When to implement an interface and when to extend a superclass?

Question

I\'ve been reading a lot about interfaces and class inheritance in Java, and I know how to do both and I think I have a good feel for both. But it seems that nobody ever rea

Answer 1

No one?

http://mindprod.com/jgloss/interfacevsabstract.html

EDIT: I should supply more than a link

Here's a situation. To build on the car example below, consider this

interface Drivable {
    void drive(float miles);
}

abstract class Car implements Drivable { 
    float gallonsOfGas;
    float odometer;
    final float mpg;
    protected Car(float mpg) { gallonsOfGas = 0; odometer = 0; this.mpg = mpg; }
    public void addGas(float gallons) { gallonsOfGas += gallons; }
    public void drive(float miles) { 
        if(miles/mpg > gallonsOfGas) throw new NotEnoughGasException();
        gallonsOfGas -= miles/mpg;
        odometer += miles;
    }
}

class LeakyCar extends Car { // still implements Drivable because of Car
    public addGas(float gallons) { super.addGas(gallons * .8); } // leaky tank
}

class ElectricCar extends Car {
    float electricMiles;
    public void drive(float miles) { // can we drive the whole way electric?
         if(electricMiles > miles) {
             electricMiles -= miles;
             odometer += miles;
             return;                 // early return here
         }
         if(electricMiles > 0) { // exhaust electric miles first
             if((miles-electricMiles)/mpg > gallonsOfGas) 
                 throw new NotEnoughGasException();
             miles -= electricMiles;
             odometer += electricMiles;
             electricMiles = 0;
         }
         // finish driving
         super.drive(miles);
    }
}