Java8 的一些使用总结

nohup java -jar -Xmx1024M china-map.jar > log.txt & 在ssh退出后， 也不会终止进程；

Object[] a = IntStream.range(0,100).
map(i->String.valueOf(i).hashCode()).
mapToObj(i->String.valueOf(i).getBytes())
.toArray();

c.forEach(System.out::println);



接口中的default方法：

public interface NewCharacter {

public void test1();

public default void test2(){
System.out.println("我是新特性1");
}

}



Lambda表达式：
Collections.sort(c,(t1,t2)->t1.compareTo(t1+t2));


Function接口：

@FunctionalInterface // 只能定义一个抽象方法； 通过Lambda去实现这个抽象方法； 相当于 func (String y) {...}
public interface MyLamda {
public void test1(String y);
}

MyLamda m = y -> System.out.println("ss"+y);


Function<Integer,Object> f = y -> y+1;

Supplier 供应商， 就相当于写好的一个 匿名func()

@FunctionalInterface
public interface Supplier<T> {

T get();
}

Optional<String> o = Optional.of("aa");
o.orElseGet(()->"none");

public T orElseGet(Supplier<? extends T> other) {
return value != null ? value : other.get();
}

等价于：
public T orElseGet(Function<T,T> func) {
return value != null ? value : func.apply();
}

stream

task是一个类, tasks 是list<task>；

final double totalPoints = tasks
.stream()
.parallel()
.map( task -> task.getPoints() ) // or map( Task::getPoints )
.reduce( 0, Integer::sum );

final Map< Status, List< Task > > map = tasks
.stream()
.collect( Collectors.groupingBy( Task::getStatus ) );

final Collection< String > result = tasks
.stream() // Stream< String >
.mapToInt( Task::getPoints ) // IntStream
.asLongStream() // LongStream
.mapToDouble( points -> points / totalPoints ) // DoubleStream
.boxed() // Stream< Double >
.mapToLong( weigth -> ( long )( weigth * 100 ) ) // LongStream
.mapToObj( percentage -> percentage + "%" ) // Stream< String>
.collect( Collectors.toList() ); // List< String >

// 文件IO
final Path path = new File( filename ).toPath();
try( Stream< String > lines = Files.lines( path, StandardCharsets.UTF_8 ) ) {
lines.onClose( () -> System.out.println("Done!") ).forEach( System.out::println );
}

并行计算；

long[] arrayOfLong = new long [ 20000 ];

Arrays.parallelSetAll( arrayOfLong,
index -> ThreadLocalRandom.current().nextInt( 1000000 ) );
Arrays.stream( arrayOfLong ).limit( 10 ).forEach(
i -> System.out.print( i + " " ) );
System.out.println();

Arrays.parallelSort( arrayOfLong );

日期比较

LocalDateTime dt = LocalDateTime.now();
DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd hh:mm:ss");
System.out.println(dtf.format(dt));

// Get duration between two dates
final LocalDateTime from = LocalDateTime.of( 2014, Month.APRIL, 16, 0, 0, 0 );
final LocalDateTime to = LocalDateTime.of( 2015, Month.APRIL, 16, 23, 59, 59 );

final Duration duration = Duration.between( from, to );
System.out.println( "Duration in days: " + duration.toDays() );
System.out.println( "Duration in hours: " + duration.toHours() );