final Button divide = new ButtonOperator(controller,
new OperatorDivide());
g.setWidget(0, 3, divide);
final Button multiply = new ButtonOperator(controller,
new OperatorMultiply());
g.setWidget(1, 3, multiply);
final Button subtract = new ButtonOperator(controller,
new OperatorSubtract());
g.setWidget(2, 3, subtract);