Demonstration of why the HeadlessFragment gist can't be done as an abstract Factory class

HeadlessFragmentFactory.java

/**
 * What you'd think an abstract factory would look like. Unfortunately, this class won't compile.
 */
public abstract class HeadlessFragmentFactory<F extends Fragment & ParentedFragment<F, ? extends P>, P> {

  protected abstract F newInstance();

  /* There will be a compiler warning on P. When joining types, only the first type is allowed to
     be a class. All others MUST be interfaces. In this situation, there is no way for the compiler
     to know that P will be an interface. If P is removed, there is no longer a compile-time check
     that the parent implements the necessary callback interface */
  public <A extends FragmentActivity & P> F attach(A parent, String tag) {
    return attach(parent.getSupportFragmentManager(), tag);
  }
  
  // Same as above
  public <A extends Fragment & P> F attach(A parent, String tag) {
    return attach(parent.getChildFragmentManager(), tag);
  }

  private F attach(FragmentManager fragmentManager, String tag) {
    F frag = ((F) fragmentManager.findFragmentByTag(tag));
    if (frag == null) {
      frag = newInstance();
      fragmentManager.beginTransaction().add(frag, tag).commitNow();
    }
    return frag;
  }

}

ParentedFragment.java

/** 
 *  Fragment that requires a parent of type P. T is a self-referential generic that also forces
 *  the implementation to extend Fragment
 */
public interface ParentedFragment<T extends Fragment & ParentedFragment<T, P>, P> {
  
  //Example parent interface
  interface FragmentParent {}
  
  //Example fragment implementation
  class ParentedFragmentImpl extends Fragment
      implements ParentedFragment<ParentedFragmentImpl, FragmentParent> {
  }
  
  //Example factory
  HeadlessFragmentFactory<ParentedFragmentImpl, FragmentParent> IMPL_FACTORY
      = new HeadlessFragmentFactory<ParentedFragmentImpl, FragmentParent>() {
    @Override
    protected ParentedFragmentImpl newInstance() {
      return new ParentedFragmentImpl();
    }
  };
}