Where is array s length property defined

Question

As you know, to determine the length of arrayList<E> using public method size(), we do this

ArrayList<Integer> arr = new ArrayList(10);
int size = arr.size();

In the same manner, using length, we can determine the length of array by this

String[] str = new String[10];
int size =  str.length;

The size() method of arrayList is defined in the arrayList class, so where is the length defined for array?