Merge 2 arrays, when one is larger and can accomodate smaller
Do not throw NullPointerException when arguments are null. NPE is thrown when you are trying to access properties and methods of null reference. Not when you are checking on some preconditions. Throw IllegalArgumentException as you do in length validation. In other method you use assertions. Chose one method and stick with it, dont mix them.

You don have to define empty constructor. It is created implicitly.