CSE115: Introduction to Computer Science I

1. CSE115: Introduction to Computer Science I Dr. Carl Alphonce 219 Bell Hall Office hours: M-F 11:00-11:50 645-4739 alphonce@buffalo.edu

2. Announcements • Exam 3 on Wednesday 11/11 • covers material from last exam up to and including Friday 11/06 • Exam review in lecture on Monday 11/09 • study over the weekend, and bring questions to class on Monday • Sample questions on web-site over the weekend

3. Agenda • Inheritance – our last relationship! • we will only introduce inheritance today • we’ll continue after the exam

4. Example from Wednesday

5. What’s wrong with this code? package pkg; public class UTA implements TA { private String _name; public UTA(String name) { _name = name; } public String job() { return "teach recitations"; } public String name() { return _name; } } package pkg; public class GTA implements TA { private String _name; public GTA(String name) { _name = name; } public String job() { return "grade labs"; } public String name() { return _name; } }

6. Code duplicationa “code smell” package pkg; public class UTA implements TA { private String _name; public UTA(String name) { _name = name; } public String job() { return "teach recitations"; } public String name() { return _name; } } package pkg; public class GTA implements TA { private String _name; public GTA(String name) { _name = name; } public String job() { return "grade labs"; } public String name() { return _name; } }

7. Number one in the stink parade is duplicated code. If you see the same code structure in more than one place, you can be sure that your program will be better if you find a way to unify them. Refactoring: Improving the Design of Existing Code, Martin Fowler, page 76

8. Inheritance • Inheritance is the last of the relationships we will study this semester. • It is a relationship between: • two classes • two interfaces • Inheritance is (syntactically) simple • Inheritance is (conceptually) messy

9. class to class inheritance

10. In code: public abstract class AbstractTA {…} public class UTA extends AbstractTA{…}

11. Abstract class • A class which mixes method specifications (abstract methods) with fully defined methods (concrete methods) is abstract. • An interface contains only abstract methods (they are labelled ‘abstract’ implicitly).

12. Abstract class • ‘abstract’ keyword in class header • cannot be instantiated

13. Inheritance(“extends”) • Source class: • subclass • child class • derived class • Target class: • superclass • parent class • base class

14. Implications of “extends” • Same type implications as for interfaces: • instance of subclass belongs to subclass type and superclass type • inheritance: non-private members of superclass can be accessed via subclass object. • e.g. it’s as if methods of superclass were defined in subclass

15. [A] common duplication problem is when you have the same expression in two sibling subclasses. You can eliminate this duplication by using Extract Method (110) in both classes then Pull Up Method (322). Refactoring: Improving the Design of Existing Code, Martin Fowler, page 76

16. Code duplicationa “code smell” package pkg; public class UTA implements TA { private String _name; public UTA(String name) { _name = name; } public String job() { return "teach recitations"; } public String name() { return _name; } } package pkg; public class GTA implements TA { private String _name; public GTA(String name) { _name = name; } public String job() { return "grade labs"; } public String name() { return _name; } }

17. Refactored code(-: a breath of fresh air :-) package pkg; public class UTA extends AbstractTA { public UTA(String name) { super(name); } public String job() { return "teach recitations"; } } package pkg; public class GTA extends AbstractTA { public GTA(String name) { super(name); } public String job() { return "grade labs"; } } package pkg; public abstract class AbstractTA { private String _name; public AbstractTA(String name) { _name = name; } public abstract String job(); public String name() { return _name; } }