Re: abstract class and instantiation

From: flaps81 (net-home_at_it.dk)
Date: 10/23/04

  • Next message: Ash: "Problem starting out with Java . . ." 
    Date: Sat, 23 Oct 2004 23:33:58 +0200

    On Sat, 23 Oct 2004 09:58:42 -0600, "Michael G" <mike-g@montana.com>
    wrote:

    >I am not sure why, in the RefinedAbstraction class, that the abstract
    >baseclass' constructor can be called. I have always thought that abstract
    >classes cannot be instantiated. It is a little confusing. Would it be better
    >programming practice to simply assign imp to this. imp in the subclass?
    >
    >
    >
    >thanks, Mike
    >
    >
    >
    >
    >
    >abstract class Abstraction{
    >
    >
    >
    > protected Implementor imp;
    >
    >
    >
    > public Abstraction(Implementor imp){
    >
    > this.imp = imp;
    > }
    >
    >
    >
    > abstract void operation();
    >
    >
    >}
    >
    >
    >class RefinedAbstraction extends Abstraction{
    >
    >
    >
    > public RefinedAbstraction(Implementor imp){
    >
    > super(imp); //not sure how this can work.
    > }
    >
    >
    >
    > public void operation(){
    >
    > imp.operationImp();
    > }
    >}
    >
    >
    >
    >
    >
    >
    >
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    Don't know where you got the code from. It looks like some sort of
    bridge pattern but something is wrong in my opinion.
    The Abstraction class should look like this:

    abstract class Abstraction {
             protected Implementor implementor;
            
             public void setImplementor(Implementor implementor) {
                     this.implementor = implementor;
             }
            
            abstract public void operation();
    }

    Then you would have an Implementor interface and a class that realizes
    it like this:

    public class ConcreteImplementor implements Implementor {
            public void operation() {
                    System.out.println("My operation");
            }
    }

    A test class could be as follows:

    public class Client {
            public static void main(String[] args) {
                    Abstraction abstraction = new RefinedAbstraction();
                    abstraction.setImplementor(new ConcreteImplementor());
                    abstraction.operation();
            }
    }

    Hopes this helps to understand that you do NOT instantiate in a bridge
    pattern, you use inheritence!

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