Java: If this is instanceof Something, display a variable that belongs only to Something?

Question

Everything here is renamed and simplified.

I have a parent class (normalThing) with a method that displays some data about the instance, and another class (specialThing) extending the parent to include another instance variable (addedPrice).

I want my display method to say whether or not the instance is a specialThing, and if it is, to display the addedPrice. But because addedPrice belongs only to specialThing, I can't figure out how to code it!

void displayInfo(){
    StringBuilder d = new StringBuilder("Am I special? ");

    if(this instanceof specialThing){
        d.append("Yes, so I cost an extra " + this.addedPrice);
        //No, I didn't expect that to work, but I had hope
    }
    else d.append("Nope.");
    JOptionPane.showMessageDialog(null, d);
}

Is there any way to do this without essentially making a "could-be-addedPrice" variable in the normalThing class?

Don't make your parent class depend on the child class. **Never.** — Sotirios Delimanolis, Feb 13 '14 at 03:24
@SotiriosDelimanolis **Except** when it's through an abstract (or overridden) method I trust! — Elliott Frisch, Feb 13 '14 at 03:34
@ElliottFrisch By dependency I meant specifically using the child type in the parent's source code. With an `abstract` method you don't do that, so it's fine. — Sotirios Delimanolis, Feb 13 '14 at 03:35

score 2 · Accepted Answer · answered Feb 13 '14 at 03:33

You can achieve this by overriding displayInfo appropriately in child class. That is a better way because parent class should not try to know what are its children because number of child classes and their names are not in the control of parent class. People can extend the parent class in unexpected/unpredictable and uncontrolled manner. It will work well if you define displayInfo contract and enforce the child classes to implement it. Number of child classes does not matter. As long as displayInfo is implemented as per contract, OOP will take care of the rest :-)

Java: If this is instanceof Something, display a variable that belongs only to Something?

1 Answers1