Re: calling own methods from constructor

On Wed, 6 Apr 2011, Andreas Leitgeb wrote:


public abstract class Library {
  private List<Document> documents;

  protected Library() {
  documents = new ArrayList<Document>();
  Collection<String> titles = listDocuments();
  for (String title: titles) {
  Document doc = loadDocument(title);
  // do other preparatory stuff with the document
  documents.add(doc);
  }
  }

  protected abstract Collection<String> listDocuments();
  protected abstract Document loadDocument(String title);
}

public class FilesystemLibrary extends Library {
  // ...
}

public class WebDavLibrary extends Library {
  // ...
}

public class JCRLibrary extends Library {
  // ...
}

What are the alternatives?

The obvious one is for the subclass constructor to prepare all the objects
and pass them upward; i think that is likely to lead to a lot of
duplication of effort.

The almost as obvious one is to push the abstract methods out into a
separate interface - DocumentStore, say - and have the subclass
constructor pass up an instance of that.

You could also push the repeated logic out into some sort of factory or
helper, and have the subclasses call that, rather than relying on code in
the supeclass, but that is repetitive, and does nothing to establish
invariants in the superclass.

tom

--
Science which is distinguishable from magic is insufficiently advanced