Java8的新特性

摘要：语言特性系列的新特性的新特性的新特性的新特性的新特性的新特性的新特性的新特性的新特性序本文主要讲的新特性，也是一个重要的版本，在语法层面有更大的改动，支持了表达式，影响堪比的泛型支持。

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

Java10的新特性

Java11的新特性

Java12的新特性

Java13的新特性

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。

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

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 codecraft 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 af = Artist::new;
        Artist a = af.create("codecraft","china");
        System.out.println(a);
    }
}

/**
 * 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 codecraft 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 list = Arrays.stream(arr).collect(ArrayList::new,ArrayList::add,ArrayList::addAll);
        System.out.println(list);

        Set set = list.stream().collect(Collectors.toSet());
        System.out.println(set);

        Map 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 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> 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 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 numbers = Arrays.asList(1,2,3,4,5,6);
        numbers.forEach(System.out::println);
    }

    @Test
    public void visitOuterVar(){
        final int num = 2;
        Function fun = (from) -> from * num;
        System.out.println(fun.apply(3));
    }
}

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)。

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);
    }

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);
    }

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> callables = IntStream.range(1,5)
                .mapToObj(x -> (Callable) () -> {
//                    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

What"s New in JDK 8

如何在Java 8中愉快地处理日期和时间