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Inhertiance??

Posted on 2003-11-27
4
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Last Modified: 2010-03-31
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.
0
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Question by:Cheney
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4 Comments
 
LVL 92

Expert Comment

by:objects
ID: 9834991
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);
   }
}
0
 
LVL 30

Accepted Solution

by:
Mayank S earned 125 total points
ID: 9835159
Can you post your entire code? I guess the second class should use MAX_SPEED = 50 ;

I tried out this piece of code. See the output:

class Engine
{
      protected final int MAX_SPEED = 10 ;

      protected int getMaxSpeed ()
      {
            return MAX_SPEED ;

      } // end of getMaxSpeed ()

} // class definition over

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

      protected int getMaxSpeed ()
      {
            return MAX_SPEED ;

      } // end of getMaxSpeed ()

} // class definition over

class Car
{
      protected Engine engine ;

      Car () // constructor ()
      {
            engine = new Engine () ;

      } // end of constructor ()

      protected Engine getEngine ()
      {
            return engine ;

      } // end of getEngine ()

} // class definition over

class SuperCar
{
      protected Engine engine ;

      SuperCar () // constructor ()
      {
            engine = new SuperEngine () ;

      } // end of constructor ()

      protected Engine getEngine ()
      {
            return engine ;

      } // end of getEngine ()

} // class definition over

public class TestClass
{
      public static void main ( String args[] )
      {
            Car car = new Car () ;
            SuperCar supercar = new SuperCar () ;
            Engine first = car.getEngine () ;
            Engine second = supercar.getEngine () ;
            System.out.println ( "CAR: " + first.getMaxSpeed () ) ;
            System.out.println ( "SUPERCAR: " + second.getMaxSpeed () ) ;

      } // end of main ()

} // class definition over

It prints:

CAR: 10
SUPERCAR: 50

However, if you print first.MAX_SPEED and second.MAX_SPEED instead of invoking the getMaxSpeed () methods, then it will print 10 for both.

Mayank.
0
 

Expert Comment

by:pjgould
ID: 9835987
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.
0
 
LVL 30

Expert Comment

by:Mayank S
ID: 9836056
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.
0

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