java subclass: multiple constructors inherited from abstract superclass

Question

Basically, I have:

public abstract class AbstractClass {
    public AbstractClass( Type arg0, Type arg1, Type arg2 ) {
        // do some stuff with all those args
    }

    public AbstractClass( Type onlyOneArg ) {
        // do different stuffs with this different arg.
    }

    protected someMethods() { /* ... */ }
}

And I have a few problems in the subclasses:

First, I have to -in most of the cases- uselessly rewrite the constructors. Not very annoying, just a bit dirty to the eye.
And, more important, I am not forced to implement both constructors (although both are used in the program).

Example of my current subclasses:

public class MyClass extends AbstractClass {
    public MyClass( Type arg0, Type arg1, Type arg2 ) {
        super( arg0, arg1, arg2 );
    }

    public MyClass( Type onlyOneArg ) {
        super( onlyOneArg );
    }
}

And

I have to be able to write some particular code in a subclass's constructor if I want.
I have too many shared code that I want to keep in the abstract class.

Can I do something about that ? Are there something I don't know about Java ? Or is my design bad ? Or.. ?

Not sure what the question is. Do you want to be forced to implement all the superclass constructors? What is it about the requirements you've stated that you can't seem to do? — Jim Garrison
– Jim Garrison, Commented Apr 2, 2012 at 3:45
I would be happy to reformulate the question but I'm not quite sure how.. I meant I want to be sure that in every AbstractClass's subclass, both constructors are available. In the program, I'm instantiating with both constructors without knowing the exact type. I only know that the object will be an AbstractClass subclass. So I have to be sure that I can always use both constructors. Is that more understandable ?... Sry, hard to express my thoughts in English ! — user978548
– user978548, Commented Apr 2, 2012 at 3:54
Beside that, I was also wondering if it was possible in some way for java to automatically use the AbstractClass constructor if it is not defined in the subclass, but that is not my main concern. — user978548
– user978548, Commented Apr 2, 2012 at 3:56
"want to be sure that in every AbstractClass's subclass, both constructors are available" Why? What if the subclass doesn't need both constructors? — Jim Garrison
– Jim Garrison, Commented Apr 2, 2012 at 16:42
The subclass represents a real (for the end-user) object which can be created, created by cloning another one, and by some other ways. — user978548
– user978548, Commented Apr 2, 2012 at 19:46

Jordão · Accepted Answer · 2012-04-02 10:30:47Z

2

The subclass has to call either one (or both, if you redefine both as in your example) of the superclass constructors; but you cannot force it to redefine constructors with the same signatures of the superclass.

The only way to guarantee that a superclass constructor is called is to have only one constructor in the superclass.

I think you should think of a way to redesign your superclass (maybe creating 2 classes) to have only one constructor if you want it to always be called.

But, if want a specific constructor to be present in a subclass, you should isolate the "construction" concern in a factory; where you can have special factory classes for each of your subclasses. Your factories would implement this interface:

interface AbstractClassFactory {
  AbstractClass create( Type arg0, Type arg1, Type arg2 );
  AbstractClass create( Type onlyOneArg );
}

edited Apr 2, 2012 at 10:30

answered Apr 2, 2012 at 3:55

Jordão

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2 Comments

user978548 Over a year ago

Just a little related question: I just tested in Eclipse and saw that I can put the factory interface nested in the abstract class, and each subclass factory nested as nested class in their respective class without problem. Is that a bad practice ?

Jordão Over a year ago

@user978548: Not necessarily, it just might confuse your users. Also, your nested factory interface and classes should be static.

Heiko Schmitz · Accepted Answer · 2012-04-02 11:03:18Z

0

Constructors aren't inherited in Java. That might be annoying sometimes, but it is like that. I don't understand, why you want every subclass to implement both constructors. You write "In the program, I'm instantiating with both constructors without knowing the exact type". What do you mean by that? You cannot instantiate an abstract class, you need to know which concrete type you instantiated by "new". If you use reflection, you can check the available constructors on runtime.

answered Apr 2, 2012 at 11:03

Heiko Schmitz

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1 Comment

user978548 Over a year ago

I am using reflection, but the need for different constructor is related to the end-user use. But I'll use factory instead of constructors.

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