public ArrayList() {
  this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

public ArrayList(int initialCapacity) {
  if (initialCapacity >0) {
    this.elementData =new Object[initialCapacity];
  }else if (initialCapacity ==0) {
    this.elementData = EMPTY_ELEMENTDATA;
  }else {
    throw new IllegalArgumentException("Illegal Capacity: "+
                      initialCapacity);
  }
}

public boolean add(E e) {
    ensureCapacityInternal(size + 1); // Increments modCount!!
    elementData[size++] = e;
    return true;
  }

private void ensureCapacityInternal(int minCapacity) {
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
  }

private static int calculateCapacity(Object[] elementData, int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
      return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    return minCapacity;
  }

private void ensureExplicitCapacity(int minCapacity) {
   modCount++;

   // overflow-conscious code
   if (minCapacity - elementData.length > 0)
     grow(minCapacity);
 }

private void grow(int minCapacity) {
   // overflow-conscious code
   int oldCapacity = elementData.length;
   int newCapacity = oldCapacity + (oldCapacity >> 1);
   if (newCapacity - minCapacity < 0)
     newCapacity = minCapacity;
   if (newCapacity - MAX_ARRAY_SIZE > 0)
     newCapacity = hugeCapacity(minCapacity);
   // minCapacity is usually close to size, so this is a win:
   elementData = Arrays.copyOf(elementData, newCapacity);
 }