Question

I have this piece of code from GWT in Action:

public void processOperator(final AbstractOperator op) {
        System.out.println("Wordt deze ooit aangeroepen?");
        if (op instanceof BinaryOperator) {
            if ((data.getLastOperator() == null) || (data.isLastOpEquals())) {
                data.setBuffer(Double.parseDouble(data.getDisplay()));
                data.setInitDisplay(true);
            } else {
                data.getLastOperator().operate(data);
            }
            data.setLastOperator(op);
        } else if (op instanceof UnaryOperator) {
            op.operate(data);
        }

        data.setLastOpEquals(false);
}

I want to eliminate the ‘instanceof’ part by using method dispatching:

public void processOperator(final BinaryOperator op) {
    if ((data.getLastOperator() == null) || (data.isLastOpEquals())) {
        data.setBuffer(Double.parseDouble(data.getDisplay()));
        data.setInitDisplay(true);
    } else {
        data.getLastOperator().operate(data);
    }
    data.setLastOperator(op); 
    data.setLastOpEquals(false);
}

public void processOperator(final UnaryOperator op) {
    op.operate(data);
    data.setLastOpEquals(false);
}

But now I run into trouble in the code below from class ButtonOperator. The following code has AbstractOperator as a type in the constructor. The code for the types UnaryOperator and BinaryOperator would look exactly the same, so it feels a little cumbersome having to make special constructors for them containing the exact same code. What is a better approach?

public ButtonOperator(final CalculatorController controller,
        final AbstractOperator op) {
        super(op.label);

        this.addClickHandler(new ClickHandler() {

            @Override
            public void onClick(ClickEvent event) {
                controller.processOperator(op);
            }

        });
        this.setStyleName(CalculatorConstants.STYLE_BUTTON);
    }

Answer 1

Unfortunately, this won’t work. When you have overloaded methods, the decision about which one is called is made at compile time based on the static type of the argument expressions. For example:

UnaryOperator uop = new UnaryOperator(...);
AbstractOperator aop = uop;

...

// This will call the "unary" version of the method
processOperator(uop);

// This will call the "abstract" version of the method
processOperator(aop);

You only get runtime dispatching of method calls when methods are overridden.