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Java.util.PriorityQueue class in Java - GeeksforGeeks

Last Updated : 06 Dec, 2021

It is a priority queue based on priority heap.

• Elements in this class are in natural order or depends on the Constructor we used at this the time of construction.
• It doesn't permit null pointers.
• It doesn't allow inserting a non-comparable object, if it relies on natural ordering.

• PriorityQueue(): Creates a PriorityQueue with the default initial capacity (11) that orders its elements according to their natural ordering.
• PriorityQueue(Collection c): Creates a PriorityQueue containing the elements in the specified collection.
• PriorityQueue(int initialCapacity): Creates a PriorityQueue with the specified initial capacity that orders its elements according to their natural ordering.
• PriorityQueue(int initialCapacity, Comparator comparator): Creates a PriorityQueue with the specified initial capacity that orders its elements according to the specified comparator.
• PriorityQueue(PriorityQueue c): Creates a PriorityQueue containing the elements in the specified priority queue.
• PriorityQueue(SortedSet c): Creates a PriorityQueue containing the elements in the specified sorted set.

public class PriorityQueue
   extends AbstractQueue
   implements Serializable

1. add(element) : java.util.PriorityQueue.add() insert the elements to the Priority Queue. Syntax :

   public boolean add(E e)
   Parameters  :
   element : the element we need to add.
   Return  :
   call return true.
   Exception : 
   -&gt ClassCastException 
   -&gt NullPointerException
   

2. comparator() : java.util.PriorityQueue.comparator() orders the elements in the queue. Syntax :

   public Comparator comparator()
   Parameters  :
   -------
   Return  :
   orders the queue or return null, if it is naturally ordered 
   Exception : 
   ----------
   

3. contains(Object obj) : java.util.PriorityQueue.contains(obj) returns true if the priority queue contains the element "obj". Syntax :

   public boolean contains(Object obj)
   Parameters  :
   obj : object to be checked
   Return  :
   true - if the object is present else, return false
   Exception : 
   
   

4. iterator() : java.util.PriorityQueue.iterator() iterates over the queue element. Syntax :

   public Iterator iterator()
   Parameters  :
   -------
   Return  :
   calls iterator over the elements in the queue.
   Exception : 
   --------
   

5. offer(element) : java.util.PriorityQueue.offer() is required to insert a specific element to the given priority queue. Syntax :

   public boolean offer(E element)
   Parameters  :
   element : specific element to  be entered.
   Return  :
   call return true.
   Exception : 
   -&gt ClassCastException 
   -&gt NullPointerException
   

6. peek() : java.util.PriorityQueue.peek() identifies the head element of the queue. Syntax :

   public E peek()    
   Parameters  :
   -------
   Return  :
   calls if head exists, else null
   Exception : 
   ------
   

7. poll() : java.util.PriorityQueue.poll() identifies the head and then removes it. Syntax :

   public E poll()    
   Parameters  :
   ---
   Return  :
   calls if head exists, else null
   Exception : 
   ------
   

8. remove(Object obj) : java.util.PriorityQueue.remove() removes a specific object from the queue. Syntax :

   public boolean remove(Object obj)
   Parameters  :
   obj : object to be removed
   Return  :
   true - if obj is removed
   Exception : 
   ------
   

9. size() : java.util.PriorityQueue.size() returns the size of elements in the Priority Queue. Syntax :

   public int size()
   Parameters  :
   ----
   Return  :
   no. of elements
   Exception : 
   ---------
   

10. toArray() : java.util.PriorityQueue.toArray() returns an array containing the elements of PriorityQueue. Syntax :

    public Object[] toArray()
    Parameters  :
    ------
    Return  :
    returns an array containing all the elements of PriorityQueue.
    Exception : 
    --------
    

11. toArray(T[] array) : java.util.PriorityQueue.toArray(T[] a) returns the array having the elements of the Priority Queue. Syntax :

    public  T[] toArray(T[] array)
    Parameters  :
    array  : the array to which are to be sorted. 
    Return  :
    call an array containing all the elements of the array. 
    Exception : 
    -> ArrayStoreException
    -> NullPointerException
    

12. clear() : java.util.PriorityQueue.clear() clears all the elements of the PriorityQueue. Syntax :

    public void clear()        
    Parameters  :
    ---
    Return  :
    ------
    Exception : 
    ------
    

Java

// Java Program illustrating the methods
// of java.utl.priorityQueue class

// add(), comparator(), conatins(), iterator(), offer()
// peek(), poll(), toArray(), size(), toArray(t[] g1),
// remove(), clear()

import java.util.*;
public class NewClass
{
    public static void main(String[] args)
    {
        // Creating a Priority Queue :
        PriorityQueue <Integer> geek = new PriorityQueue <Integer> ();

        for(int i=2; i<=20; i=i+2)
        {
            // Use of add() :
            geek.add(new Integer (i));
        }

        System.out.println("geek PriorityQueue : " + geek);

        // Use of comparator() 
        // No ordering is required here as it is naturally ordered.
        Comparator geek_comp = geek.comparator();
        System.out.println("geek PriorityQueue : " + geek_comp);

        // Use of contains() 
        boolean check = geek.contains(6);
        System.out.println("Use of contains() : " + check);

        // Use of iterator() 
        Iterator g_iterator = geek.iterator();

        System.out.print("Iterator values : ");
        while(g_iterator.hasNext())
        {
            System.out.print(g_iterator.next() + " ");
        }
        System.out.println("");

        // Use of offer() 
        geek.offer(3050);
        System.out.println("geek PriorityQueue : " + geek);

        // Use of peek() 
        System.out.println("Head of PriorityQueue via peek : " + geek.peek());

        //Use of poll() 
        int h = geek.poll();
        System.out.println("\nHead of PriorityQueue via poll : " + h);
        System.out.println("geek PriorityQueue bcz of poll() : " + geek);

        // Use of remove()
        boolean r = geek.remove(8);
        System.out.println("\nCan remove : " + r);
        System.out.println("geek PriorityQueue bcz of remove() : " + geek);

        // use of size() 
        System.out.println("\nSize of PriorityQueue : " + geek.size());

        // Use of toArray() 
        Object[] g = geek.toArray();
        System.out.print ( "Array from PriorityQueue : ");

        for ( int i = 0; i<g.length; i++ )
        {
            System.out.print (g[i].toString() + " ") ;
        }

        System.out.println("\n");

        // Use of toArray(t[] g1) :
        Integer[] g2 = new Integer[5];
        Integer[] g1 = geek.toArray(g2);
        System.out.print ( "Array from PriorityQueue of size 5 : ");

        for ( int i = 0; i<g1.length; i++ )
        {
            System.out.print (g1[i].toString() + " ") ;
        }

        System.out.println("\n");

        // Use of clear() 
        geek.clear();
        System.out.println("PriorityQueue after clear() : " + geek);

    }
}

Output :

geek PriorityQueue : [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]
geek PriorityQueue : null
Use of contains() : true
Iterator values : 2 4 6 8 10 12 14 16 18 20 
geek PriorityQueue : [2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 3050]
Head of PriorityQueue via peek : 2

Head of PriorityQueue via poll : 2
geek PriorityQueue bcz of poll() : [4, 8, 6, 16, 10, 12, 14, 3050, 18, 20]

Can remove : true
geek PriorityQueue bcz of remove() : [4, 10, 6, 16, 20, 12, 14, 3050, 18]

Size of PriorityQueue : 9
Array from PriorityQueue : 4 10 6 16 20 12 14 3050 18 

Array from PriorityQueue of size 5 : 4 10 6 16 20 12 14 3050 18 

PriorityQueue after clear() : []

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