Java8的新特性
MaximoGCU
9年前
来自： https://segmentfault.com/a/1190000004419611
Java语言特性系列
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
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lamda表达式( 重磅 )
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集合的stream操作
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提升HashMaps的性能
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Date-Time Package
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java.lang and java.util Packages
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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
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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