Kathy Sierra Chapter 2 Selftest Question 13

[Edward Seager]

Hi,

I'm struggling to understand exactly how the following code is working, regarding inheritance and instance variables.

 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();
		System.out.println(m.name + m.makeNoise());
	}
}   

I thought this code would not compile because Zebra inherits name from Mammal but then redefines it, i.e. using the keyword String before name again.

However, the code runs and prints "furby bray" because instance variables are not overriden but methods are.

I would be very greatful if someone could clarify why it is OK to redefine inherited instance variables, as you cannot redefine variables in the same class. E.g The following code does not compile.

class Mammal {
	String name = "furry ";
	String name  ="bray";
	String makeNoise() { return "generic noise"; }
}

Also, where does the book mention that instance variables are not overriden in Chapter 2?

Thanks

[Raju Champaklal]

dude...here the zebra class inherits the member string variable name from its parent class...and you are free to redefine it in your own class if you want....the only difference between inheriting superclass instance methods and variables is that overriding rules only apply to methods and not to variables....and m.name calls the mammals class variable because which instance variable to call depends on the reference variable.....and m.makeNoise() calls the method of zebra becuase which method to call depends on the object at runtime....

[Dimple John]

This is called Field hiding. there are no restrictions in hiding of fields. The type of the fields need not be the same either.It is only field name that matters in hiding of fields.