Question

As we know ArrayList increases its size by 50% when elements are added(100% incase of Vector).Where can we find the implementation for this behavior?

Thx

Answer 1

In ArrayList:

public void ensureCapacity(int minCapacity) {
  modCount++;
  int oldCapacity = elementData.length;
  if (minCapacity > oldCapacity) {
    Object oldData[] = elementData;
    int newCapacity = (oldCapacity * 3)/2 + 1;
    if (newCapacity < minCapacity)
      newCapacity = minCapacity;
    // minCapacity is usually close to size, so this is a win:
    elementData = Arrays.copyOf(elementData, newCapacity);
  }
}

and Vector:

private void ensureCapacityHelper(int minCapacity) {
  int oldCapacity = elementData.length;
  if (minCapacity > oldCapacity) {
    Object[] oldData = elementData;
    int newCapacity = (capacityIncrement > 0) ?
        (oldCapacity + capacityIncrement) : (oldCapacity * 2);
    if (newCapacity < minCapacity) {
      newCapacity = minCapacity;
    }
    elementData = Arrays.copyOf(elementData, newCapacity);
  }
}

Note: capacityIncrement defaults to 0 unless set otherwise so the default behaviour of a Vector is to double every time the backing array needs to be expanded but if you set capacityIncrement then it will be incremented by that instead.

Also in all cases (for ArrayList and Vector) the increase–regardless of what it is–is superseded if the new capacity still isn’t large enough, in which case the required capacity is used.