Question

Been working on this for a while now and I think I’ve finally cracked it, it’s working for all my tests, but I have a feeling there will be some niggling issues. This is a heavily simplified version of a double sided queue (deque) where every time a value is added, a temporary array is made to store all values, and then the new value appended on. It is easiest to explain this way, I believe. If someone could please just double-check I am correct and there is nothing glaringly wrong here, I would be extremely thankful. Thank you all very much ! 🙂

    public class ArrayBasedDeque<EltType> implements Deque<EltType> {

  private final int CAPACITY = 10;
  private int capacity;
  private int end;
  private EltType deque[];  

  public ArrayBasedDeque() {
    this.capacity = CAPACITY;
    deque = (EltType[]) (new Object[capacity]);  
  }
  public EltType first() {
    return  deque[0];
  }
  public EltType last() {
    return deque[end-1];
  }

  public boolean isEmpty() {
    return end == 0;
  }

  public int size() {
   return deque.length;
  }

  public boolean isFull() {
   return end == capacity;
  }
  public void insertFirst(EltType inserted) {
    if (!isEmpty()) {
    EltType[] tempArray;
    capacity+=1;
    tempArray = (EltType[]) new Object[capacity];
    for(int i=0;i<end;i++){
    tempArray[i+1] = deque[i]; 
    }
    deque=tempArray;
    }
    deque[0] = inserted;
    end++;
    }

  public void insertLast(EltType last) {
    if (isFull()){
          EltType[] tempArray;
          capacity+=1;
      tempArray = (EltType[]) new Object[capacity];
      for (int i=0;i<end;i++) {
        tempArray[i] = deque[i]; 
      }
//      System.out.print(deque[end]);
    }
    deque[end] = last;   
    end++;
  }

  public EltType removeFirst() {
    EltType[] tempArray;
    EltType returned = deque[0];
    tempArray = (EltType[]) new Object[capacity];
      for (int i=1;i<capacity;i++) {
        tempArray[i-1] = deque[i]; 
      }
      deque = tempArray;
      end--;
    return returned;
  }

  public EltType removeLast() {
    EltType[] tempArray;
    EltType returned = deque[end-1];

    tempArray = (EltType[]) new Object[capacity];
      for (int i=0;i<capacity;i++) {
        tempArray[i] = deque[i]; 
      }
      deque = tempArray;
      end--;
    return returned;
  }




}

Answer 1

A few comments:

• I would use T or E as the name of the type parameter, rather than EltType
• I’d rename the constant CAPACITY to DEFAULT_CAPACITY, and make it static.
• first() will return a value even if the deque is logically empty
• last(), removeLast() and removeFirst() should throw an appropriate exception if end is 0
• There’s no point in having a capacity separate from the size unless you’re using that to avoid creating a new array each time. If you’re always going to expand/shrink the array on any change, just use the array on its own – you can tell the size just from the array’s length
• In removeFirst and removeLast your loop bound is capacity instead of end
• Use System.arraycopy as a simpler way to copy arrays
• You haven’t got an assignment to deque in insertLast – hence the exception you’re seeing in the comments.

I’m not sure I see the benefit of having this over just using ArrayList<T> though… the main point of having a separate Deque implementation would be to make adding to both head and tail cheap… here we have neither!

… or of course just use ArrayDeque or LinkedList 🙂