121 lines
2.8 KiB
Java
121 lines
2.8 KiB
Java
package DataStructures.Queues;
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/**
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* This class implements a PriorityQueue.
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*
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* <p>A priority queue adds elements into positions based on their priority. So the most important
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* elements are placed at the front/on the top. In this example I give numbers that are bigger, a
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* higher priority. Queues in theory have no fixed size but when using an array implementation it
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* does.
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*/
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class PriorityQueue {
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/** The max size of the queue */
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private int maxSize;
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/** The array for the queue */
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private int[] queueArray;
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/** How many items are in the queue */
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private int nItems;
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/**
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* Constructor
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*
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* @param size Size of the queue
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*/
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public PriorityQueue(int size) {
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maxSize = size;
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queueArray = new int[size];
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nItems = 0;
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}
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/**
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* Inserts an element in it's appropriate place
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*
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* @param value Value to be inserted
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*/
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public void insert(int value) {
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if (isFull()) {
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throw new RuntimeException("Queue is full");
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} else {
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int j = nItems - 1; // index of last element
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while (j >= 0 && queueArray[j] > value) {
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queueArray[j + 1] = queueArray[j]; // Shifts every element up to make room for insertion
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j--;
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}
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queueArray[j + 1] = value; // Once the correct position is found the value is inserted
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nItems++;
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}
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}
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/**
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* Remove the element from the front of the queue
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*
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* @return The element removed
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*/
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public int remove() {
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return queueArray[--nItems];
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}
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/**
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* Checks what's at the front of the queue
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*
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* @return element at the front of the queue
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*/
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public int peek() {
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return queueArray[nItems - 1];
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}
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/**
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* Returns true if the queue is empty
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*
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* @return true if the queue is empty
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*/
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public boolean isEmpty() {
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return (nItems == 0);
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}
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/**
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* Returns true if the queue is full
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*
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* @return true if the queue is full
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*/
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public boolean isFull() {
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return (nItems == maxSize);
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}
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/**
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* Returns the number of elements in the queue
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*
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* @return number of elements in the queue
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*/
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public int getSize() {
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return nItems;
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}
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}
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/**
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* This class implements the PriorityQueue class above.
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*
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* @author Unknown
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*/
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public class PriorityQueues {
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/**
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* Main method
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*
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* @param args Command Line Arguments
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*/
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public static void main(String[] args) {
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PriorityQueue myQueue = new PriorityQueue(4);
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myQueue.insert(10);
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myQueue.insert(2);
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myQueue.insert(5);
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myQueue.insert(3);
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// [2, 3, 5, 10] Here higher numbers have higher priority, so they are on the top
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for (int i = 3; i >= 0; i--)
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System.out.print(
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myQueue.remove() + " "); // will print the queue in reverse order [10, 5, 3, 2]
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// As you can see, a Priority Queue can be used as a sorting algotithm
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}
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}
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