Bug 125420 [Commands] parameters: allow commands to specify return type
bundles/org.eclipse.core.commands/src/org/eclipse/core/commands/Command.java
Tracing.printTrace("COMMANDS", "execute" + Tracing.SEPARATOR 
					+ "starting: id=" + getId() + "; event=" + event); 
		}
		if (executionListeners != null) {
			final Object[] listeners = executionListeners.getListeners();
			for (int i = 0; i < listeners.length; i++) {
				final IExecutionListener listener = (IExecutionListener) listeners[i];
				listener.preExecute(getId(), event);
			}
		}
	}
	/**
	 * Returns the category for this command.
	 * 
	 * @return The category for this command; never <code>null</code>.
	 * @throws NotDefinedException
	 *             If the handle is not currently defined.
	 */
	public final Category getCategory() throws NotDefinedException {
		if (!isDefined()) {
			throw new NotDefinedException(
					"Cannot get the category from an undefined command"); 
		}
		return category;
	}
	/**
	 * Returns the parameter with the provided id or <code>null</code> if this
	 * command does not have a parameter with the id.
	 * 
	 * @param parameterId