Inheritance Polymorphism Briana B. Morrison CSE 1302C Spring 2010

2 CSE 1302C Topics Inheritance Concepts Inheritance Design –Inherited Members of a Class –Subclass Constructors –Adding Specialization to the Subclass –Overriding Inherited Methods The protected Access Modifier

3 CSE 1302C Inheritance Concepts A common form of reuse of classes is inheritance. We can organize classes into hierarchies of functionality. The class at the top of the hierarchy (superclass) defines instance variables and methods common to all classes in the hierarchy. We derive a subclass, which inherits behavior and fields from the superclass.

4 CSE 1302C Inheritance Inheritance allows a software developer to derive a new class from an existing one The existing class is called the parent class, or superclass, or base class The derived class is called the child class, subclass, or derived class As the name implies, the child inherits characteristics of the parent That is, the child class inherits the methods and data defined by the parent class

5 CSE 1302C A Sample Vehicle Hierarchy This hierarchy is depicted using a Unified Modeling Language (UML) diagram. In UML diagrams, arrows point from the subclass to the superclass.

6 CSE 1302C Superclasses and Subclasses A superclass can have multiple subclasses. Subclasses can be superclasses of other subclasses. A subclass can inherit directly from only one superclass. All classes inherit from the Object class.

7 CSE 1302C Superclasses and Subclasses A big advantage of inheritance is that we can write common code once and reuse it in subclasses. A subclass can define new methods and instance variables, some of which may override (hide) those of a superclass.

8 CSE 1302C Specifying Inheritance The syntax for defining a subclass is to use the : symbol in the class header, as in accessModifier class SubclassName: SuperclassName { // class definition } The superclass name specified after the : is called the direct superclass. As mentioned, a subclass can have many superclasses, but only one direct superclass.

9 CSE 1302C Class Hierarchies A child class of one parent can be the parent of another child, forming a class hierarchy Business KMartMacys ServiceBusiness Kinkos RetailBusiness

10 CSE 1302C Class Hierarchies Two children of the same parent are called siblings Common features should be put as high in the hierarchy as is reasonable An inherited member is passed continually down the line Therefore, a child class inherits from all its ancestor classes There is no single class hierarchy that is appropriate for all situations

11 CSE 1302C The Object Class A class called Object exists All classes are derived from the Object class If a class is not explicitly defined to be the child of an existing class, it is assumed to be the child of the Object class Therefore, the Object class is the ultimate root of all class hierarchies

12 CSE 1302C The Object Class The Object class contains a few useful methods, which are inherited by all classes: Constructor Equals GetHashCode GetType ReferenceEquals ToString (virtual) Finalize (overridden, destructor) MemberwiseClone (shallow copy of object)

13 CSE 1302C Example namespace ConsoleApplication1 { public class ObjectExample { static void Main(string[] args) { Object o1 = new Object(); Console.WriteLine("This is an object " + o1); Temp t = new Temp(); Console.WriteLine("This is a temp object " + t); } public class Temp { private int a; private char ch; } Output This is an object System.Object This is a temp object ConsoleApplication1.Temp

14 CSE 1302C The Bank Account Hierarchy The BankAccount class is the superclass. –Instance variables: balance (double) –Methods: Default and overloaded constructors deposit and withdraw methods balance accessor toString

15 CSE 1302C ScreenManager Hierarchy //**pull from A2**//

16 CSE 1302C private Members Superclass members declared as private are part of the subclass, but not accessible by the subclass /** rework based on a2 **/

17 CSE 1302C Subclass Constructors Constructors are not inherited. The first task of a constructor is to call a base class constructor. Use this syntax: base( argument list ); Or: public constructor(arg1 list):base( argument list );

18 CSE 1302C The base Reference A child’s constructor is responsible for calling the parent’s constructor Normally the first line of a child’s constructor should use the base reference to call the parent’s constructor The base reference can also be used to reference other variables and methods defined in the parent’s class

19 CSE 1302C Adding Specialization A subclass can define new fields and methods.

20 CSE 1302C Software Engineering Tip The superclasses in a class hierarchy should contain fields and methods common to all subclasses. The subclasses should add specialized fields and methods.

21 CSE 1302C Overriding Inherited Methods A subclass can override (or replace) an inherited method by providing a new version of the method. Base class must include virtual Descendant must include override The API of the new version must match the inherited method. When the client calls the method, it will call the overridden version. The overridden (original) method is invisible to the client of the subclass, but the subclass methods can still call the overridden (original) method using this syntax: base.methodName( argument list )

22 CSE 1302C Overloading vs. Overriding Overloading deals with multiple methods with the same name in the same class, but with different signatures Overriding deals with two methods, one in a parent class and one in a child class, that have the same signature Overloading lets you define a similar operation in different ways for different parameters Overriding lets you define a similar operation in different ways for different object types

23 CSE 1302C Common Error Trap Do not confuse overriding a method with overloading a method. Overriding a method: –A subclass provides a new version of that method (same signature), which hides the superclass version from the client. Overloading a method: –A class provides a version of the method, which varies in the number and/or type of parameters (different signature). A client of the class can call any of the public versions of overloaded methods.

24 CSE 1302C protected Members protected members are accessible by subclasses (like public members), while still being hidden from client classes (like private members). Also, any class in the same package as the superclass can directly access a protected field, even if that class is not a subclass. Disadvantage: –Because more than one class can directly access a protected field, protected access compromises encapsulation and complicates maintenance of a program. –For that reason, try to use private, rather than protected, for instance variables.

25 CSE 1302C The protected Access Modifier Declaring fields as private preserves encapsulation. –Subclass methods call superclass methods to set the values of the fields, and the superclass methods enforce the validation rules for the data. –But calling methods incurs processing overhead. Declaring fields as protected allows them to be accessed directly by subclass methods. –Classes outside the hierarchy and package must use accessors and mutators for protected fields.

26 CSE 1302C protected fields: Tradeoffs Advantage: –protected fields can be accessed directly by subclasses, so there is no method-invocation overhead. Disadvantage: –Maintenance is complicated because the subclass also needs to enforce validation rules. Recommendation: –Define protected fields only when high performance is necessary. –Avoid directly setting the values of protected fields in the subclass.

27 CSE 1302C Inheritance Rules Superclass Members Inherited by subclass? Directly Accessible by Subclass? Directly Accessible by Client of Subclass? public fieldsyesyes, by using field name yes public methodsyesyes, by calling method from subclass methods yes protected fieldsyesyes, by using field name no, must call accessors and mutators protected methods yesyes, by calling method from subclass methods no private fieldsyes* - no access no, must call accessors and mutators private methodsyes* - no access no

28 CSE 1302C Questions?