Extracting something from a selection

That's a somewhat tedious task in JFace programming: you receive an object that represents the selection on a tree or table viewer; the selection object is of type ISelection (a JFace interface), but that means it could be either an IStructuredSelection (one or more elements from a tree, table or list) or an ITextSelection (some text marked in a text editor); thus you need to check the subtype of ISelection first, then you have to check whether the selection is empty, then you can get the first element (for it might be a multi-selection, too), then you have to cast that into the correct type. All in all, you often end up with something long-winded like this:

selectionService.addPostSelectionListener( new ISelectionListener() {
  public void selectionChanged( IWorkbenchPart part, ISelection selection ) {
    if( selection instanceof IStructuredSelection && !selection.isEmpty() ) {
      IStructuredSelection strusel = (IStructuredSelection)selection;
      Object element = strusel.getFirstElement();
      if( element instanceof MyCoolType ) {
        MyCoolType coolThing = (MyCoolType)element;
        // ... and now we finally can do something with the cool thing
      }
    }
  }
} );

And you think: oh well, shouldn't this be a little easier to do?

You are right to ask that question. Here's one way to do it a little more elegantly:

public void selectionChanged( IWorkbenchPart part, ISelection selection ) {
  MyCoolType coolThing = new ElementFrom( selection ).as( MyCoolType.class );
  // ... now do something with coolThing already
}

And this would be the implementation of the ElementFrom utility:

/** Extract the first element (if any) from a JFace selection, in a type-safe
  * manner.
  */
public class ElementFrom {

  private final ISelection selection;

  /** constructs a new extraction operation.
   *
   * @param selection a JFace selection object from which to extract the
   * selected element. Can be null, in which case this extraction
   * operation yields null.
   */
  public ElementFrom( ISelection selection ) {
    this.selection = selection;
  }

  /** retrieves the extracted element typed as specified.
   *
   * @param cls the class representing the expected type of the extracted element.
   *            Must not be null. 
   * @param  the expected type of the extracted element.
   * @return    the element under the expected type, or null
   */
  public  T as( Class cls ) {
    return extractElement( cls );
  }

  private  T extractElement( Class cls ) {
    T result = null;
    if( selectionIsGood() ) {
      IStructuredSelection strusel = (IStructuredSelection)selection;
      Object element = strusel.getFirstElement();
      if( elementTypeIsGood( element, cls ) ) {
        result = cls.cast( element );
      }
    }
    return result;
  }

  private  boolean elementTypeIsGood( Object element, Class cls ) {
    return cls.isAssignableFrom( element.getClass() );
  }

  private boolean selectionIsGood() {
    return selection instanceof IStructuredSelection && !selection.isEmpty();
  }
}

Have fun :-)