/**
Searches for the first occurrence of the given argument,
testing for equality using the equals method.
@param elem an object.
@return the index of the first occurrence
of the argument in this list; returns -1 if
the object is not found.
*/
public int indexOf(Object elem)
{
if (elem == null)
{
for (int i = 0; i < size; i++)
if (elementData[i] == null) return i;
}
else
{
for (int i = 0; i < size; i++)
if (elem.equals(elementData[i])) return i;
}
return -1;
}