@Override
 public void add(T element) {
 Objects.requireNonNull(element);
 if (elements.length != size) {
 elements[size] = element;
 } else {
 Object[] copy = new Object[size * 2];
 System.arraycopy(elements, 0, copy, 0, size);
 elements = copy;
 }
 size++;
 }

We should check at least 5 elements (if InitialCapaciti is 5):

 void add() {
 arrayList.add(10);
 arrayList.add(15);
 arrayList.add(20);
 arrayList.add(21);
 arrayList.add(24);
 arrayList.add(845);
 Object[] internalArray = getTestArray();
 assertThat(internalArray[0]).isEqualTo(10);
 assertThat(internalArray[1]).isEqualTo(15);
 assertThat(internalArray[2]).isEqualTo(20);
 assertThat(internalArray[3]).isEqualTo(21);
 assertThat(internalArray[4]).isEqualTo(24);
 assertThat(internalArray[5]).isEqualTo(845);
 }