Upcasting doesn't work as intended; clarification needed

[Sandra Bachan]

Hello,

I have taken code from Sierra/Bates, end of chapter 2, and am playing around with it:

 class Mammal {
   String name = "furry ";
   String makeNoise() { return "generic noise"; }
 }
 class Zebra extends Mammal {
   String name = "stripes ";
   String makeNoise() { return "bray"; }
 }
 public class ZooKeeper {
   public static void main(String[] args) { new ZooKeeper().go(); }
   void go() {
     Mammal m = new Zebra();
     Zebra z = new Zebra();
     System.out.println(m.name + ((Mammal)m).makeNoise());//I want output to be "furry generic noise"
     System.out.println(z.name + z.makeNoise());
   }
 }

Output is:

furry bray
stripes bray

How do I make output such as:

furry generic noise
stripes bray


Thanks!

[Frits Walraven]

Hi Sandra,

You can't.

Two important rules apply here:

A reference variables type determines the methods that can be invoked on the object the variable is referencing

What method is called is decided at runtime based on the object type.

In your case the the method makeNoise() is allowed because the class of the reference variable Mammal has a method called makeNoise(). The actual method to be called is the method on the Zebra (because it overrides the one in Mammal)

Regards,
Frits

[Sandra Bachan]

Totally forgot about the overriding part; now I understand

[Sandra Bachan]

By the way, I noticed when I comment out makeNoise() in Zebra, it invokes makeNoise() in Mammal:

 class Mammal {
   String name = "furry ";
   String makeNoise() { return "generic noise"; }
 }
 class Zebra extends Mammal {
   String name = "stripes ";
   //String makeNoise() { return "bray"; }
 }
 public class ZooKeeper {
   public static void main(String[] args) { new ZooKeeper().go(); }
   void go() {
     Mammal m = new Zebra();
     Zebra z = new Zebra();
     System.out.println(m.name + ((Mammal)m).makeNoise());//I want output to be "furry generic noise"
     //System.out.println(z.name + z.makeNoise());
   }
 }

Output:

furry generic noise

[Frits Walraven]

correct, now it just inherits the method from its super class.

Regards,
Frits

[Tahir Akram]

Sandra Bachan wrote:Hello,

I have taken code from Sierra/Bates, end of chapter 2, and am playing around with it:

 class Mammal {
   String name = "furry ";
   String makeNoise() { return "generic noise"; }
 }
 class Zebra extends Mammal {
   String name = "stripes ";
   String makeNoise() { return "bray"; }
 }
 public class ZooKeeper {
   public static void main(String[] args) { new ZooKeeper().go(); }
   void go() {
     Mammal m = new Zebra();
     Zebra z = new Zebra();
     System.out.println(m.name + ((Mammal)m).makeNoise());//I want output to be "furry generic noise"
     System.out.println(z.name + z.makeNoise());
   }
 }

Output is:

furry bray
stripes bray

How do I make output such as:

furry generic noise
stripes bray


Thanks!

Mammal m = new Zebra();
Zebra z = new Zebra();

System.out.println(m.name + ((Mammal)m).makeNoise());
System.out.println(m.name + ((Zebra)m).makeNoise());
System.out.println(m.name + m.makeNoise());

Output

furry bray
furry bray
furry bray