DelayQueue in Java

A DelayQueue in Java is a specialized implementation of a blocking queue that supports delayed elements. Elements can only be taken from the queue when their delay has expired. This makes it useful for scenarios where tasks need to be executed after a certain delay.

Key Properties of DelayQueue

Elements in the DelayQueue must implement the Delayed interface, which requires implementing the getDelay method to specify the delay time and the compareTo method for ordering.

The queue blocks retrieval operations until the delay of the head element has expired.

Example: Suppose you are developing a system where tasks need to be executed after a certain delay, such as sending out delayed notifications.

class Notification implements Delayed {  
    private final String name;  
    private final long startTime;  

    public Notification(String name, long delay, TimeUnit unit) {  
        this.name = name;  
        this.startTime = System.currentTimeMillis() + TimeUnit.MILLISECONDS.convert(delay, unit);  
    }  

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

    @Override  
    public int compareTo(Delayed other) {  
        return Long.compare(this.startTime, ((Notification) other).startTime);  
    }  

    @Override  
    public String toString() {  
        return "Task: {name='" + name + "', startTime=" + startTime + '}';  
    }  
}  

public class DelayQueueExample {  
    public static void main(String[] args) throws InterruptedException {  
        DelayQueue<Notification> notifications = new DelayQueue<>();  
        notifications.add(new Notification("Task 1", 5, TimeUnit.SECONDS));  
        notifications.add(new Notification("Task 2", 10, TimeUnit.SECONDS));  
        notifications.add(new Notification("Task 3", 3, TimeUnit.SECONDS));  

        // Processing tasks as they expire  
        while (!notifications.isEmpty()) {  
            Notification task = notifications.take();  
            System.out.println("Notifying " + task);  
        }  
    }  
}

Note that the take() method retrieves and removes tasks from the queue when their delay has expired, blocking until a task is available.

Output:

Notifying Task: {name='Task 3', startTime=1721688863846}
Notifying Task: {name='Task 1', startTime=1721688865846}
Notifying Task: {name='Task 2', startTime=1721688870846}