java8 Stream使用案例
阿新 • • 發佈：2018-12-17
1. 原理
Stream 不是集合元素，它不是資料結構並不儲存資料，它是有關演算法和計算的，它更像一個高階版本的 Iterator。
原始版本的 Iterator，使用者只能顯式地一個一個遍歷元素並對其執行某些操作；
高階版本的 Stream，使用者只要給出需要對其包含的元素執行什麼操作，比如:
- 所有元素求和
- 過濾掉長度大於 10 的字串
- 獲取每個字串的首字母
Stream 就如同一個迭代器（Iterator），單向，不可往復，資料只能遍歷一次，遍歷過一次後即用盡了，就好比流水從面前流過，一去不復返。
而和迭代器又不同的是，Stream 可以並行化操作
Stream 的另外一大特點是，資料來源本身可以是無限的
2.使用步驟
獲取一個數據源（source）→ 資料轉換→執行操作獲取想要的結果
每次轉換原有 Stream 物件不改變，返回一個新的 Stream物件（可以有多次轉換），這就允許對其操作可以像鏈條一樣排列，變成一個管道，如下圖所示。
3. Stream的構造
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、DoubleStreamIntStream 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)); }
結果
a b c 23 abc efg 1 2 3 3 1.0 2.0 3.0 3.4
4. Stream的轉換
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); }
結果
2 [Ljava.lang.String;@17f052a3 [1, 2, 3] abcmn
5.一個 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
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); }
結果
1 2 1 2 3
10.併發
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
11. 新的Stream繼續操作
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)); }
結果
<<<<<< Optional[ abcd ] abcd >>>>>>>>>>>> <<<<<< Optional[] >>>>>>>>>>>> <<<<<< Optional.empty >>>>>>>>>>>> 6 0 -1
13. 字串拼接、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
40
16.找出全文的單詞，轉小寫，並排序
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
* */ /** // 2018/10/24 21:40 = @author: @date: @description: class getname() int name)