CSC152 2004F, Class 17: Inheritance

Admin:
* HW due.
  * A note on casting.  
  * Suppose A and B are in different classes.  Should the be equal if they have the same string representation?  If so, use
  return this.toString().equals(other.toString());
    * Good idea: More general, "March" might reasonably be the same as the thing created by new Month(3).
    * Bad idea: If something's in a different class, logic tells us that it's different, and therefore not equal.  (Equality of name says nothing about actual equality.)
* HW assigned.
* Exam distributed (and discussed).
  * Ask questions in class.
  * Email me questions.
  * Visit me in the office to ask questions.
  * Call me at home if necessary (before 10 p.m.)
* Grading still not complete.  Apologies.

Overview:
* Code reuse
* Inheritance: Reusing code from class to class
* Inheritance in Java
  * Polymorphism
  * Single inheritance
* A problem: Constructors
* Example

/Code Reuse/

* An important goal of software engineering: Programmers should not have to "reinvent the wheel".
  * If you have to solve a problem similar to one already solved, you should be able to use as much of the previous code as possible.
* Most OO languages provides two forms of code reuse:
  * Polymorphism: If you write code that uses a "general" set of methods, it can work with anything that provides those methods.  (A graduate student can test anything that provides the Vec2D methods; we can compute the square of anything that provides a multiplyBy method.)
  * Inheritance: Once we've built one class, we can build a new, similar, class that adds or replaces capabilities without copying code.

/Inheritance in General/

* We tend to talk about inheritance in terms of the "is a" relationship.
  * Every fraction is a number.
  * Every circle is a drawable object.
  * Every library book is a book.
  * Every colored point is a point.
* Terminology: The class on the left is a subclass, the class on the right is a superclass
  * Fraction is a subclass of Number
  * Point is a superclass of ColoredPoint
* Language design decision: Can one class have more than one superclass?
  * Yes: Models reality well.
  * No: Can be confusing.  What if both superclasses have a method with the same name but different meanings?
  * Java says "only one superclass"
* Benefits of inheritance (the "is a" relationship).
  * The subclass automatically "inherits" all the fields of the superclass.
  * The subclass automatically "inherits" all the methods of the superclass
* However,
  * The subclass may add new fields and methods.
  * The subclass can "override" existing methods, giving them different definitions.

/Inheritance in Java/

* Write
	public class SUBCLASS
		extends SUPERCLASS
	{
	}

* Forget about the homework assignment.  Just work on the exam.