Object-Oriented Programming More Inheritance Ewan Klein School of Informatics Inf1 :: 2009/10 Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 1 / 45
1 Inheritance Flat Hierarchy Layered Hierarchy 2 Polymorphism 3 Overriding and Overloading 4 Abstract Classes 5 Recap Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 2 / 45
Flat Animal Hierarchy Animal sleep() makenoise() roam() Lion Wolf makenoise() Cat makenoise() Dog makenoise() makenoise() Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 3 / 45
Animals Example, 1 Our base class: Animal Animal public class Animal { public void sleep() { Systemoutprintln( Sleeping: Zzzzz ); public void makenoise() { Systemoutprintln( Noises ); public void roam() { Systemoutprintln( Roamin on the plain ); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 4 / 45
Animals Example, 2 1 Lion IS-A Animal 2 Override the makenoise() method Lion public class Lion extends Animal { public void makenoise() { Systemoutprintln( Roaring: Rrrrrr! ); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 5 / 45
Animals Example, 3 1 Cat IS-A Animal 2 Override the makenoise() method Cat public class Cat extends Animal { public void makenoise() { Systemoutprintln( Miaowing: Miaooo! ); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 6 / 45
Animals Example, 4 1 Wolf IS-A Animal 2 Override the makenoise() method Wolf public class Wolf extends Animal { public void makenoise() { Systemoutprintln( Howling: Ouooooo! ); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 7 / 45
Animals Example, 5 1 Dog IS-A Animal 2 Override the makenoise() method Dog public class Dog extends Animal { public void makenoise() { Systemoutprintln( Barking: Woof Woof! ); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 8 / 45
Animals Example, 6 The Launcher public class AnimalLauncher { public static void main(string[] args) { Systemoutprintln( \nwolf\n===== ); Wolf wolfie = new Wolf(); wolfiemakenoise() ; // from Wolf wolfieroam(); // from Animal wolfiesleep(); // from Animal Systemoutprintln( \nlion\n===== ); Lion leo = new Lion(); leomakenoise(); // from Lion leoroam(); // from Animal leosleep(); // from Animal Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 9 / 45
Animals Example, 7 Output Wolf ===== Howling: Ouooooo! Roamin on the plain Sleeping: Zzzzz Lion ===== Roaring: Rrrrrr! Roamin on the plain Sleeping: Zzzzz Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 10 / 45
Nested Animal Hierarchy Lions and cats can be grouped together into Felines, with common roam() behaviours Dogs and wolves can be grouped together into Canines, with common roam() behaviours Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 11 / 45
Nested Animal Hierarchy Animal sleep() makenoise() roam() Feline roam() Canine roam() Lion makenoise() Cat makenoise() Wolf makenoise() Dog makenoise() Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 12 / 45
Animals Example, 1 Same as before Animal public class Animal { public void sleep() { Systemoutprintln( Sleeping: Zzzzz ); public void makenoise() { Systemoutprintln( Noises ); public void roam() { Systemoutprintln( Roamin on the plain ); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 13 / 45
Animals Example, 2 The new class Feline Feline public class Feline extends Animal { public void roam() { // Override roam() Systemoutprintln( Roaming: I m roaming alone ); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 14 / 45
Animals Example, 3 The new class Canine Canine public class Canine extends Animal { public void roam() { // Override roam() Systemoutprintln( Roaming: I m with my pack ); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 15 / 45
Animals Example, 4 1 Lion IS-A Feline 2 Override the makenoise() method Lion public class Lion extends Feline { public void makenoise() { Systemoutprintln( Roaring: Rrrrrr! ); Similarly for Cat Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 16 / 45
Animals Example, 5 1 Wolf IS-A Canine 2 Override the makenoise() method Wolf public class Wolf extends Canine { public void makenoise() { Systemoutprintln( Howling: Ouooooo! ); Similarly for Dog Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 17 / 45
Which method gets called? Animal sleep() makenoise() roam() 1 Wolf wolfie = new Wolf(); 2 wolfiemakenoise(); 3 wolfieroam(); Canine roam() 4 wolfiesleep(); Wolf makenoise() Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 18 / 45
Animals Example, 6 The Launcher public class AnimalLauncher { public static void main(string[] args) { Systemoutprintln( \nwolf\n===== ); Wolf wolfie = new Wolf(); wolfiemakenoise(); // from Wolf wolfieroam(); // from Canine wolfiesleep(); // from Animal Systemoutprintln( \nlion\n===== ); Lion leo = new Lion(); leomakenoise(); // from Lion leoroam(); // from Feline leosleep(); // from Animal Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 19 / 45
Animals Example, 7 Output Wolf ===== Howling: Ouooooo! Roaming: I m with my pack Sleeping: Zzzzz Lion ===== Roaring: Rrrrrr! Roaming: I m roaming alone Sleeping: Zzzzz Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 20 / 45
Typing and Polymorphism polymorphism (= many shapes ): the same piece of code can be assigned multiple types A class defines a type, namely the signatures of its methods S is a subtype of T, written S <: T, if a value of type S can be used in any context where a value of type T is expected The relation <: is reflexive: T <: T The relation <: is transitive: if S <: T and T <: U, then S <: U NB: We say T is a supertype of S if S is a subtype of T Inclusion polymorphism: objects of different types S1, S2, may be treated uniformly as instances of a common supertype T Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 21 / 45
Declaring and Initializing a Reference Variable create a Wolf object Wolf wolfie = new Wolf(); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 22 / 45
Declaring and Initializing a Reference Variable declare a reference variable Wolf wolfie = new Wolf(); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 23 / 45
Declaring and Initializing a Reference Variable link the object to the reference Wolf wolfie = new Wolf(); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 24 / 45
Declaring and Initializing a Reference Variable supertype object of subtype Animal wolfie = new Wolf(); Reference type can be supertype of the object type Eg, Wolf <: Animal Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 25 / 45
Polymorphic ArrayList The Launcher public class AnimalLauncher2 { public static void main(string[] args) { Wolf wolfie = new Wolf(); Lion leo = new Lion(); Cat felix = new Cat(); Dog rover = new Dog(); ArrayList< Animal > animals = new ArrayList<Animal>(); animalsadd(wolfie); animalsadd(leo); animalsadd(felix); animalsadd(rover); for ( Animal a : animals) { amakenoise(); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 26 / 45
Polymorphic Arrays ArrayList<Animal> is polymorphic animalsadd(wolfie) add an object of type Wolf OK since Wolf <: Animal for (Animal a : animals) for each object a of type T such that T <: Animal amakenoise() if a is of type T, use T s makenoise() method Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 27 / 45
Method Overriding, 1 If a class C overrides a method m of superclass D, then: Parameter lists must be same and return type must be compatible: 1 signature of m in C must be same as signature of m in D; ie same name, same parameter list, and 2 return type S of m in C must such that S <: T, where T is return type of m in D m must be at least as accessible in C as m is in D Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 28 / 45
Method Overriding, 2 method in Animal public void makenoise() { Wrong: method in Wolf public void makenoise( int volume ) { Wrong: method in Wolf private void makenoise() { Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 29 / 45
Method Overloading, 1 Overloading: two methods with same name but different parameter lists Overloaded makenoise public void makenoise() { public void makenoise(int volume) { Overloaded println Systemoutprintln(3); // int Systemoutprintln(30); // double Systemoutprintln((float) 30); // cast to float Systemoutprintln( 30 ); // String Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 30 / 45
Method Overloading, 2 1 Return types can be different 2 You can t just change the return type gets treated as an invalid override 3 Access levels can be varied up or down Incorrect override of makenoise public String makenoise() { String howl = Ouooooo! ; return howl; Exception in thread main javalangerror: Unresolved compilation problem: The return type is incompatible with AnimalmakeNoise() at week06wolfmakenoise(wolfjava:15) at week06animallaunchermain(animallauncherjava:11) Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 31 / 45
Animal Objects? Creating new objects Wolf wolfie = new Wolf(); Animal leo = new Lion(); Animal weird = new Animal(); Animal class is meant to contain information that all animals have in common But this is not enough to define any one specific animal Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 32 / 45
Concrete vs Abstract Concrete Abstract Examples: Cat, Wolf Specific enough to be instantiated Examples: Animal, Feline Not intended to have instances Only useful if extended Any instances will have to be instances of a subclass of the abstract class Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 33 / 45
The Abstract Animal, 1 Animal abstract class Animal { public void sleep() { Systemoutprintln( Sleeping: Zzzzz ); public void makenoise() { Systemoutprintln( Noises ); public void roam() { Systemoutprintln( Roamin on the plain ); Just put the keyword abstract before the class declaration Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 34 / 45
The Abstract Animal, 2 An abstract class can be extended by other abstract classes Canine and Feline can (and should) both be abstract Animal abstract class Animal { public void sleep() { Systemoutprintln( Sleeping: Zzzzz ); public void makenoise() { Systemoutprintln( Noises ); public void roam() { Systemoutprintln( Roamin on the plain ); Just put the keyword abstract before the class declaration Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 35 / 45
The Abstract Animal, 2 Animal abstract class Animal { public void sleep() { Systemoutprintln( Sleeping: Zzzzz ); public abstract void roam(); public abstract void makenoise(); Now has abstract methods! Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 36 / 45
The Abstract Animal, 3 roam() and makenoise() are abstract methods (we ve already seen these in interfaces): no body; must be implemented in any concrete subclass (implemented overriden); don t have to be implemented by an abstract subclass; can only be declared in an abstract class; sleep() is not abstract, so can be straightforwardly inherited Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 37 / 45
Abstract Classes in Animal Hierarchy <<Abstract>> Animal sleep() makenoise() roam() <<Abstract>> roam() Feline <<Abstract>> roam() Canine Lion makenoise() Cat makenoise() Wolf makenoise() Dog makenoise() Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 38 / 45
Using Abstract Classes Use an abstract class when you have several similar classes that: have a lot in common the implemented parts of the abstract class have some differences the abstract methods Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 39 / 45
Digression: Constructor Chaining Constructor of the immediate superclass invoked with super() If you don t explicitly call this, the compiler will Case 1 No constructor explicitly declared Compiler inserts public Foo() { super(); Case 2 You have already declared one or more constructors Compiler inserts the following as first statement in each one: super(); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 40 / 45
Recap: Implementing/Overriding, 1 If a class C implements or overrides a method m of superclass D, then: Parameter lists must be same and return type must be compatible: 1 signature of m in C must be same as signature of m in D; ie same name, same parameter list, and 2 return type S of m in C must such that S <: T, where T is return type of m in D Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 41 / 45
Method Implementation, 1 Abstract method in Animal public abstract void makenoise(); Wrong: method in Wolf public void makenoise( int volume ) { Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 42 / 45
Method Overloading Overloading: two methods with same name but different parameter lists Overloaded makenoise public void makenoise(int volume) { public void makenoise() { makenoise(int 1); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 43 / 45
Method Implementation, 2 You cannot implement or override a method by providing just an overloaded method Declaration of sleep() in Animal public void sleep() { Systemoutprintln( Sleeping: Zzzzz ); Right: overriding sleep() in Wolf public void sleep() { Systemoutprintln( Sleeping: XXXXX ); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 44 / 45
Method Implementation, 3 Wrong: overloading sleep() in Wolf public void sleep(int duration) { String sleepnoise = Sleeping: Zzzzz ; if (duration > 2) sleepnoise = Still Sleeping: XXXXX ; Systemoutprintln(sleepNoise); Right: overloading and overriding public void sleep(int duration) { public void sleep() { Systemoutprintln( Sleeping: XXXXX ); Ewan Klein (School of Informatics) OOP: More Inheritance Inf1 :: 2009/10 45 / 45