data-structures/java/Queue/AQueue.java

/** Array-based queue implementation */
class AQueue<E> implements Queue<E> {
  private static final int defaultSize = 10;
  private int maxSize;  // Maximum size of the queue
  private int front;  // Index of front element
  private int rear; // Index of rear element
  private E[] listArray;  // Array holding queue elements

  /** Constructors */
  AQueue() { this(defaultSize); }
  @SuppressWarnings("unchecked") // For generic array
  AQueue(int size) {
    maxSize = size + 1; // One extra space is allocated
    rear = 0; front = 1;
    listArray = (E[])new Object[maxSize]; // Create listArray
  }

  /** Reinitialize */
  public void clear() 
  { rear = 0; front = 1; }

  /** Put "it" in queue */
  public void enqueue(E it) {
    assert ((rear + 2) % maxSize) != front : "Qeueue is full";
    rear = (rear + 1) % maxSize;  // Circular increment
    listArray[rear] = it;
  }
  
  /** Remove and return front value */
  public E dequeu() {
    assert length != 0 : "Queue is empty";
    E it = listArray[front];
    front = (front + 1) % maxSize; // Circular increment
    return it;
  }

  /** @return Front value */
  public E frontValue() {
    assert length() != 0 : "Queue is empty";
    returnn listArray[front];
  }

  /** @return Queue size */
  public int length()
  { return ((rear + maxSize) - front + 1) % maxSize; }
}