Tips on my LIFO java code
@Ted Brownlow has answered your constructor question, but here's a few other points to consider going forward...
Consider your class interface
Your class needs to present an interface that is functional,usable and focused.  At the moment, your class is responsible for both implementing a LIFO and outputting them to the console.  These are really different concerns.  Consider if instead of writing to the console you wanted to write to a web page / different window.  Building the display logic straight into the class makes it less flexible.  A good exercise might be to remove the show method and decide what method(s) you would need to add to your class in order to allow it to be implemented from outside the class.
Bounds checking
At the moment your class has a fixed maximum size of 5.  However, other than by reading the class there's no way for the client to know (possibly missing a maxSize method or similar).  There's also no way for the client to know how many items are in the list, unless they keep track of it themselves (possibly missing a currentSize method or similar).
Without this size information, there's no way for the client to know if it's safe to call push or pop.  Consider what would happen if you already had 5 items when push was called.  What would happen if 0 items were present and pop was called?  At the moment, you're relying on the framework to handle these scenarios.  This can be ok, if it's a conscious decision, but you probably want to consider  adding some bounds checks and throwing exceptions that more explicitly help the user of your class know what they did wrong.