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DelayQueue peek() method in Java with Examples

  • Last Updated : 07 May, 2021

The peek() method of DelayQueue is used to retrieve head of the DelayQueue, but does not remove it, as in case of poll() method where head is removed from the DelayQueue.

Syntax: 

public E peek ()

Parameters: 
This method does not accept any parameter.

Returns: 
This method returns the head of the DelayQueue and does not remove it from this DelayQueue.

Exception: 
Null Pointer Exception: if head is not present, this function will return null.



Below program to illustrate DelayQueue peek() in Java:

Example 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<DelayObject> DQ
            = new DelayQueue<DelayObject>();
 
        // 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);
    }
}
Output: 
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}]

 

Example 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<DelayObject> DQ
            = new DelayQueue<DelayObject>();
 
        // 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());
    }
}
Output: 
Original DelayQueue: [
{ A, time=1545817342666}, 
{ B, time=1545817342667}]

Head of the DelayQueue: null

 

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