Abstract Class - clarification please

debugcn Published at Dev

user3799584

I read the tutorial stuff about abstract classes and am a little unclear on something..

I have an abstract class

abstract class Master implements ActionListener {
    static Point p;
    static JFrame glass;

    /* code to define glass and a mouseListener to it.  
       The mouselistener essentially registers the mouseclick 
       and point P where it happened  */
}

And the I extend some others

class FirstMaster extends Master {
/* some definitions and stuff */
}

class SecondMaster extends Master {
/* some definitions and stuff */
}

My questions

Is the static variable p and JFrame specific to both FirstMaster and SecondMaster, that is Master.p, FirstMaster.p and SecondMaster.p are the one and the same Point with a single memory location?
Each (instance of) First and Second Master is an independent ActionListener and needs to define the associated method - correct?
Without the modifiers static it would be equivalent to simply copy and paste the code in Master into the First and Second and get rid of the extends?

Sorry for the newbie question but I didnt see an example to figure it out definitively.

EDIT: Since there is a lot of info below I thought I would summarize my understanding of the answers for the benefit of subsequent readers (and please correct if I am mistaken)

There is only one p. The extension does not create a new static p for each subclass and this fact is independent of the abstract modifier.
the method actionPerformed() can be specified in Master or in BOTH First and Second definitions. Specifying it in First and Second allows the response to the action to be unique to each subclass.
Ignoring the complication with the implement ActionListener, the answer is yes, although this would result in redundant code.

Thanks to all.

ajb

Question 1: When you declare a static member object (field) in a top-level class, there is only one of that object in the entire program, period. If the object is x and it's in class Class1, Class1.x will refer to that object. If you have a subclass Class2 that extends Class1, Class2.x is another way to refer to the same object, if x isn't hidden by another declaration of some other x. But a new static object is not created for the subclass. None of this depends on whether the classes are abstract or not.

Question 2: Any methods declared in ActionListener need to be implemented, by writing bodies for the methods. You can implement them in Master or not. Whatever methods you don't implement in Master, you need to implement in both FirstMaster and SecondMaster. If you implement them in Master, you could still write overriding methods in FirstMaster or SecondMaster, but if you don't, they'll inherit the methods you wrote in Master. I'm not sure what you mean by "each instance" having to define the associated method; you do have to define it in each (non-abstract) subclass.

Question 3: Getting rid of extends would create a completely different program. The way you've written it, if you have a method that needs an ActionListener:

public void addListener(ActionListener listener);

you can pass it an instance of a FirstMaster or a SecondMaster, since instances of those classes are also indirectly instances of ActionListener. If you get rid of extends Master, you can't do that. Even if you get rid of extends Master and add implements ActionListener, you could no longer write a method like

public void addMasterListener(Master m);

In order to write a method like this and have it work, you need to be able to pass it instances of FirstMaster or SecondMaster. So (as Thomas Uhrig commented), using the extends is a way to improve your program by using polymorphism, not just a way to avoid code duplication.

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edited at2021-06-28

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