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Inhertiance??

I know this is a lame question but...

I have a class called Engine:

public class Engine extends Thread
{
protected final int MAX_SPEED = 10;

public void run()
{
.... Run code here
}

... Rest of class...
}

and a derived class

public class SuperEngine extends Engine
{
protected final int MAX_SPEED = 50;
}

Now I implement two other classes

public class Car
{

protected Engine engine;

public Car()
{
engine = new Engine();
engine.start();
}
}

and...

public class SuperCar extends Car
{
public SuperCar()
{
engine = new SuperEngine();
engine.start();
}
}

Some other code I have written shows the cars moving in an applet window:

public class Drawer
{
public void draw()
{
Engine engine1 = car.getEngine();
Engine engine2 = supercar.getEngine();

// ... Do stuff - the point is that this class doesnt know what type of engines each of the cars have so it uses the parent
// class
}
}

My problem is that the both the Car and the SuperCar have the same max speed! grrrr. How do I force the SuperCar class to use the SuperEngine's MAX_SPEED variable rather than the Engine's MAX_SPEED even though it is basically declared as "Engine engine = new SuperEngine();" and referenced as an instance of Engine?
(In cpp I'd use virtual variables but Java doesnt do those!)

Thanks for your help..
PS. Casting is a no go.

Mick Barry

public class Engine extends Thread
{
protected int MAX_SPEED = 10;

public Engine(int maxspeed)
{
MAX_SPEED = maxspeed;
}

public void run()
{
.... Run code here
}

... Rest of class...
}

and a derived class

public class SuperEngine extends Engine
{
public SuperEngine()
{
suoer(50);
}
}

ASKER CERTIFIED SOLUTION

Mayank S

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pjgould

The problem is to do with 'Variable Shadowing' of the field MAX_SPEED in the extended SuperEngine class.

In case you're interested, here's a link to a page giving an explanation of why the above solution works (variable shadowing vs. method overriding):

http://builder.com.com/5100-6370-5031837.html#Listing%20B

It has an example that reflects your problem exactly.

All you need to change in your code is the addition of the getMaxSpeed(){ return MAX_SPEED; } to Engine and SuperEngine.

Mayank S

Yes, of course, because when you access the MAX_SPEED variable directly with 'first' or 'second', then it will print the value of 10. However, while invoking the method, the dynamic dispatch will come into play. That's why I told him that first.getMaxSpeed () and second.getMaxSpeed () will work, but first.MAX_SPEED and second.MAX_SPEED will give the value 10 for both. (Another reason why I prefer never to access class-members directly, unless it is mandatory to do so.)

Cheers,
Mayank.