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Session 18

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Presentation Transcript

  1. Session 18 Introducing Inheritance

  2. When do we use inheritance? • When we want to reduce code duplication by pulling together common code from two or more classes. • When we want to set up a mechanism for substitutability between two or more classes. • When we want to add additional features to an existing class.

  3. 1) Reducing Code Duplication • Create a new parent class which contains all of the “common” code from your set of existing classes • We looked at what both DVD and CD had in common (title, running time, comments, got it) and pulled those variables and accessor/mutator methods into a new class called Item • Next, we have DVD and CD EXTEND Item

  4. Developing an Extended Class (a class which inherits from another) • There are typically four steps in developing an extended class. • declare the class • declare the new data • create the constructors   • adjust the methods if needed

  5. Developing an Extended Class • declare the class(es) public class CD extends Item { … public class DVD extends Item { …

  6. Developing an Extended Class • declare the new data … private String artist; private int numberOfTracks; … private String director;

  7. Developing an Extended Class • create the constructor public CD(String theTitle, String the Artist, int tracks, int time) { super(theTitle, time); artist = theArtist; numberOfTracks= tracks; }

  8. Developing an Extended Class •  adjust the methods • Leave inherited methods alone • Add completely new methods • Modify/Override inherited methods

  9. Developing an Extended Class •   Leave inherited methods alone • setComment() and setOwn() are both inherited from Item and there is no need to change them.

  10. Developing an Extended Class • Add completely new methods • We need an accessor method for tracks public void getNumberOfTracks() { return numberOfTracks; }

  11. Developing an Extended Class • Modify/Override inherited methods • print() is inherited, but it doesn’t really do what we want it to do. • Let’s come back to this in a minute…

  12. 2) Set up substitutability • You can use different data types in a single location if they are all substitutable for the same thing. • A subclass is always substitutable for it’s superclass • Therefore, we can adapt Database to handle a single ArrayList of Items (or things substitutable for Items)

  13. 3) Adding Behavior to a Class • Any time that we need to add behavior to a class we have at least three options: • Add code to the class itself, keeping the original class. • Copy all the old code into a new class and add code to this new class. • Create a subclass that extends the original class' behavior.

  14. Pros and Cons “Add code to the class itself, keeping the original class. “ • Pros: Quick. Convenient. Simple. • Cons: May change the behavior of the class. Thus, it isn’t always an option.

  15. Pros and Cons “Copy all the old code into a new class and add code to this new class. “ • Pros: Quick. Convenient. Simple. • Cons: Duplicated code. Error trap! Error trap!

  16. Pros and Cons “Create a subclass that extends the original class' behavior.“ • Pros: Doesn’t break existing code. Virtually eliminates duplicate code. Provides the most flexibility. • Cons: Slightly more time consuming.

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