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Java8的新特性

Java语言特性系列

Java5的新特性
Java6的新特性
Java7的新特性
Java8的新特性
Java9的新特性

序

本文主要讲Java8的新特性，Java8也是一个重要的版本，在语法层面有更大的改动，支持了lamda表达式，影响堪比Java5的泛型支持。

特性列表

lamda表达式( 重磅 )
集合的stream操作
提升HashMaps的性能
Date-Time Package
java.lang and java.util Packages
Concurrency

lamda表达式( `重磅` )

方法引用

/**  * 静态方法引用：ClassName::methodName  * 实例上的实例方法引用：instanceReference::methodName  * 超类上的实例方法引用：super::methodName  * 类型上的实例方法引用：ClassName::methodName  * 构造方法引用：Class::new  * 数组构造方法引用：TypeName[]::new  * Created by patterncat on 2016-02-05.  */ public class MethodReference {      @Test     public void methodRef(){         SampleData.getThreeArtists().stream()                 .map(Artist::getName)                 .forEach(System.out::println);     }      @Test     public void constructorRef(){         ArtistFactory<Artist> af = Artist::new;         Artist a = af.create("patterncat","china");         System.out.println(a);     } }

集合的stream操作

/**  * 主要接口  * 1,predicate  * 2,Unary/BinaryOperator:传入参数和返回值必然是同一种数据类型  * 3,Int/Double/LongFunction/BiFunction:函数接口并不要求传入参数和返回值之间的数据类型必须一样  * 4,Int/Long/DoubleConsumer/BiConsumer:消费数据  * 5,Int/Long/DoubleSupplier:生产数据  *  * 主要方法:  * 1,filter  * 2,map  * 3,reduce  * 4,collect  * 5,peek  * -Djdk.internal.lambda.dumpProxyClasses  * Created by patterncat on 2016-02-05.  */ public class LamdaDemo {      int[] arr = {4,12,1,3,5,7,9};      @Test     public void filter(){         Arrays.stream(arr).filter((x) -> x%2 !=0).forEach(System.out::println);     }      @Test     public void map(){         Arrays.stream(arr).map((x) -> x * x).forEach(System.out::println);     }      @Test     public void reduce(){         Arrays.stream(arr).reduce((x,y) -> x+y).ifPresent(System.out::println);         System.out.println(Arrays.stream(arr).reduce(-10, (x, y) -> x + y));     }      @Test     public void collect(){         List<Integer> list = Arrays.stream(arr).collect(ArrayList::new,ArrayList::add,ArrayList::addAll);         System.out.println(list);          Set<Integer> set = list.stream().collect(Collectors.toSet());         System.out.println(set);          Map<String,Artist> map = SampleData.getThreeArtists().stream()                 .collect(Collectors.toMap(a -> a.getName(),a -> a));         System.out.println(map);     }      @Test     public void peek(){         long count = Arrays.stream(arr).filter(x -> x > 2).peek(System.out::println).count();         System.out.println(count);     }      @Test     public void average(){         Arrays.stream(arr).average().ifPresent(System.out::println);     }      @Test     public void sum(){         System.out.println(Arrays.stream(arr).sum());     }      @Test     public void max(){         Arrays.stream(arr).max().ifPresent(System.out::println);     }      @Test     public void min(){         Arrays.stream(arr).min().ifPresent(System.out::println);     }      @Test     public void sorted(){         Comparator<Artist> asc = (x,y) -> x.getName().compareTo(y.getName());         SampleData.getThreeArtists().stream().sorted(asc).forEach(System.out::println);         SampleData.getThreeArtists().stream().sorted(asc.reversed()).forEach(System.out::println);         SampleData.getThreeArtists().stream().sorted(Comparator.comparing(Artist::getName)).forEach(System.out::println);         SampleData.getThreeArtists().stream().sorted(Comparator.comparing(Artist::getName).reversed()).forEach(System.out::println);          SampleData.getThreeArtists().stream().sorted(Comparator.comparing(Artist::getName).thenComparing(Artist::getNationality)).forEach(System.out::println);     }      @Test     public void groupBy(){         Map<String,List<Artist>> rs = SampleData.getThreeArtists().stream().collect(Collectors.groupingBy(Artist::getNationality));         System.out.println(rs);     }      @Test     public void join(){         String joinedNames = SampleData.getThreeArtists().stream().map(Artist::getName).collect(Collectors.joining(","));         System.out.println(joinedNames);         joinedNames.chars().mapToObj(c -> (char) Character.toUpperCase(c)).forEach(System.out::println);     }      @Test     public void flatMap(){         Set<Artist> rs = SampleData.getThreeArtists().stream().flatMap(a -> a.getMembers()).collect(Collectors.toSet());         rs.stream().forEach(System.out::println);     }      @Test     public void arrStream(){         Arrays.stream(arr).forEach(System.out::println);     }      @Test     public void then(){ //        IntConsumer out = System.out::println; //        IntConsumer err = System.err::println;         IntConsumer out = (x) -> System.out.println("out consume:"+x);         IntConsumer err = (x) -> System.err.println("err consume:"+x); //        Arrays.stream(arr).forEach(out.andThen(err));         Arrays.stream(arr).forEach(err.andThen(out));     }       @Test     public void foreach(){         List<Integer> numbers = Arrays.asList(1,2,3,4,5,6);         numbers.forEach(System.out::println);     }      @Test     public void visitOuterVar(){         final int num = 2;         Function<Integer,Integer> fun = (from) -> from * num;         System.out.println(fun.apply(3));     } }

提升HashMaps的性能

当hash冲突时，以前都是用链表存储，在java8里头，当节点个数>=TREEIFY_THRESHOLD - 1时，HashMap将采用红黑树存储，这样最坏的情况下即所有的key都Hash冲突，采用链表的话查找时间为O(n)，而采用红黑树为O(logn)。

Date-Time Package

Java 8新增了LocalDate和LocalTime接口，一方面把月份和星期都改成了enum防止出错，另一方面把LocalDate和LocalTime变成不可变类型，这样就线程安全了。

@Test     public void today(){         LocalDate today = LocalDate.now();         System.out.println(today);     }      @Test     public void parseString(){         // 严格按照ISO yyyy-MM-dd验证，02写成2都不行，当然也有一个重载方法允许自己定义格式         LocalDate date = LocalDate.parse("2016-02-05");         System.out.println(date);     }      @Test     public void calculate(){         LocalDate today = LocalDate.now();         LocalDate firstDayOfThisMonth = today.with(TemporalAdjusters.firstDayOfMonth());         System.out.println(firstDayOfThisMonth);          // 取本月第2天：         LocalDate secondDayOfThisMonth = today.withDayOfMonth(2);         System.out.println(secondDayOfThisMonth);          // 取本月最后一天，再也不用计算是28，29，30还是31：         LocalDate lastDayOfThisMonth = today.with(TemporalAdjusters.lastDayOfMonth());         System.out.println(lastDayOfThisMonth);          // 取下一天：         LocalDate nextDay = lastDayOfThisMonth.plusDays(1);         System.out.println(nextDay);          // 取2015年1月第一个周一，这个计算用Calendar要死掉很多脑细胞：         LocalDate firstMondayOf2015 = LocalDate.parse("2015-01-01").with(TemporalAdjusters.firstInMonth(DayOfWeek.MONDAY));         System.out.println(firstMondayOf2015);     }      @Test     public void getTime(){         LocalTime now = LocalTime.now();         System.out.println(now);     }      @Test     public void getTimeWithoutMillis(){         LocalTime now = LocalTime.now().withNano(0);         System.out.println(now);     }      @Test     public void parseTime(){         LocalTime zero = LocalTime.of(0, 0, 0); // 00:00:00         System.out.println(zero);          LocalTime mid = LocalTime.parse("12:00:00"); // 12:00:00         System.out.println(mid);     }

java.lang and java.util Packages

比如数组的并行排序

public class UtilDemo {      int[] data = {4,12,1,3,5,7,9};      @Test     public void parallelSort(){         Arrays.parallelSort(data);         System.out.println(Arrays.toString(data));     }      @Test     public void testCollectPrallel() {         //[4, 16, 17, 20, 25, 32, 41]         Arrays.parallelPrefix(data, Integer::sum);         System.out.println(Arrays.toString(data));     } }

比如文件遍历

 @Test     public void list() throws IOException {         Files.list(Paths.get(".")).filter(Files::isDirectory).forEach(System.out::println);     }      @Test     public void walk() throws IOException {         Files.walk(Paths.get("."), FileVisitOption.FOLLOW_LINKS).forEach(System.out::println);     }

Concurrency

StampedLock

public class BankAccountWithStampedLock {      private final StampedLock lock = new StampedLock();     private double balance;      public void deposit(double amount) {         long stamp = lock.writeLock();         try {             balance = balance + amount;         } finally {             lock.unlockWrite(stamp);         }     }      public double getBalance() {         long stamp = lock.readLock();         try {             return balance;         } finally {             lock.unlockRead(stamp);         }     } }

测试

@Test     public void bench() throws InterruptedException {         BankAccountWithStampedLock account = new BankAccountWithStampedLock();         ExecutorService pool = Executors.newCachedThreadPool();         List<Callable<Double>> callables = IntStream.range(1,5)                 .mapToObj(x -> (Callable<Double>) () -> { //                    if (x % 2 == 0) { //                        return account.getBalance(); //                    } else { //                        account.deposit(x); //                        return 0d; //                    }                     account.deposit(x);                     return 0d;                 })                 .collect(Collectors.toList());         pool.invokeAll(callables).forEach(x -> {             try {                 System.out.println(x.get());             } catch (InterruptedException e) {                 e.printStackTrace();             } catch (ExecutionException e) {                 e.printStackTrace();             }         });         pool.shutdown();         System.out.println(account.getBalance());     }