• 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());      }

• ConcurrentHashMap的stream支持

参考

• What's New in JDK 8

• 如何在Java 8中愉快地处理日期和时间

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