Join depending on generic type

Question

My RefineByBasicTaskType should add the same criteria for different CriteriaQuery types. T could be a Task or a Appointment. The only difference is, that there is no join needed if T is a Task.

How can I use my RefineByBasicTaskType class for multiple types?

public class RefineByBasicTaskType<T> extends AbstractRefinement<T> {

private BasicTaskType basicTaskType;

public Predicate addCriteriaQuery(CriteriaQuery<T> criteria, CriteriaBuilder builder, Root<T> taskRoot) {
    
    if (T instanceof Task) {
        return refineByBasicTaskType(builder,
                                     taskRoot);
    }

    return refineByBasicTaskType(builder,
                                 fetchOrCreateFirstJoin(taskRoot,"task"));
}...

Answer 1

From a proper object oriented point of view you are trying to re-invent mechanisms that are already possible with inheritance:

public abstract class RefineByBasicTaskType<T> extends AbstractRefinement<T> {

  public abstract Predicate addCriteriaQuery(CriteriaQuery<T> criteria, CriteriaBuilder builder, Root<T> root);
}

public class RefineByTask extends RefineByBasicTaskType<Task> {
  
  @Override
  public Predicate addCriteriaQuery(CriteriaQuery<Task> criteria, CriteriaBuilder builder, Root<Task> root) {
    return refineByBasicTaskType(builder, root);
  }
}

public class RefineByWhatever extends RefineByBasicTaskType<Whatever> {
  
  @Override
  public Predicate addCriteriaQuery(CriteriaQuery<Whatever> criteria, CriteriaBuilder builder, Root<Whatever> root) {
    return refineByBasicTaskType(builder, fetchOrCreateFirstJoin(root, "task"));
  }
}

Edit: If you don't want to use inheritance you should be able to get the Java type from the Root object so you could also replace your if-statement with:

if (taskRoot.getJavaType().equals(Task.class)) {
  // ...
}