How do I shuffle certain parts of an array in java?

In my program, I have an array that changes sizes (16 elements, 36 elements, and 64 elements) that I need to randomly shuffle. I want to have one block of code or method so that it will shuffle only those elements of the array but not include the "null" values. So is there a way to only shuffle specific parts of an array Ex. first 16, 36, or 64?