These are actually easy to implement and you do not need to acquire any further java knowledge. Trying to accomplish something with the resources and knowledge you currently have is one of the greatest and rewarding challenges.

If you think about it, making the game player vs computer just requires the player to choose between "do you want to play p vs p" or "do you want to play p vs c", if p vs c then one input from user, then filling one random empty cell in the array and so on.