Java中的 DelayQueue peek() 方法及示例

DelayQueue的peek()方法用于检索 DelayQueue 的头，但不会将其移除，如 poll() 方法中头从 DelayQueue 中移除的情况。

句法：

public E peek ()

参数：
此方法不接受任何参数。

回报：
此方法返回DelayQueue 的头部，并且不会将其从该DelayQueue 中移除。

例外：
Null 指针异常：如果 head 不存在，此函数将返回 null。

下面的程序来说明Java中的 DelayQueue peek() ：

示例 1：

Java

// Java Program Demonstrate DelayQueue peek() 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");
        // peek() method for returning head of the DelayQueue
 
        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

// Java Program Demonstrate DelayQueue peek() 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));
        // Print delayqueue
        System.out.println("Original DelayQueue: "
                           + DQ + "\n");
        // removing all elements
        DQ.clear();
        // peek() method for returning head of the DelayQueue
        System.out.println("Head of the DelayQueue: " + DQ.peek());
    }
}

输出：

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

Head of the DelayQueue: null