在lambdas中,能夠看到lambda表達式讓代碼更加簡潔、乾淨、容易理解,並容許不須要建立一個類就能夠達到目的。lambdas很好的幫助開發人員更好的表達片斷代碼的意思,Stream對於集合提供一個抽象計算和Fluent接口更能讓程序猿變得爽歪歪。java
###1. 構建stream 初始化stream的幾個方法編程
//1. Stream generate 生成無限個無序的stream Stream<String> stream = Stream.generate(() -> UUID.randomUUID().toString()); // 2. of 構造stream Stream<Integer> integerStream = Stream.of(1, 2, 4, 6, 3, 9, 7); //3.集合構造出stream Stream<List<Integer>> inputStream = Stream.of( Arrays.asList(1), Arrays.asList(2, 3), Arrays.asList(4, 5, 6) );
###2. 能夠怎樣用streamapi
/** * jdk8 stream 用法 * Created by kaishui on 2016/9/3. */ public class JDK_07StreamTest { static class User { private String name; private String password; public User(String name, String password) { this.name = name; this.password = password; } //省略getter and setter } public static void main(String[] args) { List<User> userList = new ArrayList<User>(); userList.add(new User("name1", "passowrd1")); userList.add(new User("name4321", "passowrd4")); userList.add(new User("name321", "passowrd3")); userList.add(new User("kaishui", "kaishuiPassword")); userList.add(new User("name21", "passowrd2")); //1. 在jdk7以前的代碼中,也許咱們常常會這樣寫 List<User> otherUsers = new ArrayList<>(); for (User u : userList) { //獲取含有 "name"的User if (null != u.getName() && u.getName().indexOf("name") >= 0) { otherUsers.add(u); } } //按照名字長度排序 Collections.sort(otherUsers, new Comparator<User>() { @Override public int compare(User u1, User u2) { return u1.getName().length() - u2.getName().length(); } }); //打印 for (User u : otherUsers) { System.out.println(u.getName()); } System.out.println("-------我是一條分割線1---------"); System.out.println("-------jdk8 示例---------"); // 2. jdk8 使用stream操做集合 List<String> nameList = userList.stream() .filter(u -> null != u.getName() && u.getName().indexOf("name") >= 0) //過濾結果 .sorted((u1, u2) -> u1.getName().length() - u2.getName().length()) //排序 .map(u -> u.getName()) //提取 .collect(Collectors.toList()); //把上述步驟後的結果轉換成list //打印 nameList.forEach(System.out::println); } }
執行結果:app
name1 name21 name321 name4321 -------我是一條分割線1--------- -------jdk8--------- name1 name21 name321 name4321
上面的例子,大概瞭解stream的用法,下面分點介紹一下: JDK8 collection中已經實現了一個default的stream方法dom
1. stream() 在集合collection中新建一個stream的管道,有點相似Linux命令中 grep or awk的用法。 2. filter(Predicate) 過濾條件, 看到Predicate應該能夠想到就是過濾數據。 3. sorted(Comparator) 根據Comparator排序,英語國家的就是爽,寫接口均可以省註釋了. 4. map(Function) 提取計算,根據Funtion的做用,咱們也知道,<T,R>輸入一個T,返回一個R。 5. collect(Collectors.toList()) 把上述操做stream的結果轉爲另一個集合。
若是不瞭解predicate、function的用法,能夠系列篇之函數式接口編程ide
###3. stream 懶加載函數
// 1. of 構造 Stream<Integer> Stream<Integer> integerStream = Stream.of(1, 2, 4, 6, 3, 9, 7).map(x -> x / 0); System.out.println("-------我是一條分割線--------");
執行結果:ui
-------我是一條分割線--------
上述看到,map(x -> x/0)並無執行,若是換成下面代碼:this
// 1. of 構造 Stream<Integer> Stream<Integer> integerStream = Stream.of(1, 2, 4, 6, 3, 9, 7).map(x -> x / 0); List<Integer> list = integerStream.collect(Collectors.toList()); System.out.println("-------我是一條分割線--------");
執行結果:lua
Exception in thread "main" java.lang.ArithmeticException: / by zero at com.iu.jdk8.JDK_09StreamLazyLoadTest.lambda$main$0(JDK_09StreamLazyLoadTest.java:19) at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193) at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948) at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481) at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499) at com.iu.jdk8.JDK_09StreamLazyLoadTest.main(JDK_09StreamLazyLoadTest.java:20) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:497) at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
stream只有真正使用到的時候纔去真正的執行。
###4. stream 鏈式編程何時纔是結束
forEach、 forEachOrdered、 toArray、 reduce、 collect、 min、 max、 count、 anyMatch、 allMatch、 noneMatch、 findFirst、 findAny、 iterator
當steam趕上這些方法纔算是結束,其餘的方法,例如:
map (mapToInt, flatMap 等)、 filter、 distinct、 sorted、 peek、 limit、 skip、 parallel、 sequential、 unordered
仍是能夠在上一個操做結果上繼續操做,例如:根據條件filter -> sort -> map
List<User> userList = new ArrayList<User>(); for (int i = 0; i < 10; i++) { userList.add(new User("name" + i, "passowrd" + i, i)); } List<String> nameList = userList.stream() .filter(u -> null != u.getName() && u.getName().indexOf("name") >= 0) //過濾結果 .sorted((u1, u2) -> u1.getName().length() - u2.getName().length()) //排序 .map(u -> u.getName()) //提取 .collect(Collectors.toList()); //把上述步驟後的結果轉換成list
###5. stream api 例子
####5.1 distinct 去重
Stream.of(1, 2, 4, 6, 3, 9, 7, 2, 4, 9, 7).distinct().sorted().forEach(System.out::print); //執行結果: 1234679
####5.2 skip 跳過條數 limit 限定結果返回幾條
Stream.of(1, 2, 4, 6, 3, 9, 7, 2, 4, 9, 7).skip(2).limit(3).forEach(System.out::print); //執行結果:463
####5.3 fitler 參數爲:Predicate 過濾知足條件的結果, Count 統計條數
//獲取集合中7的個數 long sum = Stream.of(1, 2, 4, 6, 3, 9, 7, 2, 4, 9, 7).filter(x -> x == 7).count(); System.out.println("sum = " + sum); //執行結果: sum = 2
####5.4 flatMap 使數據扁平化處理 不少情景,咱們都會用到一個類中有List<Object>屬性,怎樣轉化成Strean<Object>呢,這是flatMap就該出手了
public class JDK_08StreamFlatMapTest { static class User { private String name; private String password; private int no; public User(String name, String password, int no) { this.name = name; this.password = password; this.no = no; } //省略getter and setter } //班級 static class Clazz{ List<User> users; //省略getter and setter } public static void main(String[] args) { List<User> girlList = new ArrayList<User>(); List<User> boyList = new ArrayList<User>(); for (int i = 0; i < 10; i++) { girlList.add(new User("girl" + i, "passowrd" + i, i)); boyList.add(new User("boy" + i, "passowrd" + i, i)); } //班級1 Clazz clazz = new Clazz(); //班級2 Clazz clazz2 = new Clazz(); girlList.addAll(boyList); clazz.setUsers(girlList); clazz2.setUsers(girlList); //clazz stream Stream<Clazz> clazzSteam = Stream.of(clazz, clazz2); //clazz stream -> user stream 扁平化 Stream<User> allUserStream = clazzSteam.flatMap(c -> c.getUsers().stream()); System.out.println("兩個班級一共多少人:" + allUserStream.count());; } }
//執行結果:
兩個班級一共多少人:40
####5.5 anyMatch
//是否存在x*x = 81的結果 boolean flag = Stream.of(1, 2, 4, 6, 3, 9, 7, 2, 4, 9, 7).map(x -> x * x).anyMatch(x -> x == 81); System.out.println(flag); //執行結果:true
####5.6 reduce Optional<T> reduce(BinaryOperator<T> accumulator) 這個方法的主要做用是把 Stream 元素組合起來,Optional使用方法,請參考網紅篇:Java函數式開發——優雅的Optional空指針處理
//clazz stream -> user stream Stream<User> allUserStream = clazzSteam.flatMap(c -> c.getUsers().stream()); //串起全部名字 想了解optional用法能夠參考:http://my.oschina.net/chkui/blog/739034 Optional<String> names = allUserStream. map(User::getName).sorted((first, second) -> first.compareTo(second)) .reduce((first, second) -> first + " *** " + second); System.out.println(names.get());
執行結果:
boy0 *** boy0 *** boy1 *** boy1 *** boy2 *** boy2 *** boy3 *** boy3 *** boy4 *** boy4 *** boy5 *** boy5 *** boy6 *** boy6 *** boy7 *** boy7 *** boy8 *** boy8 *** boy9 *** boy9 *** girl0 *** girl0 *** girl1 *** girl1 *** girl2 *** girl2 *** girl3 *** girl3 *** girl4 *** girl4 *** girl5 *** girl5 *** girl6 *** girl6 *** girl7 *** girl7 *** girl8 *** girl8 *** girl9 *** girl9
####5.7 Parallel 並行運行 vs 普通stream操做
long startTime = System.nanoTime(); Map<String, List<Integer>> numbersPerThread = IntStream.rangeClosed(1, 100000) .parallel() .boxed() .collect(Collectors.groupingBy(i -> Thread.currentThread().getName())); long runTime = System.nanoTime() - startTime; System.out.println("運行時間:" + String.valueOf(runTime)); //運行時間:159695489
long startTime = System.nanoTime(); Map<String, List<Integer>> numbersPerThread = IntStream.rangeClosed(1, 100000) .boxed() .collect(Collectors.groupingBy(i -> Thread.currentThread().getName())); long runTime = System.nanoTime() - startTime; System.out.println("運行時間:" + String.valueOf(runTime)); //運行時間:94576511