級版本的 Stream，用戶只要給出需要對其包含的元素執(zhí)行什么操作，比如:

所有元素求和
過濾掉長度大于 10 的字符串
獲取每個(gè)字符串的首字母

Stream 就如同一個(gè)迭代器（Iterator），單向，不可往復(fù)，數(shù)據(jù)只能遍歷一次，遍歷過一次后即用盡了，就好比流水從面前流過，一去不復(fù)返。

而和迭代器又不同的是，Stream 可以并行化操作

Stream 的另外一大特點(diǎn)是，數(shù)據(jù)源本身可以是無限的

2.使用步驟

獲取一個(gè)數(shù)據(jù)源（source）→ 數(shù)據(jù)轉(zhuǎn)換→執(zhí)行操作獲取想要的結(jié)果

每次轉(zhuǎn)換原有 Stream 對象不改變，返回一個(gè)新的 Stream對象（可以有多次轉(zhuǎn)換），這就允許對其操作可以像鏈條一樣排列，變成一個(gè)管道，如下圖所示。

3. Stream的構(gòu)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));
 }

結(jié)果

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

4. Stream的轉(zhuǎn)換

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);
 }

結(jié)果

2

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

5.一個(gè) 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());
}

輸出

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.轉(zhuǎn)換大寫

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);
 }

輸出

[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);

 }

輸出

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

8.找偶數(shù)

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);
 }

輸出

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

9. 區(qū)間值

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);
 }

結(jié)果

1
2

1
2
3

10.并發(fā)

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

輸出

3
7
1
5
2
8
10
6
9
4　　

是否并發(fā)思考

11. 新的Stream繼續(xù)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());
 }

結(jié)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));
 }

結(jié)果

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

<<<<<<
Optional[]

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

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

6
0
-1

13. 字符串拼接、最值、求和、過濾

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);
 }

結(jié)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);
 }

輸出

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

15.找出最長一行的長度

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);
 }

輸出

16.找出全文的單詞，轉(zhuǎn)小寫，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));
 }

輸出