I don't think there is such a method, I am afraid. An array has a fixed length.
There is a potential workaround; the elements of an array are a bit like instance fields in that they have default values; booleans default to false, numbers default to 0 (or 0L 0d or 0f) and reference types default to null. You can iterate through the array counting how many elements are different from their default types. Of course if you "fill" an array with 0s that technique will give a totally misleading answer.
Alternative: create a class which incorporates an array and methods to insert and retrieve values. You can have counters for how many values have been inserted, or an array of booleans which change from false to true or vice versa whenever a value is inserted first time, and use those to count insertions.
subject: Is there a method to find what part of array has been filled