Java 用谓词限制流
是否有Java 8流操作限制(可能无限)Java 用谓词限制流,java,java-8,java-stream,Java,Java 8,Java Stream,是否有Java 8流操作限制(可能无限)流,直到第一个元素无法匹配谓词 在Java9中,我们可以使用takeWhile打印所有小于10的数字,如下例所示 IntStream .iterate(1, n -> n + 1) .takeWhile(n -> n < 10) .forEach(System.out::println); IntStream .迭代(1,n->n+1) .takeWhile(n->nn+1); TakeWhile.stream(
流
,直到第一个元素无法匹配谓词
在Java9中,我们可以使用takeWhile
打印所有小于10的数字,如下例所示
IntStream
.iterate(1, n -> n + 1)
.takeWhile(n -> n < 10)
.forEach(System.out::println);
IntStream
.迭代(1,n->n+1)
.takeWhile(n->n<10)
.forEach(System.out::println);
由于Java8中没有这样的操作,一般来说,实现它的最佳方法是什么 对于Java 8
流,这样的操作应该是可能的,但它不一定能有效地完成——例如,您不一定能并行化这样的操作,因为您必须按顺序查看元素
API并没有提供一种简单的实现方法,但最简单的方法可能是使用Stream.iterator()
,将迭代器
包装成“take while”实现,然后返回到拆分器
,然后是流
。或者——也许——包装拆分器
,尽管在这个实现中它不能再被拆分了
下面是一个未经测试的takeWhile
在拆分器上的实现:
static <T> Spliterator<T> takeWhile(
Spliterator<T> splitr, Predicate<? super T> predicate) {
return new Spliterators.AbstractSpliterator<T>(splitr.estimateSize(), 0) {
boolean stillGoing = true;
@Override public boolean tryAdvance(Consumer<? super T> consumer) {
if (stillGoing) {
boolean hadNext = splitr.tryAdvance(elem -> {
if (predicate.test(elem)) {
consumer.accept(elem);
} else {
stillGoing = false;
}
});
return hadNext && stillGoing;
}
return false;
}
};
}
static <T> Stream<T> takeWhile(Stream<T> stream, Predicate<? super T> predicate) {
return StreamSupport.stream(takeWhile(stream.spliterator(), predicate), false);
}
静态拆分器(
Spliterator splitr,Predicate更新:Java9流现在提供了一个方法。
不需要黑客或其他解决方案。只要使用它
我相信这可以大大改进:
(也许有人可以让它线程安全)
Stream=Stream.iterate(0,n->n+1);
TakeWhile.stream(流,n->n<10000)
.forEach(n->System.out.print((n==0?“+n:”,“+n));
一个黑客肯定…不优雅-但它的作品~:D
类TakeWhile实现迭代器{
私有最终迭代器;
私有最终谓词;
下一步;
private volatile boolean keepGoing=true;
公共TakeWhile(流s,谓词p){
this.iterator=s.iterator();
this.predicate=p;
}
@凌驾
公共布尔hasNext(){
如果(!继续){
返回false;
}
如果(下一步!=null){
返回true;
}
if(iterator.hasNext()){
next=迭代器.next();
keepGoing=predicate.test(下一步);
如果(!继续){
next=null;
}
}
返回下一步!=null;
}
@凌驾
公共交通工具{
if(next==null){
如果(!hasNext()){
抛出新的NoSuchElementException(“对不起,没什么适合你的。”);
}
}
T温度=下一个;
next=null;
返回温度;
}
公共静态流(流s,谓词p){
TakeWhile tw=新TakeWhile(s,p);
Spliterator split=Spliterators.Spliterator(tw,Integer.MAX_值,Spliterator.ORDERED);
返回StreamSupport.stream(拆分,false);
}
}
这是一个在ints上完成的版本-如问题中所述
用法:
StreamUtil.takeWhile(IntStream.iterate(1, n -> n + 1), n -> n < 10);
takeWhile
是由提供的功能之一
Stream infinitements=Stream.iterate(0,i->i+1);
streamfiniteints=StreamUtils.takeWhile(无限元素,i->i<10);
断言(finiteInts.collect(Collectors.toList()),
hasSize(10));
通过实现这一点,我有了另一个快速的解决方案(事实上这是不干净的,但你明白了):
publicstaticvoidmain(字符串[]args){
System.out.println(StreamUtil.iterate(1,o->o+1).terminateOn(15)
.map(o->o.toString()).collect(collector.joining(“,”));
}
静态接口终止流{
流终止(te);
}
静态类StreamUtil{
静态终止流迭代(T种子,一元运算符op){
返回新的TerminatedStream(){
公共流终止(TE){
Builder=Stream.Builder().add(种子);
T电流=种子;
而(!current.equals(e)){
电流=操作应用(电流);
builder.add(当前);
}
返回builder.build();
}
};
}
}
allMatch()
是一个短路函数,因此您可以使用它来停止处理。主要缺点是您必须执行两次测试:一次是查看是否应该处理它,另一次是查看是否继续
IntStream
.iterate(1, n -> n + 1)
.peek(n->{if (n<10) System.out.println(n);})
.allMatch(n->n < 10);
IntStream
.迭代(1,n->n+1)
.peek(n->{if(nn<10);
您可以使用java8+
import java.util.stream.IntStream;
进口接收。可观察;
//例1)
IntStream IntStream=IntStream.iterate(1,n->n+1);
可观察的.from(()->intStream.iterator())
.takeWhile(n->
{
系统输出println(n);
返回n<10;
}
).subscribe();
//例2
IntStream IntStream=IntStream.iterate(1,n->n+1);
可观察的.from(()->intStream.iterator())
.takeWhile(n->n<10)
.forEach(n->System.out.println(n));
以下是我仅使用Java流库的尝试
IntStream.iterate(0, i -> i + 1)
.filter(n -> {
if (n < 10) {
System.out.println(n);
return false;
} else {
return true;
}
})
.findAny();
IntStream.iterate(0,i->i+1)
.过滤器(n->{
如果(n<10){
系统输出println(n);
返回false;
}否则{
返回true;
}
})
.findAny();
操作takeWhile
和dropWhile
已添加到JDK 9中。您的示例代码
IntStream
.iterate(1, n -> n + 1)
.takeWhile(n -> n < 10)
.forEach(System.out::println);
IntStream
.迭代(1,n->n+1)
.takeWhile(n->n<10)
.forEach(System.out::println);
在JDK 9下编译和运行时,它的行为将完全符合您的预期
JDK 9已发布。可在此处下载:。作为后续。我的库具有与当前JDK-9实现兼容的操作。在JDK-9下运行时,它
Stream<Integer> infiniteInts = Stream.iterate(0, i -> i + 1);
Stream<Integer> finiteInts = StreamUtils.takeWhile(infiniteInts, i -> i < 10);
assertThat(finiteInts.collect(Collectors.toList()),
hasSize(10));
public static void main(String[] args) {
System.out.println(StreamUtil.iterate(1, o -> o + 1).terminateOn(15)
.map(o -> o.toString()).collect(Collectors.joining(", ")));
}
static interface TerminatedStream<T> {
Stream<T> terminateOn(T e);
}
static class StreamUtil {
static <T> TerminatedStream<T> iterate(T seed, UnaryOperator<T> op) {
return new TerminatedStream<T>() {
public Stream<T> terminateOn(T e) {
Builder<T> builder = Stream.<T> builder().add(seed);
T current = seed;
while (!current.equals(e)) {
current = op.apply(current);
builder.add(current);
}
return builder.build();
}
};
}
}
IntStream
.iterate(1, n -> n + 1)
.peek(n->{if (n<10) System.out.println(n);})
.allMatch(n->n < 10);
import java.util.stream.IntStream;
import rx.Observable;
// Example 1)
IntStream intStream = IntStream.iterate(1, n -> n + 1);
Observable.from(() -> intStream.iterator())
.takeWhile(n ->
{
System.out.println(n);
return n < 10;
}
).subscribe() ;
// Example 2
IntStream intStream = IntStream.iterate(1, n -> n + 1);
Observable.from(() -> intStream.iterator())
.takeWhile(n -> n < 10)
.forEach( n -> System.out.println(n));
IntStream.iterate(0, i -> i + 1)
.filter(n -> {
if (n < 10) {
System.out.println(n);
return false;
} else {
return true;
}
})
.findAny();
IntStream
.iterate(1, n -> n + 1)
.takeWhile(n -> n < 10)
.forEach(System.out::println);
IntStreamEx.iterate(1, n -> n + 1)
.takeWhile(n -> n < 10)
.forEach(System.out::println);
IntStream.iterate(1, n -> n + 1).takeWhile(n -> n < 10).forEach(System.out::println);
import java.util.Objects;
class ThingProcessor
{
static Thing returnNullOnCondition(Thing thing)
{ return( (*** is condition met ***)? null : thing); }
void processThings(Collection<Thing> thingsCollection)
{
thingsCollection.stream()
*** regular stream processing ***
.map(ThingProcessor::returnNullOnCondition)
.filter(Objects::nonNull)
*** continue stream processing ***
}
} // class ThingProcessor
IntStream.iterate(2, (i) -> i + 2).peek(System.out::println).allMatch(i -> i < 20);
IntStream.iterate(2, (i) -> i + 2).peek(System.out::println).anyMatch(i -> i >= 20);
2
4
6
8
10
12
14
16
18
20
IntStream
.iterate(1, n -> n + 1)
.limit(10)
.forEach(System.out::println);
static <T> Stream<T> takeWhile(Stream<T> stream, Predicate<? super T> p) {
class Taking extends Spliterators.AbstractSpliterator<T> implements Consumer<T> {
private static final int CANCEL_CHECK_COUNT = 63;
private final Spliterator<T> s;
private int count;
private T t;
private final AtomicBoolean cancel = new AtomicBoolean();
private boolean takeOrDrop = true;
Taking(Spliterator<T> s) {
super(s.estimateSize(), s.characteristics() & ~(Spliterator.SIZED | Spliterator.SUBSIZED));
this.s = s;
}
@Override
public boolean tryAdvance(Consumer<? super T> action) {
boolean test = true;
if (takeOrDrop && // If can take
(count != 0 || !cancel.get()) && // and if not cancelled
s.tryAdvance(this) && // and if advanced one element
(test = p.test(t))) { // and test on element passes
action.accept(t); // then accept element
return true;
} else {
// Taking is finished
takeOrDrop = false;
// Cancel all further traversal and splitting operations
// only if test of element failed (short-circuited)
if (!test)
cancel.set(true);
return false;
}
}
@Override
public Comparator<? super T> getComparator() {
return s.getComparator();
}
@Override
public void accept(T t) {
count = (count + 1) & CANCEL_CHECK_COUNT;
this.t = t;
}
@Override
public Spliterator<T> trySplit() {
return null;
}
}
return StreamSupport.stream(new Taking(stream.spliterator()), stream.isParallel()).onClose(stream::close);
}
import java.util.stream.IntStream;
import io.netty.util.internal.ThreadLocalRandom;
class TrialStreamMatch {
public static void main(String[] args) {
if(!IntStream.range(1,3).anyMatch(integ -> webServiceCall(integ))){
//Code for sending email notifications
}
}
public static boolean webServiceCall(int i){
//For time being, I have written a code for generating boolean randomly
//This whole piece needs to be replaced by actual web-service client code
boolean bool = ThreadLocalRandom.current().nextBoolean();
System.out.println("Iteration index :: "+i+" bool :: "+bool);
//Return success status -- true or false
return bool;
}
static <T> List<T> take(List<T> l, int n) {
if (n <= 0) {
return newArrayList();
} else {
int takeTo = Math.min(Math.max(n, 0), l.size());
return l.subList(0, takeTo);
}
}
assertEquals(newArrayList(1, 2, 3), take(newArrayList(1, 2, 3, 4, 5), 3));
assertEquals(newArrayList(1, 2, 3), take(newArrayList(1, 2, 3), 5));
assertEquals(newArrayList(), take(newArrayList(1, 2, 3), -1));
assertEquals(newArrayList(), take(newArrayList(1, 2, 3), 0));
static <T> List<T> takeWhile(List<T> l, Predicate<T> p) {
return l.stream().
filter(p.negate()).findFirst(). // find first element when p is false
map(l::indexOf). // find the index of that element
map(i -> take(l, i)). // take up to the index
orElse(l); // return full list if p is true for all elements
}
assertEquals(newArrayList(1, 2, 3), takeWhile(newArrayList(1, 2, 3, 4, 3, 2, 1), i -> i < 4));
IntStream
.iterate(1, n -> n + 1)
.limit(10)
.forEach(System.out::println);
IntStream.iterate(1, n -> n + 1)
.peek(System.out::println) //it will be executed 9 times
.filter(n->n>=9)
.findAny();
IntStream.iterate(1, n -> n + 1)
.mapToObj(n->{ //it will be executed 9 times
if(n<9)
return "";
return "Loop repeats " + n + " times";});
.filter(message->!message.isEmpty())
.findAny()
.ifPresent(System.out::println);