Inheritance, part 1

1. Inheritance, part 1 COMP 401, Spring 2014 Lecture 8 2/4/2014

2. Static Class Fields • Indicated by static modifier to field declaration. • Can be accessed via the class name • access modifiers can also be used to control visibility • Contrast with instance fields • Each object has its own copy of an instance field • Value of the instance fields are the state of the object • Only one copy of the class field • Associated with the class as a whole • Common uses • As named constant values associated with the class • In this case, modify the declaration of the field with the keyword final • By convention, named constants are ALL_CAPS • As collective information associated with the class as a whole. • Avoid accessing static class fields through an object. • It does work, but it is misleading and not good style • lec8.ex1

3. Static Class Methods • Methods associated with the class but that are not associated with a specific object. • No this reference within the function. • Common uses • Getters / Setters for static class fields • Helper / auxiliary functions • Factory methods for creating new instances • We’ll revisit this in more detail later later in the class. • lec8.ex2

4. Motivating Enumerations • Often need to model part of an object as one value from a set of finite choices • Examples: • Suite of a playing card • Day of week • Directions of a compass • One approach is to use named constants • lec8.ex3 • Drawbacks of this approach • No type safety • No value safety

5. Simple Java Enumerations • General syntax: access_typeenumEnumName {symbol, symbol, ...}; • Example: • public enum Genre {POP, RAP, JAZZ, INDIE, CLASSICAL} • Enumeration name acts as the data type for the enumerated values. • Enumerated values available as EnumName.symbol as in: Genre.POP • Outside of the class • Fully qualified name required as in: Song.Genre.POP • Symbol names don’t have to all caps, but that is traditional • lec8.ex4

6. Not so simple enumerations • Java enumerations are actually much more powerful than this. • Check out this tutorial for more: http://javarevisited.blogspot.com/2011/08/enum-in-java-example-tutorial.html

7. Recap of Interfaces • A “contract” for behavior. • Defined by a set of method signatures. • Acts as a data type. • No implementation. • Specific classes implement the interface

8. Song and Video as Media public interface Media { intgetLengthInSeconds(); double getLengthInMinutes(); intgetRating(); void setRating(intnew_rating); String getName(); } We expect Song and Video to have methods matching those specified in Media. public class Song implements Media { .... public class Video implements Media { ....

9. Is-A and casting • A class that implements an interface creates an “is-a” relationship between class data type and the interface data type. • A implements B => A “is a” B • Song is a Media • Video is a Media • Casting allowed across an is-a relationship. Song s = new Song(); Media m; m = (Media) s; Video v = new Video(); Media m; m = (Media) v; Video v = new Video(); Song s; s = (Song) v;

10. Inheritance • What is inheritance in real life? • Characteristics / resources that you receive from your parents • Get these automatically. • Part of who you are. • Similar idea in object-oriented programming. • In Java, concept of inheritance applied to both interfaces and classes. • Both signaled by the keyword “extends” • Similar in concept, but details are distinctly different. • Class inheritance more complex.

11. Extending Interfaces • Adds methods to contract. • Original: • parent interface, super interface • New: • subinterface, child interface, extended interface • Created by using the “extends” keyword. public interface CompressedMedia extends Media { intgetCompressedSize(); intgetUncompressedSize(); Media uncompress(); }

12. Extension Creates Hierarchy Media • Is-A relationship is transitive up the hierarchy. extends Compressed Media Methods for both must be provided. public class Song implements CompressedMedia { ... Song s = new Song(); CompressedMedia cm = (CompressedMedia) s; Media m = (Media) s; Song s2 = (Song) m; OK because s “is a” Compressed Media OK because s is a Media by virtue of extension. Casting from interface back to specific object type is allowed, but at runtime, if the object’s type does not actually match, a runtime exception will be thrown.

13. Extension vs. Composition • Interface extension appropriate when additional methods make no sense without methods of the parent interface. • Alternatively, can compose multiple interfaces together as facets of an object.

14. Extension vs. Composition public interface Compressed { intgetCompressedSize(); intgetUncompressedSize(); Media uncompress(); } public interface Media { intgetLengthInSeconds(); double getLengthInMinutes(); intgetRating(); void setRating(intnew_rating); String getName(); } Instead of extending Media, Compressed is a separate interface and Song implements both. public class Song implements Compressed, Media { ... Song s = new Song(); Media m = (Media) s; Compressed c = (Compressed) s; Song “is a” Media AND Song “is a” Compressed.

15. Subinterfacing Odds and Ends • Multiple inheritance for interfaces is allowed. • A subinterface can extend more than one existing interface. • In this case, just a union of all methods declared in all of the parent interfaces. • Plus, of course, the ones added by the subinterfaces. • Interfaces can define enumerations.

16. Assignment 3 • Pixel iterator should NOT make a copy of the pixels in the frame. • SHOULD maintain a reference to the frame and any info needed to keep track of the traversal. • setPixel method will require a cast • You can assume that the specific object provided to the method will be the appropriate type.

17. Midterm Logistics • Strongly considering moving midterm to either be Thursday or Friday during recitation. • Will talk to TAs on Wed. evening and will have a decision posted that night. • Closed book • No need for exam book or scantron • Learning accommodations • Let me know if this applies. • Take the test at learning services. • Drop it off at my office.

18. Midterm • Part 1: Multiple choice, true/false • Basic Java language syntax • OO concepts • More than one answer may be correct. • Think of it as a group of related true/false questions. • Example: • Which of these is NOT a valid built-in Java data type? • a) int • b) unsigned short • c) char • d) double • e) true

19. Midterm • Part 2: Short answer / calculation • Evaluate small snippets of code • Example: • What is the value of the variable bar after the following code executes? int bar; int[] a = {1, 2, 3, 4, 5}; int[] b = a; b[2] = a[1]; bar = a[2];

20. Midterm • Part III: Understanding Code • I’ll provide the code for one or more classes and then ask questions like: • Identify all of the instance fields of class A. • Identify all of the class methods of class A. • What is the return type of method m of class B?

21. Midterm • Part IV: Writing Code • Asked to define one or more simple classes according to some specification. • May be provided with a partial implementation that you have to complete.

22. Topics / Concepts • Basic Java syntax • Variable names, built-in data types • Reference types vs. value types • Arrays • Concepts of abstraction and encapsulation • JavaBeans conventions • Interfaces • Including extending interfaces covered today • Is-A relationship with interfaces • Enumerations • Iterator • Constructor Chaining / Method Overloading