Java中的DelayQueue poll()方法及其示例

poll() 方法用于检索DelayQueue的头部。此方法还从DelayQueue中删除头部。因此，调用poll方法时，DelayQueue的大小减小1。

语法：

public E poll ()

参数：

此方法不接受任何参数。

返回值：

此方法返回DelayQueue的头部，并将其从此DelayQueue中删除。

异常：

Null Pointer Exception： 如果头部不存在，此函数将返回 null。

以下程序演示Java中的DelayQueue poll()：

// Java Program Demonstrate DelayQueue poll() method

import java.util.concurrent.*;
import java.util.*;

// The DelayObject for DelayQueue
// It must implement Delayed and
// its getDelay() and compareTo() method
class DelayObject implements Delayed {

    private String name;
    private long time;

    // Constructor of DelayObject
    public DelayObject(String name, long delayTime)
    {
        this.name = name;
        this.time = System.currentTimeMillis()
                    + delayTime;
    }

    // Implementing getDelay() method of Delayed
    @Override
    public long getDelay(TimeUnit unit)
    {
        long diff = time - System.currentTimeMillis();
        return unit.convert(diff, TimeUnit.MILLISECONDS);
    }

    // Implementing compareTo() method of Delayed
    @Override
    public int compareTo(Delayed obj)
    {
        if (this.time < ((DelayObject)obj).time) {
            return -1;
        }
        if (this.time > ((DelayObject)obj).time) {
            return 1;
        }
        return 0;
    }

    // Implementing toString() method of Delayed
    @Override
    public String toString()
    {
        return "\n{"
            + " " + name + ", time=" + time + "}";
    }
}

// Driver Class
public class GFG {
    public static void main(String[] args) throws InterruptedException
    {

        // create object of DelayQueue
        // using DelayQueue() constructor
        BlockingQueue DQ
            = new DelayQueue();

        // Add numbers to end of DelayQueue
        // using add() method
        DQ.add(new DelayObject("A", 1));
        DQ.add(new DelayObject("B", 2));
        DQ.add(new DelayObject("C", 3));
        DQ.add(new DelayObject("D", 4));

        // Print delayqueue
        System.out.println("Original DelayQueue: "
                                + DQ + "\n");
        // Print size of original DelayQueue
        System.out.println("Original DelayQueue size: "
                                + DQ.size() + "\n");

        // poll() method for returning head of the DelayQueue
        System.out.println("Head of the DelayQueue: "
                                + DQ.poll());
        // print DelayQueue size after poll method
        System.out.println("\nAfter poll() method, DelayQueue size: "
                                + DQ.size());
    }
}

输出:

Original DelayQueue: [
{ A, time=1545817370442}, 
{ B, time=1545817370443}, 
{ C, time=1545817370444}, 
{ D, time=1545817370445}]

Original DelayQueue size: 4

Head of the DelayQueue: 
{ A, time=1545817370442}

After poll() method, DelayQueue size: 3

示例：

// Java程序演示DelayQueue poll()方法
import java.util.concurrent.*;
import java.util.*;

// DelayQueue的DelayObject
//必须实现Delayed接口，且实现getDelay()和compareTo()方法
class DelayObject implements Delayed {
    private String name;
    private long time;

    // DelayObject的构造函数
    public DelayObject(String name, long delayTime)
    {
        this.name = name;
        this.time = System.currentTimeMillis()
                    + delayTime;
    }

    // 实现getDelay()方法
    @Override
    public long getDelay(TimeUnit unit)
    {
        long diff = time - System.currentTimeMillis();
        return unit.convert(diff, TimeUnit.MILLISECONDS);
    }

    // 实现compareTo()方法
    @Override
    public int compareTo(Delayed obj)
    {
        if (this.time < ((DelayObject)obj).time) {
            return -1;
        }
        if (this.time > ((DelayObject)obj).time) {
            return 1;
        }
        return 0;
    }

    // 实现toString()方法
    @Override
    public String toString()
    {
        return "\n{"
            + " " + name + ", time=" + time + "}";
    }
}

// Driver Class
public class GFG {
    public static void main(String[] args) throws InterruptedException
    {
        // 创建DelayQueue对象
        //使用DelayQueue()构造函数
        BlockingQueue<DelayObject> DQ
            = new DelayQueue<DelayObject>();

        // 打印原始DelayQueue的大小
        System.out.println("原始DelayQueue的大小："
                           + DQ.size() + "\n");

        // poll()方法返回DelayQueue的头
        System.out.println("DelayQueue的头："
                           + DQ.poll());
    }
}

输出：

原始DelayQueue的大小：0

DelayQueue的头：null