Inheritance and Polymorphism. CS180 Fall 2007

Transcription

1 Inheritance and Polymorphism CS180 Fall 2007

2 Definitions Inheritance object oriented way to form new classes from pre-existing ones Superclass The parent class If class is final, cannot inherit from this class Subclass Class which inherits all non-private properties from the superclass Generally a specific instance of a superclass Polymorphism a object can take many forms Combining the two, can create subclasses with different properties

3 Inheriting From the Superclass Two ways Use the protected keyword instead of the private keyword Variables and methods which are protected are accessible by the subclass, but not the world Use public for methods to inherit, private for methods to not inherit and all variables Variables no longer accessible directly in subclass Use accessors and mutators to grab and modify them Override methods as necessary If a method is declared final, then it cannot be overridden in a subclass Can indicate Not required, but a good check Constructors Not inherited! Must rewrite them Use super keyword to call superconstructor from parent class Can only use super as first line of a constructor (much like this) If no superconstructor is explicitly called, then super() (the default superconstructor) is automatically called as the first line Common error: if you don t call super explicitly, make sure your superclass has a default constructor! Must be first line of constructor super keyword can also be used to call instances of methods from superclass

4 Syntax To extend a class, use the extends keyword The class will now inherit all non-private characteristics of the superclass Do not have to rewrite methods which are the same Can introduce new methods and/or variables public class A { private int x; public A() { set(0); public A(int x) { set(x); public void set(int x) { this.x = x; public int get() { return x; public void add1() { bump(); private void bump() { x++; public class B extends A { public B() { super(); public B(int x) { super(x); public void add1() { set(get()+1);

5 Inheriting From the Superclass public class A { private int x; public A() { set(0); public A(int x) { set(x); public void set(int x) { this.x = x; public int get() { return x; public void add1() { bump(); private void bump() { x++; public class B extends A { public B() { super(); public B(int x) { super(x); public void add1() { set(get()+1); public class A { protected int x; public A() { set(0); public A(int x) { set(x); public void set(int x) { this.x = x; public int get() { return x; public void add1() { bump(); private void bump() { x++; public class B extends A { public B() { super(); public B(int x) { super(x); The add1() method is overridden here!

6 Inheriting From the Superclass On the superclass constructor calls, constructors wind up being executed top-down on the hierarchy. public class A { A() { SOP( A ); public class B extends A { public B() { SOP( B ); public class C extends A { public C() { SOP( C ); public class D extends B { public D() { SOP( D ); A = new A(); Output: A B = new B(); Output: AB C = new C(); Output: AC D = new D(); Output: ABD

7 Object Class Class Object is the superclass of all superclasses in Java All classes defined implicitly inherit from Object if not explicitly extending from another class Even if explicitly extending another class, that other class either extends from another class or implicitly extends from Object Some useful methods which are commonly overwritten equals for comparisons (defaults to reference address comparision) tostring translates your object into an appropriate String (defaults to the reference address)

8 Polymorphism Allows a variable to take on many forms A variable of type Animal may point to may different types of animals Only methods scoped within the variable type are available Not all animals fly, so even though an Animal identifier may point to a Bat object, you cannot call the fly() method directly public class Animal { protected int age; public void eat() { SOP( Yummy! ); public class Giraffe extends Animal { public Giraffe() { super(); public void eat() { SOP( Eating leaves! ); public void gallop() { SOP( Galloping away! ); public class Bat extends Animal { public Bat() { super(); public void eat() { SOP( Drinking blood! ); public void fly() { SOP( Flapping wings! ); Animal a = new Bat(); a.eat(); a.fly(); ((Bat)a).fly(); ((Giraffe)a).gallop(); // legal! All animals make sounds! // illegal! Not all animals know how to fly! // legal! A bat knows how to fly! // compiles, but runtime error!

9 Dynamic Binding On the call the eat(), what happens? If Animal object a is a Giraffe, then Eating leaves! If Animal object a is a Bat, then Drinking blood! Why not Yummy!? dynamic binding! Objects have two types Static type the type of the variable name Dynamic type the actual type of the variable in memory Dynamic binding Resolved at runtime Used for method lookup Look up most specific instance of method call (look up the method in the dynamic type if it exists) Most specific instance of a method may be in a parent class (and therefore inherited) Static binding (optional) Resolved at compile time Used for variable lookup Also all static variables and methods are resolved during compilation

10 Dynamic Binding public class A { protected int x; public A() { set(0); public A(int x) { set(x); public void set(int x) { this.x = x; public int get() { return x; public void bump() { x+=2; static type dynamic type public class B extends A { public B() { super(); public B(int x) { super(x); A a = new B(); a.bump(); SOP( a= +a.get()); Object a is of static type A, dynamic type B. On a method call, use dynamic type first! public void bump() { x++;

11 Determining Class Type In order to access a method in a subclass from an object with a static type of the superclass, need to downcast Downcasting is unsafe! Need to know that downcast is legal. Use instanceof keyword or other methods x instanceof A true if x is an instance of class A or a subclass of A x.getclass().equals(a.class) true if x is an instance of class A A.class.isAssignableFrom(x.getClass()) is true if x is an instance of class A or a subclass of A

12 Abstract Classes Sometimes a superclass never needs to be instantiated you only make instances of the subclasses Make class abstract Cannot be instantiated May have some methods be abstract Is like a blueprint for subclasses, with methods that need to be filled in public abstract class Animal { protected int age; public abstract void eat(); public abstract class Mammal { public Mammal() { super(); public abstract void eat(); // other mammal-like behavior public class Giraffe extends Mammal { public Giraffe() { super(); public void eat() { SOP( Eating leaves! );

13 Interfaces Contains method headers Cannot instantiate interface Cannot have any non-final variables A class can implement many interfaces Interfaces can extend from each other Example: Comparable Contains one method, compareto, which takes two objects and compares them Returns a negative number if less than Returns 0 if equal to Returns a positive number if greater than Comparable is an interface and not an abstract class because not all things comparable are related Numbers and people are both comparable, but have no reasonable relationship between them

14 Interfaces public interface I { public void dostuff(); public interface J extends I { public void domorestuff(); J extends I, so interface J actually has 2 methods: dostuff() and domorestuff(). public interface K { public void doevenmorestuff(); public class A implements J, K { public void dostuff() { public void domorestuff() { public void doevenmorestuff { A implements J and K, so A must have a method for each method in the interfaces it implements.

15 Abstract Class vs. Interface Abstract classes are blueprints, interfaces are contracts Interface: Here s some methods. If your class implements this interface, you must provide an implementation of each method in the interface in the class. Method headers final variables A class can implement multiple interfaces Cannot instantiate Abstract class: Here s a blueprint for a class. If your class extends this abstract class, you can use any functionality here and add onto it, but you must fill in what I have not. Abstract method headers Methods Variables A class can extend only one class Cannot instantiate

16 Animal (Planet?) Animal Bird Mammal Fish Owl Penguin Bat Giraffe Whale Shark Salmon

17 Animal (Planet?) Our animals: public class Owl { public class Penguin { public class Bat { public class Giraffe { public class Whale { public class Shark { public class Salmon {

18 Animal (Planet?) Create appropriate superclasses: public class Animal { public class Bird { public class Mammal { public class Fish { public class Owl { public class Penguin { public class Bat { public class Giraffe { public class Whale { public class Shark { public class Salmon {

19 Animal (Planet?) Connect the hierarchy: public class Animal { public class Bird extends Animal { public class Mammal extends Animal { public class Fish extends Animal { public class Owl extends Bird { public class Penguin extends Bird { public class Bat extends Mammal { public class Giraffe extends Mammal { public class Whale extends Mammal { public class Shark extends Fish { public class Salmon extends Fish {

20 Animal (Planet?) Add in appropriate interfaces at the highest levels: public class Animal { public class Bird extends Animal { public class Mammal extends Animal { public class Fish extends Animal implements Swimmer { public class Owl extends Bird implements Flyer { public class Penguin extends Bird { public class Bat extends Mammal implements Flyer { public class Giraffe extends Mammal { public class Whale extends Mammal implements Swimmer { public class Shark extends Fish { public class Salmon extends Fish {

21 Animal (Planet?) Make appropriate classes abstract: abstract public class Animal { abstract public class Bird extends Animal { abstract public class Mammal extends Animal { abstract public class Fish extends Animal implements Swimmer { public class Owl extends Bird implements Flyer { public class Penguin extends Bird { public class Bat extends Mammal implements Flyer { public class Giraffe extends Mammal { public class Whale extends Mammal implements Swimmer { public class Shark extends Fish { public class Salmon extends Fish {

22 Animal (Planet?) abstract public class Animal { abstract public class Bird extends Animal { abstract public class Mammal extends Animal { abstract public class Fish extends Animal implements Swimmer { public class Owl extends Bird implements Flyer { public class Penguin extends Bird { public class Bat extends Mammal implements Flyer { public class Giraffe extends Mammal { public class Whale extends Mammal implements Swimmer { public class Shark extends Fish { public class Salmon extends Fish { Which are valid instantiations? Animal a = new Animal(); Animal b = new Fish(); Animal c = new Flyer(); Mammal d = new Bat(); Fish e = new Swimmer(); Swimmer f = new Shark(); Flyer g = new Owl(); Swimmer h = new Whale(); Swimmer i = new Fish();