Bug 125420 [Commands] parameters: allow commands to specify return type
bundles/org.eclipse.core.commands/src/org/eclipse/core/commands/Command.java
}
	/**
	 * Adds a listener to this command that will be notified when this command
	 * is about to execute.
	 * 
	 * @param executionListener
	 *            The listener to be added; must not be <code>null</code>.
	 */
	public final void addExecutionListener(
			final IExecutionListener executionListener) {
		if (executionListener == null) {
			throw new NullPointerException(
					"Cannot add a null execution listener"); 
		}
		if (executionListeners == null) {
			executionListeners = new ListenerList(ListenerList.IDENTITY);
		}
		executionListeners.add(executionListener);
	}
	/**
	 * <p>
	 * Adds a state to this command. This will add this state to the active
	 * handler, if the active handler is an instance of {@link IObjectWithState}.
	 * </p>
	 * 
	 * @param id
	 *            The identifier of the state to add; must not be
	 *            <code>null</code>.
	 * @param state
	 *            The state to add; must not be <code>null</code>.