我注意到在以下场景中有一些奇怪的行为:
迭代器 -> 流 -> map() -> iterator() -> 迭代
原始迭代器的hasNext()方法在已经返回false后又被调用了一次。
这正常吗?
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()方法或者将最后的iterator()替换成类似于count()或collect()的方法,就可以避免冗余调用。
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!!!!
next
可以被调用者随意调用... - Holger