Re: Forcing sub-classes to implement Constructor

At 08:14 AM 8/9/2007, you wrote:
>Brian Maso wrote:
>>No.
>>
>>Constructors are more like static methods than regular methods: they
>>can't be declared abstract and thus force subclasses to have them. You
>>can add tools to your build to recognize certain annotations that
>>trigger verifier rules. For example, you could define an annotation
>>such as:
>
>Hmm. Brian appears to be correct after a quick test.
>
>Whats weird is that I *know* I've seen situations where the definition
>of a super class contrutctir forced the definition of sub class one...

A subclass constructor always *must* invoke a 
superclass constructor. If you have a superclass 
with only non-default constructors visible to a 
subclass, then the subclass cannot rely on the 
compiler-provided default constructor and must 
instead explicitly define a constructor that 
explicitly invokes a superclass constructor. For example:

public class Base {
   //...only public constructor...
   public Base(int x) {
     ...
   }
   ...
}


public class Sub extends Base {
   //...must explicitly class superclass constructor...
   public Sub() {
     super(0);
     ...
   }
   ...

   //...more typically, subclasses will have c-tors with same signature as
   //   superclass and just pass-through the c-tor args, like so...
   public Sub(int x) {
     super(x);
   }
   ...
}

Brian Maso



>JK
>>
>>@SubclassConstructorRequiredPattern(presense=REQUIRED,
>>constructor=PUBLIC_DEFAULT)
>>
>>which would be applied to the base class. Some part of your build then
>>would scan code either jsut before or just after compilation to see if
>>it breaks all the rules defined by such annotations.
>>
>>Sounds like an awful lot of work, though. What's your use-case? Why do
>>you ned subclasses to supply a public default constructor?
>>
>>Brian Maso
>>
>>At 07:13 AM 8/9/2007, you wrote:
>>>Is there a way to force subclasses of a class to implement a certain
>>>(read
>>>empty) constructor.
>>>
>>>mvh roger j
>>>
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