Question

Given a generic tree based (Java) data model. Let’s say

abstract class XModel {
  long id;
  XModel parent;
}

class ProjectModel extends XModel {
  String customer;
}

class FileModel extends XModel {
  String name;
}

class FolderModel extends XModel {
  String name;
  String attributes;
}

My challenge is to make sure that FileModels are only used in FolderModels (or PrjectModels) and FolderModels are only used in ProjectModels.
Since the model should be extensible – is there any generic way to do this constraint validation (like XML Schema does) without hardcoding the parent-child-realtions in a validation method?

Answer 1

I would use generics and interfaces that describe the restrictions

  interface Model {

  }

  abstract class AbstractModel<P extends Model> implements Model {

    Long id;
    P    parent;
  }

  class ProjectModel extends AbstractModel implements HasFileModel,
      HasFolderModel {

  }

  interface HasFileModel extends Model {
  }

  static class FileModel extends AbstractModel<HasFileModel> {

  }

  interface HasFolderModel extends Model {

  }

  class FolderModel extends AbstractModel<HasFolderModel> implements
      HasFileModel {

  }

This way the child only restricts the parent to an interface not a concrete type