Java中的DelayQueue peek()方法及其示例

peek() 方法用于检索DelayQueue的头部，但是不像poll()方法那样从DelayQueue中删除它。

语法:

public E peek ()

参数:

此方法不接受任何参数。

返回值:

此方法返回DelayQueue的头部，并且不从此DelayQueue中移除它。

异常:

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

以下程序演示了Java中的DelayQueue peek():

示例 1:

// Java程序演示DelayQueue peek()方法

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

// DelayQueue的DelayObject
//它必须实现Delayed和
//其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Delayed的getDelay()方法
    @Override
    public long getDelay(TimeUnit unit)
    {
        long diff = time - System.currentTimeMillis();
        return unit.convert(diff, TimeUnit.MILLISECONDS);
    }

    // 实现Delayed的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Delayed的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的对象
        //使用DelayQueue()构造函数
        BlockingQueue DQ
            = new DelayQueue();

        // 使用add()方法将数字添加到DelayQueue的末尾
        DQ.add(new DelayObject("A", 1));
        DQ.add(new DelayObject("B", 2));
        DQ.add(new DelayObject("C", 3));
        DQ.add(new DelayObject("D", 4));
        // 打印DelayQueue
        System.out.println("Original DelayQueue: "
                                   + DQ + "\n");
        //通过peek()方法返回DelayQueue的头部

        System.out.println("Head of the DelayQueue: "
                                   + DQ.peek());
        // print DelayQueue
        System.out.println("\nAfter peek() method, DelayQueue: "
                                   + DQ);
    }
}

输出:

Original DelayQueue: [
{ A, time=1545817334485}, 
{ B, time=1545817334486}, 
{ C, time=1545817334487}, 
{ D, time=1545817334488}]

Head of the DelayQueue: 
{ A, time=1545817334485}

After peek() method, DelayQueue: [
{ A, time=1545817334485}, 
{ B, time=1545817334486}, 
{ C, time=1545817334487}, 
{ D, time=1545817334488}]

示例 2:

// Java程序演示DelayQueue peek()方法
 
import java.util.concurrent.*;
import java.util.*;
 
// DelayQueue的DelayObject
// 它必须实现Delayed和其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Delayed的getDelay()方法
    @Override
    public long getDelay(TimeUnit unit)
    {
        long diff = time - System.currentTimeMillis();
        return unit.convert(diff, TimeUnit.MILLISECONDS);
    }
 
    // 实现Delayed的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Delayed的toString()方法
    @Override
    public String toString()
    {
        return "\n{"
            + " " + name + ", time=" + time + "}";
    }
}
 
// Driver类
public class GFG {
    public static void main(String[] args) throws InterruptedException
    {
 
        // 使用DelayQueue()构造函数创建DelayQueue对象
        BlockingQueue<DelayObject> DQ
            = new DelayQueue<DelayObject>();
 
        // 使用add()方法向DelayQueue末尾添加数字
        DQ.add(new DelayObject("A", 1));
        DQ.add(new DelayObject("B", 2));
        // 打印DelayQueue
        System.out.println("原始DelayQueue: "
                           + DQ + "\n");
        // 删除所有元素
        DQ.clear();
        // 返回DelayQueue的头部的peek()方法
        System.out.println("DelayQueue头部: " + DQ.peek());
    }
}

输出：

原始DelayQueue: [
{ A, time=1545817342666}, 
{ B, time=1545817342667}]

DelayQueue头部: null