JavaAlgorithms/DataStructures/Queues/PriorityQueues.java

package DataStructures.Queues;

/**
 * This class implements a PriorityQueue.
 * <p>
 * A priority queue adds elements into positions based on their priority.
 * So the most important elements are placed at the front/on the top.
 * In this example I give numbers that are bigger, a higher priority.
 * Queues in theory have no fixed size but when using an array
 * implementation it does.
 */
class PriorityQueue {
    /**
     * The max size of the queue
     */
    private int maxSize;
    /**
     * The array for the queue
     */
    private int[] queueArray;
    /**
     * How many items are in the queue
     */
    private int nItems;

    /**
     * Constructor
     *
     * @param size Size of the queue
     */
    public PriorityQueue(int size) {
        maxSize = size;
        queueArray = new int[size];
        nItems = 0;
    }

    /**
     * Inserts an element in it's appropriate place
     *
     * @param value Value to be inserted
     */
    public void insert(int value) {
        if (isFull()) {
            throw new RuntimeException("Queue is full");
        } else {
            int j = nItems - 1; // index of last element
            while (j >= 0 && queueArray[j] > value) {
                queueArray[j + 1] = queueArray[j]; // Shifts every element up to make room for insertion
                j--;
            }
            queueArray[j + 1] = value; // Once the correct position is found the value is inserted
            nItems++;
        } 
    }

    /**
     * Remove the element from the front of the queue
     *
     * @return The element removed
     */
    public int remove() {
        return queueArray[--nItems];
    }

    /**
     * Checks what's at the front of the queue
     *
     * @return element at the front of the queue
     */
    public int peek() {
        return queueArray[nItems - 1];
    }

    /**
     * Returns true if the queue is empty
     *
     * @return true if the queue is empty
     */
    public boolean isEmpty() {
        return (nItems == 0);
    }

    /**
     * Returns true if the queue is full
     *
     * @return true if the queue is full
     */
    public boolean isFull() {
        return (nItems == maxSize);
    }

    /**
     * Returns the number of elements in the queue
     *
     * @return number of elements in the queue
     */
    public int getSize() {
        return nItems;
    }
}

/**
 * This class implements the PriorityQueue class above.
 *
 * @author Unknown
 */
public class PriorityQueues {
    /**
     * Main method
     *
     * @param args Command Line Arguments
     */
    public static void main(String[] args) {
        PriorityQueue myQueue = new PriorityQueue(4);
        myQueue.insert(10);
        myQueue.insert(2);
        myQueue.insert(5);
        myQueue.insert(3);
        // [2, 3, 5, 10] Here higher numbers have higher priority, so they are on the top

        for (int i = 3; i >= 0; i--)
            System.out.print(myQueue.remove() + " "); // will print the queue in reverse order [10, 5, 3, 2]

        // As you can see, a Priority Queue can be used as a sorting algotithm
    }
}