package DataStructures.Queues;

import java.util.ArrayList;

/**
 * This class implements a GenericArrayListQueue.
 * <p>
 * A GenericArrayListQueue data structure functions the same as any specific-typed queue.
 * The GenericArrayListQueue holds elemets of types to-be-specified at runtime.
 * The elements that are added first are the first to be removed (FIFO)
 * New elements are added to the back/rear of the queue.
 *
 */
public class GenericArrayListQueue<T> {
 /**
 * The generic ArrayList for the queue
 * T is the generic element
 */
 ArrayList<T> _queue = new ArrayList<T>();

 /**
 * Checks if the queue has elements (not empty)
 *
 * @return True if the queue has elements. False otherwise.
 */
 private boolean hasElements() {
 return !_queue.isEmpty();
 }

 /**
 * Checks what's at the front of the queue
 *
 * @return If queue is not empty, element at the front of the queue. Otherwise, null
 */
 public T peek() {
 T result = null;
 if(this.hasElements()) { result = _queue.get(0); }
 return result;
 }

 /**
 * Inserts an element of type T to the queue.
 *
 * @param element of type T to be added
 * @return True if the element was added successfully
 */
 public boolean add(T element) {
 return _queue.add(element);
 }

 /**
 * Retrieve what's at the front of the queue
 *
 * @return If queue is not empty, element retrieved. Otherwise, null
 */
 public T poll() {
 T result = null;
 if(this.hasElements()) { result = _queue.remove(0); }
 return result;
 }

 /**
 * Main method
 *
 * @param args Command line arguments
 */
 public static void main(String[] args) {
 GenericArrayListQueue<Integer> queue = new GenericArrayListQueue<Integer>();
 System.out.println("Running...");
 assert queue.peek() == null;
 assert queue.poll() == null;
 assert queue.add(1) == true;
 assert queue.peek() == 1;
 assert queue.add(2) == true;
 assert queue.peek() == 1;
 assert queue.poll() == 1;
 assert queue.peek() == 2;
 assert queue.poll() == 2;
 assert queue.peek() == null;
 assert queue.poll() == null;
 System.out.println("Finished.");
 }
}