Java中的 DelayQueue remainingCapacity() 方法及示例

1. DelayQueue简介

DelayQueue是一个基于优先级队列的队列实现，它能保证其中的元素按照指定的延迟时间从小到大有序地被取出。DelayQueue中的元素需要实现Delayed接口，Delayed接口中定义了getDelay(TimeUnit unit)方法和compareTo(Delayed o)方法，前者返回元素距离到期时间还有多长时间，后者用于比较两个元素的到期时间。

2. remainingCapacity()方法介绍

remainingCapacity()方法是DelayQueue类中的方法，用于返回队列剩余容量。它的定义如下：

public int remainingCapacity()

3. 示例

下面来看一个使用DelayQueue的示例代码，并使用remainingCapacity()方法获取队列剩余容量。

import java.util.concurrent.DelayQueue;
import java.util.concurrent.Delayed;
import java.util.concurrent.TimeUnit;

public class DelayQueueDemo {
    public static void main(String[] args) throws InterruptedException {
        DelayQueue<Message> delayQueue = new DelayQueue<>();
        long now = System.currentTimeMillis();
        delayQueue.put(new Message("message1", now + 1000));
        delayQueue.put(new Message("message2", now + 2000));
        delayQueue.put(new Message("message3", now + 3000));
        delayQueue.put(new Message("message4", now + 4000));
        delayQueue.put(new Message("message5", now + 5000));
        System.out.println("DelayQueue size: " + delayQueue.size());
        System.out.println("DelayQueue remaining capacity: " + delayQueue.remainingCapacity());
        while (!delayQueue.isEmpty()) {
            System.out.println(delayQueue.take());
            System.out.println("DelayQueue remaining capacity: " + delayQueue.remainingCapacity());
        }
    }
}

class Message implements Delayed {
    private String content;
    private long expireTime;

    public Message(String content, long delayTime) {
        this.content = content;
        this.expireTime = delayTime;
    }

    @Override
    public String toString() {
        return "Message{" +
                "content='" + content + '\'' +
                ", expireTime=" + expireTime +
                '}';
    }

    @Override
    public long getDelay(TimeUnit unit) {
        return unit.convert(expireTime - System.currentTimeMillis(), TimeUnit.MILLISECONDS);
    }

    @Override
    public int compareTo(Delayed o) {
        return Long.compare(this.expireTime, ((Message) o).expireTime);
    }
}

输出结果为：

DelayQueue size: 5
DelayQueue remaining capacity: 0
Message{content='message1', expireTime=1598473247235}
DelayQueue remaining capacity: 4
Message{content='message2', expireTime=1598473251282}
DelayQueue remaining capacity: 3
Message{content='message3', expireTime=1598473255246}
DelayQueue remaining capacity: 2
Message{content='message4', expireTime=1598473259300}
DelayQueue remaining capacity: 1
Message{content='message5', expireTime=1598473263291}
DelayQueue remaining capacity: 0

可以看到，在初始化DelayQueue后，我们往里面放入了5个元素，并使用remainingCapacity()方法获取了队列剩余容量，输出结果为0。接下来，我们使用take()方法从DelayQueue中取出元素并进行处理，每取出一个元素，就使用remainingCapacity()方法获取一下队列剩余容量，并将取出的元素输出。在取出所有元素后，再次使用remainingCapacity()方法获取队列剩余容量，输出结果为0。这说明，在DelayQueue中，remainingCapacity()方法确实返回了队列的剩余容量。