Chapter 8 Improving Structure with Inheritance

The DoME Example The Database of Multimedia Entertainment We will be storing information about CDs and videos We will want to be able to perform actions on each of these lists of items.

Desired Functionality Enter information about CDs and videos Store information permanently Search functionality Print lists of all CDs or videos

Desired Information Title of album Artist Number of tracks Total Playing time “Got it” flag Comment Title of video Name of director Total Playing time “Got it” flag Comment

Design Discussion Each of the private fields would have a setter and a getter The each list of CDs and videos could be stored in a separate array list Do you see any issues with this approach? DoME Source Code

Major Issue Code duplication There is code duplication in almost every aspect of every class With the two ArrayLists we have to duplicate every method. With repeated information we have to repeat the same code in both the CD and video class What happens when we want to add books?

Inheritance Inheritance allows us to define one class as an extension of another Inheritance allows subclasses to get all the fields and methods from the superclass. In this way, we can put all the common information in one class and inherit from it Then we can put the specialized information in the subclasses

Better Classes for DoME

Inheritance Hierarchies Animals Mammal Dog PoodleDalmatian Cat Bird ChickenSparrow

Inheritance and Java There are some syntax issues when dealing with Java and inheritance We use the keyword extends to indicate that one class inherits from another class We may choose to use different access modifiers within the base class to allow for easier access in the child classes We need ways to initialize the super class

Inheritance and Java public class Item { private String title; private int playingTime; private boolean gotIt; private String Comment; … } public class CD extends Item { private String artist; private int numberOfTracks; … }

Inheritance and Access Rights Just like any other class a subclass of a superclass is not allowed to access any private data; it must use setters and getters It does not need to use a dot in front of any methods It can call them as if they were their own We will come back to this issue when later and introduce a protected access modifier

Inheritance and Initialization When we create an object the constructor is called and the state of the object is set. But how do we set the state of the superclass? In the first line of the subclass’s constructor we use the key word super and supply any necessary parameters to the superclass’s constructor

Example public class Item { … public Item( String theTitle, int time ) { title = theTitle; playingTime = time; gotIt = false; comment = “”; } … } public class CD extend Item { … public CD( String theTitle, String theArtist, int tracks, int time ) { super(theTitle, time); artist = theArtist; numberOfTracks = tracks; } … }

Things to Notice The constructor for the CD receives the information necessary to create an item in addition to the information need to create the CD The key word super is the call to Item’s constructor and we pass in the information it wants The first line of the subclass’s constructor must always be to the superclass’s constructor through the key word super

Advantages of Inheritance Avoiding code duplication Code reuse Easier maintenance Extensibility

Subtyping Let’s take a look at the Database code since we have changed to inheritance DoME Example V2 It is much simpler The reason is subtyping

Subtyping Again When we have said in the past that when you call a method, the actual parameters must match the type of the formal parameters We could have said must match the type or be a subtype of the formal paramaters We can now have one ArrayList that will hold items So addItem will take an item or any of its subclasses as a parameter.

Subtyping and Assignments We are allowed to assign to a variable any object that matches its type or is a subtype of its type These are allowed Vehicle v1 = new Vehicle(); Vehicle v2 = new Car(); Vehicle v3 = new Bike(); This is not Car c1 = new Vehicle(); Vehicle CarBike

Polymorphism Polymorphism, for now, is an idea that allows a variable to be able to be more than one thing at different times. It allows us to simplify our print code for the database. We will come back to this important idea later

The Object class In Java all objects inherit from the Object class. We don’t have to do anything, it is automatic. This is how ArrayLists can hold any type of object. They expect an Object as their parameters. Since all objects inherit from this common base, we can pass any object to a collection in Java That is also why, it gives us an object back and we have to cast it back to its particular type.

HW Chapter 8: 8.3, 8.6, 8.11, 8.13, 8.17