CIS110
Inheritance Practice
Assume that statement below compile and runs without errors:
> A data = new C();
1. What can we infer about relationship between A and C?
A is supertype and C is subtype (C is a A). This is due to type rules – a reference
variable of type t may refer to an object of its own type or any subtype.
2. Assume B is a class that inherit from A. Class B has a method b() but C does not.
Class C has a methods c() but B does not. Consider the following:
> A data = new C();
For the following interaction answer yes or no if the code compiles with error. If it
compiles does it have run-time/semantic errors. Explain.
> ((C)data).c();
Compiles. Without the cast it would be compile error as c() is not in class A and
variable data is of type A. It also executes without errors as variable data is
referring to an instance of C.
> ((B)data).b();
Compiles. Without the cast it would be compile error as b() is not in A and
variable data is of type A. However, it does not execute as variable data is referring
to an instance of C. We cannot convert an instance of type C to a B.
3. Consider toString() in classes A and C. Consider the following:
> A data = new C();
Consider the execution
> data //or can also do data.toString()
From which class is the toString method executed?
From class C. Variable data is referring to object of type C. Due to overriding, java
picks the method lowest in the inheritance chain. If toString( ) was not in class C,
then the method written in class A would be executed by object of type C as it would
inherit the method from class A. If both A and C did not have toString() method,
then method from Object class would be executed (parent of all) and whose default
behavior is to return’s objects heap address.
