Where is the length property of the array defined?

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

In order to obtain the length of the ArrayList in this way, the size() method is used.

And

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

This is how you get the length of the Array object.

ArrayList's size() class is defined in the ArrayList class So where is the length property of the array defined? Is it implemented by JVM or defined in the java API class?