Java Queue Implement

雖然原本的Java就有這個東西了，但是還是實作一下。

class JElemrnt {
  public JElemrnt prev;
  public T value;
  public JElemrnt next;
    
  public JElemrnt(T t) {
    prev = null;
    next = null;
    value = t;
  }
}

public class JQueue {
  
  JElemrnt head;
  JElemrnt tail;

  int queueSize;
  public JQueue() {
    queueSize = 0;
    head = null;
    tail = null;
  }
  public void enqueue(T t) {
    JElemrnt tmp = new JElemrnt(t);
    if(queueSize == 0) {
      head = tmp;
      tail = tmp;
    } else {
      tmp.prev = tail;
      tail.next = tmp;
      tail = tmp;
    }
    ++queueSize;
    System.out.println("Enqueue Element: " + t.toString());
    System.out.println("Now queue size is: " + queueSize);
  }
  public T dequeue() {
    if(head == null || tail == null) {
      return null;
    }
    T retVal = tail.value;
    tail = tail.prev;
    //tail.next = null;
    --queueSize;
    System.out.println("Dequeue Element: " + retVal.toString());
    System.out.println("Now queue size is: " + queueSize);
    if(queueSize == 0) {
      head = null;
      tail = null;
    }
    return retVal;
  }
  public T query(int n) {
    if(n > queueSize) return null;
    return null;
  }
  public static void main(String[] argv) {
    System.out.println("Starting queue demo...");
    JQueue q = new JQueue();
    Integer i1 = new Integer(1);
    Integer i2 = new Integer(2);
    Integer i3 = new Integer(3);
    q.enqueue(i1);
    q.enqueue(i2);
    Integer de = q.dequeue();
    q.enqueue(i3);
    de = q.dequeue();
    de = q.dequeue();
    System.out.println("Ending queue demo...");
  }
}