如何自定义PriorityQueue.stream().foreach以优先级顺序进行迭代 [英] How to customize PriorityQueue.stream().foreach to iterate in priority order

问题描述

我有一个带有PriorityQueue字段的类:

I have a class with PriorityQueue field inside:

public class MyClass<T>{
    Queue<T> queue = new PriorityQueue<>();

我想以某种方式从MyClass获取流并使用foreach，并希望序列按PriorityQueue的优先级顺序运行.最简单的方法是重写stream()方法:

I want somehow get a stream from MyClass and use foreach and want the sequence behave in priority order of my PriorityQueue. The easiest way is to override stream() method:

@Override
public Stream stream() {
    return queue.stream();
}

，但这不会按优先级顺序显示队列元素.因此，问题是:如何使foreach流方法的行为类似于:

but this will not expose the queue element in priority order. So the question is: how to make foreach stream method behave like:

    while(!queue.isEmpty()) 
        queue.poll();

推荐答案

您可以使用 Queue :: poll 方法来创建 Stream ，其中包含 PriorityQueue 中的元素并保持其顺序:

You could use Stream::generate and Queue::poll method to get create a Stream with elements from PriorityQueue with keeping their order:

@Override
public Stream<T> stream() {
    return Stream.generate(queue::poll);
}

但是这可能很危险，因为 Stream :: generate 将不断调用 poll ，因此它可能是无限的Stream.因此，应考虑在队列大小中使用 Stream :: limit :

However this might be dangerous because Stream::generate will be invoking poll constantly so it is potentially an inifinite Stream. Therfore using Stream::limit with the queue size should be considered :

@Override
public Stream<T> stream() {
    return Stream.generate(queue::poll)
        .limit(queue.size());
}

或者您可以简单地返回已排序的流:

Or you could simply return sorted stream :

@Override
public Stream<T> stream() {
    return queue.stream()
            .sorted(comparator);
}

比较器是您的比较器.

在 Java 9 中，您可以使用

In Java 9 you could use Stream::takeWhile with predicate that rejects nulls. As Queue::poll will return null when queue is empty - the resulting Stream will contain elements from the queue in their order (this is alternative to using limit as described in the first solution) :

@Override
public Stream<T> stream() {
    return Stream.generate(queue::poll)
            .takeWhile(Objects::nonNull);
}

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