Subclassing, con.nued Method overriding, virtual methods, abstract classes/methods. COMP 401, Spring 2015 Lecture 9 2/19/2015

Transcription

1 Subclassing, con.nued Method overriding, virtual methods, abstract classes/methods COMP 401, Spring 2015 Lecture 9 2/19/2015

2 Subclassing So Far A subclass inherits implementa.on details from its superclass Fields Direct access to public and protected fields No direct access to private fields Methods Access to public and protected methods No access to private methods Subclass constructors Should call superclass constructor with super() as first line. Or, chain to a different constructor Or, rely on implicit call to super() constructor with no parameters.

3 Subclass Method Polymorphism Subclass can overload methods in superclass. Remember, overloading is providing a different version of an exis.ng method. An example of polymorphism Method signature is different in some way. lec09.ex1

4 Overriding Methods A subclass can override a super class method by providing its own defini.on. Method signature must be the same. Original method must be visible from subclass i.e., public, protected, or package- level access lec09.ex2

5 @Override direc.ve So what s with the line that Eclipse includes when genera.ng a stub? Known as a compiler direc.ve. Completely op.onal, but useful Indicates that the method is intended to override a superclass method. Compiler will complain if it does not detect a visible superclass or interface method with the same method signature. Helpful when you misspell a method name or a\empt to override a method not visible to the subclass. lec09.ex3

6 Class Polymorphism Previously introduced the idea of is- a rela.onships Between a class and interfaces implemented. Between a class and its superclass hierarchy. This is also an example of polymorphism Covariance Trea.ng an instance of a subclass as a reference typed as the parent class. This can be typed checked at compile type. Contravariance Trea.ng a reference typed as the parent class as an instance of a subclass. Contravariance can not be type checked in advance at compile.me. Fails if the object is actually invariant with respect to the subclass. lec09.ex4, lec09.ex4main Also demonstrates protected base class constructor

7 A Covariant Conundrum Problem: What should happen when an overridden method is called on a covariant reference? C c_obj = new C();! B b_obj = (B) c_obj;! A a_obj = (A) c_obj;!! System.out.println(c_obj.m());! System.out.println(b_obj.m());! System.out.println(a_obj.m());! class A {! public int m() {return 0;}! }!! class B extends A {! public int m() {return 1;}! }!! class C extends B {! public int m() {return 2;}! }! What should these lines print?

8 Solu.on 1: Non- virtual methods Let type of reference dictate which method defini.on is used. C c_obj = new C();! B b_obj = (B) c_obj;! A a_obj = (A) c_obj;!! System.out.println(c_obj.m());! System.out.println(b_obj.m());! System.out.println(a_obj.m());! class A {! public int m() {return 0;}! }!! class B extends A {! public int m() {return 1;}! }!! class C extends B {! public int m() {return 2;}! }! If methods are non- virtual then these lines expected to print: 2 1 0

9 Solu.on 2: Virtual methods Use method defined by the actual type of object (even if reference is covariant) C c_obj = new C();! B b_obj = (B) c_obj;! A a_obj = (A) c_obj;!! System.out.println(c_obj.m());! System.out.println(b_obj.m());! System.out.println(a_obj.m());! class A {! public int m() {return 0;}! }!! class B extends A {! public int m() {return 1;}! }!! class C extends B {! public int m() {return 2;}! }! With virtual methods, these lines expected to print: 2 2 2

10 Virtual Methods Different OOP languages choose to solve this problem in different ways. C++, C# Default is non- virtual solu.on. Programmer can force virtual solu.on by marking a method with a special virtual keyword Java Methods are always virtual. No special keyword needed. lec09.ex5

11 A virtual problem Drawback to the always virtual approach. Consider the situa.on in which a subclass just needs a method to do just a li\le more. In other words, wants to execute a method as defined in the superclass and then tweak the result. Or maybe do something in advance of execu.ng a method as defined in the superclass. Because methods are always virtual, cas.ng this reference to superclass in order to get to method as defined by the superclass won t work. lec09.ex6 (won t work)

12 It s a bird, it s a plane, it s the super keyword. The super keyword provides exactly this ability to invoke methods on an instance as it is understood at the superclass. Think of it as a version of this that is restricted to just what is provided by the superclass. Note: Only goes up one level in class hierarchy Essen.ally suspends virtualness of methods. lec09.ex7

13 Whence inheritance Related classes with common internals Common fields used as part of methods with a common implementa.on. Note, not just common behavior Specializa.on of exis.ng classes aler the fact.

14 Inheritance Recap Subinterfacing Adding methods to create a new contract Subclassing Inherits fields and methods from parent class Visibility controlled by access modifier Adds subclass specific fields and methods Constructor rela.onship Overloading parent methods Version of a method provided in subclass with different signatures than the version(s) in the parent class. Overriding parent methods Subclass methods with same signature as in parent class. Always virtual super keyword provides a mechanism to call parent class version of a method

15 Using inheritance Example starts with several related classes that are implemented independently. BlackBear PolarBear Python Robin lec09.ex8.v1

16 Common behavior Refactor common behaviors into a common parent interface. Animal interface lec09.ex8.v2

17 Common object state Refactor common object fields into a common parent class AnimalImpl Pull common fields id and loca.on to here Need to make protected in order to allow subclass access. lec09.ex8.v3

18 Common implementa.on Refactor common method implementa.ons into parent class. Use overriding if subclasses need to do something special or different. AnimalImpl Constructor in parent class used by subclass constructor to ini.alize common fields at this level. getid() and getloca.on() moved here Common por.on of move() put here Subclass- specific override of move Calls common por.on through super keyword lec09.ex8.v4

19 Common behavior with uncommon implementa.on No.ce speak() is a common behavior. So we want it to be part of Animal interface That way if we can have a reference to an Animal object and ask it to speak() without having to know what subclass it is. But no common implementa.on. Each subclass of animal will have a different way of speaking. No good default or commonali.es in way of speaking that can be specified at parent class.

20 Abstract Classes and Methods Parent class has no meaningful implementa.on of a method. But method is part of an interface implemented by parent class Expect subclass to provide it. In these situa.ons, we never expect (or want) the parent class to be instan.ated directly. We always make new objects using a subclass. Syntax Use abstract modifier when declaring parent class Declare any methods that must be provided by subclass in parent Add abstract modifier to method signature. Follow signature with semicolon instead of method defini.on

21 Example Revisited AnimalImpl Declare implementa.on of Animal here where it belongs since AnimalImpl is matching implementa.on. Declare AnimalImpl as abstract Prevents direct instan.a.on Declare speak() method as abstract No common implementa.on, but needs to be declared here as part of common interface. Declaring as abstract forces subclass to override. lec09.ex8.v5

22 Repeat Exercise with Bears lec09.ex8.v6 Draws out common bear behavior into Bear interface and common bear implementa.on in BearImpl Bear extends Animal Adds getcolor() BearImpl extends AnimalImpl Provides lumber(), trek(), move(), and speak(). Declared abstract because subclass must provide getcolor()