Java says that the default length of a JAVA array at the time of initialization will be 10.
private static final int DEFAULT_CAPACITY = 10;
But the size() method returns the number of inserted elements in the array, and since at the time of initialization, if you have not inserted any element in the array, it will return zero.
private int size;
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,size - index);
elementData[index] = element;
size++;
}