Merge sort Java implementation for an interview
Here are some thoughts on things you might improve:

Always use curly braces, even when they're optional. This is a common source of errors when existing code is modified.

int i == 0; //For left array should be int leftArrayIndex = 0. Don’t use a comment to label a variable .. use the variable’s name to do that. Same thing for j and k.

mid should be midpoint, perhaps? originalArray instead of a? Variable names should be clear and descriptive. Avoid abbreviations where possible.

Almost all Java style guides put a space between if, for, or while and their opening paren. This easily differentiates them from method calls.

Variables that don’t change should be declared final.

Learn and use the libraries. In particular, System.arraycopy() is preferable to looping to manually copy array entries.

merge should be private, not public.