@Override
public void activate() {
	super.activate();
	
	AbstractEMFOperation emfOp = new AbstractEMFOperation(getEditingDomain(), "Location setting") {
		@Override
		protected IStatus doExecute(IProgressMonitor monitor, IAdaptable info) throws ExecutionException {
			Location lc = (Location) ((Node) getModel()).getLayoutConstraint();
			int x=10;
			int y=10;
			//TODO: ---here the logic to calculate node position
			lc.setX(x);
			lc.setY(y);
			return Status.OK_STATUS;
		}
	};

	try {
		OperationHistoryFactory.getOperationHistory().execute(emfOp, null, null);
	} catch (ExecutionException e) {
		e.printStackTrace();
	}
}

        //here we are in the EditPart of the node we want set the initial position
        
        @Override
	public void activate() {
		super.activate();
		
                //The new rectangle rapresenting the new position and size of the node
                //size to -1 means "use preferredSize"
		Rectangle r = new Rectangle(0, 0, -1, -1);
                ((GraphicalEditPart) getParent()).setLayoutConstraint(this, getFigure(), r);	
	}