Engr 691-10: Special Topics in Engineering Science
Software Architecture
Spring Semester 2002
Lecture Notes

Multiple Inheritance

This Web document was based on a set of "slides" created by the instructor for use in the Fall 1997 and spring 1999 offerings of the object-oriented design and programming course. That set was, in turn, based on a set of slides created by Timothy Budd to supplement chapter 13 of his textbook An Introduction to Object-Oriented Programming, Second Edition (Addison-Wesley, 1997).

Orthogonal Classifications

Objects often characterized in different ways that are orthogonal to each other

For example, the instructor is

• North American

• Male

• Professor

• Computer scientist

None of these are proper subsets of each other

Cannot be placed into an inheritance hierarchy

CS Example -- Complex Numbers

Two abstract classifications:

Magnitude:

things that can be compared to each other

Number:

things that can perform arithmetic

Three specific classes:

Integer:

comparable and arithmetic

Char:

comparable but not arithmetic

Complex:

arithmetic but not comparable

Solutions

1. Make Number subclass of Magnitude, but redefine comparison operators in class Complex to print error message

   -- subclassing for limitation

2. Don't use inheritance at all -- redefine all operators in all classes

   -- flattening the inheritance tree

3. Use inheritance for some relationships, but simulate others -- use Number, but have each number implement all relational operators

4. Make Number and Magnitude independent, and have Integer inherit from both

   -- multiple inheritance

Inheritance as a Form of Combination

                   ____ Char
              ____|
             |
Magnitude <--|
             |
             |____
                  |____
                   ____ Integer
              ____|
             |
   Number <--|
             |
             |____
                  |____ Complex

Another Example -- Cascading Menus

• A Menu is structure charged with displaying itself when selected by users

• A Menu maintains collection of MenuItems

• Each MenuItem knows how to respond when selected

• A cascading menu is both a MenuItem and Menu

Multiple Inheritance: Name Ambiguity Problem

What happens when same name is used in both parent classes?

• A CardDeck knows how to draw (select) a Card

• A GraphicalItem knows how to draw (display) an image on screen

• A GraphicalCardDeck should be able to draw -- but which?

Multiple Inheritance: Common Ancestors?

What happens when parent classes have common root ancestor?

Does new object have one or two instances of common ancestor?

                ____ InStream  ____
           ____|                   |____
          |                             | 
Stream <--|                             |<-- InOutStream
          |____                     ____|
               |____ OutStream ____|

Multiple Inheritance in Java

Java supports multiple inheritance of interfaces (subtypes) but not of classes (subclasses)

interface A { ... }

interface B { ... }

interface AB extends A, B { ... }

interface C { ... }

class X { ... }

class Y extends X implements AB, C { ... }

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