Question

2. Identify whether the following statements are TRUE or FALSE:

(a) Subtyping supports code reuse from one class, inside of another class.

(b) Subclasses depend only on the public interfaces of objects, not their implementations.

(c) Interfaces contain fully defined methods and can extend other classes.

(d) If B is a subclass of A, then a variable of type A can refer to either an object of type A or an object of type B.

Answer 1

Question a:

Subclasses allow one to reuse the code inside classes of both instance variable declarations and method definitions. So they are useful in supporting code reuse inside a class. Sub typing on the other hand is helpful in supporting salvage externally, giving rise to a form of polymorphism.

Question B:

The subtype relation depends only on the public interfaces of objects, not their implementations. In particular if one type is a subtype of another, it is not necessary for objects of those types to have come up from classes that are in the subclass relation. With the same argument, objects of the same type need not have come up from the same class. They may have been generated by classes with different collections of instance variables which are hidden and method bodies, but whose methods are visible have the same types. This is similar to the brunt of information hiding in programming languages which support abstract data types.

Question C:

Interfaces are not classes and an interface can extend more than one parent interface. Methods in interfaces do not have body, they have to be implemented by the class before you can access them. The class that implements interface must implement all the methods of that interface.

Question D:

Polymorphism defined as that different objects can retort to the same method in different ways, depending on the actual type of the object. This can occur because a method can be overridden in a subclass. So, objects belonging to the subclass will respond to the method differently from objects belonging to the superclass.