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
- Java14的新特性
- Java15的新特性
- Java16的新特性
- Java17的新特性
- Java18的新特性
- Java19的新特性
- Java20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 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);
}
} 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 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));
}
} 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) () -> { // 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