Question

在以下情况中我注意到一些奇怪的行为：

迭代器 - ＆gt;流 - ＆gt; map（） - ＆gt; iterator（） - ＆gt;迭代

原始迭代器的hasNext（）在返回false后被称为额外时间。

这是正常的吗？

package com.test.iterators;

import java.util.Iterator;
import java.util.Spliterators;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

public class TestIterator {

    private static int counter = 2;

    public static void main(String[] args) {

        class AdapterIterator implements Iterator<Integer> {
            boolean active = true;

            @Override
            public boolean hasNext() {
                System.out.println("hasNext() called");

                if (!active) {
                    System.out.println("Ignoring duplicate call to hasNext!!!!");
                    return false;
                }

                boolean hasNext = counter >= 0;
                System.out.println("actually has next:" + active);

                if (!hasNext) {
                    active = false;
                }

                return hasNext;
            }

            @Override
            public Integer next() {
                System.out.println("next() called");
                return counter--;
            }
        }

        Stream<Integer> stream = StreamSupport.stream(Spliterators.spliteratorUnknownSize(new AdapterIterator(), 0), false);
        stream.map(num -> num + 1).iterator().forEachRemaining(num -> {
            System.out.println(num);
        });
    }
}

如果我删除map（）或用count（）或collect（）替换最终的itearator（），它就可以在没有冗余调用的情况下工作。

输出

hasNext() called
actually has next:true
next() called
3
hasNext() called
actually has next:true
next() called
2
hasNext() called
actually has next:true
next() called
1
hasNext() called
actually has next:true
hasNext() called
Ignoring duplicate call to hasNext!!!!

Answer 1

是的，这很正常。冗余调用发生在StreamSpliterators.AbstractWrappingSpliterator.fillBuffer()中，该调用是从hasNext()返回的迭代器的stream.map(num -> num + 1).iterator()方法调用的。来自JDK 8来源：

/**
 * If the buffer is empty, push elements into the sink chain until
 * the source is empty or cancellation is requested.
 * @return whether there are elements to consume from the buffer
 */
private boolean fillBuffer() {
    while (buffer.count() == 0) {
        if (bufferSink.cancellationRequested() || !pusher.getAsBoolean()) {
            if (finished)
                return false;
            else {
                bufferSink.end(); // might trigger more elements
                finished = true;
            }
        }
    }
    return true;
}

对原始pusher.getAsBoolean()个实例hasNext()的{{1}}来电AdapterIterator。如果为true，则将下一个元素添加到bufferSink并返回true，否则返回false。当原始迭代器用完项目并返回false时，此方法将调用bufferSink.end()并重试填充缓冲区，从而导致冗余的hasNext()调用。

在这种情况下，bufferSink.end()没有效果，第二次尝试填充缓冲区是不必要的，但正如源注释所解释的那样，它可能触发更多元素＆＃34;在另一种情况下。这只是深入Java 8流复杂内部工作的实现细节。

流到迭代器的Java 8 Iterator导致对hasNext（）的冗余调用

1 个答案: