Java Interfaces and Abstract Classes (3)

Java Interfaces and Abstract Classes (3) Chris Loftus

Use of abstract class instead • Q. Was the Animal interface the best solution for our Pets problem? • A. An abstract class is probably better since we still have the duplication problem • We can put any duplicated code in the abstract superclass...

UML class diagram using an Abstract class -myPets * Pets -myPets: Pet[] -numPets: int +addPet(newPet: Pet):void +showSound():void TestPets +main(…) {abstract} Pet - name: String +Pet() +getName():String {final} +sound():String Cat +sound():String … Fish +getKind():String … Dog +sound():String … HappyCat +HappyCat() +sound():String

Demo (notes) • Pet is now an abstract class • Abstract means we cannot create an object of this class, cannot use new Pet() • I want to create instances of Cats, Dogs etc but not the abstract concept of Pet • Pet is used to store the pet name and provide getName and sound methods in one place... • Why use private for name rather than protected?...

Demo (notes) • Pet constructor takes a name • Let’s see how this is used in the Cat subclass... • Default implementations for getName and sound • getName is final: it cannot be overridden in subclass. Let’s try... • sound default implementation means we won’t have to override in Fish (or other mute animals)

Demo (notes) • Can also make methods abstract • I demonstrated this with sound... • Rather like methods in an interface so can still act like a contract... • Notice how Fish now does not compile since it must have a sound implementation...

Abstract classes vs interfaces • Use abstract classes instead of or as well as interfaces if you have default implementations and somewhere to store common data • Interfaces are good when you don’t know what the implementation will look like and want to specify a contract that future implementation classes (including abstract classes) must meet...

Demo (notes): HappyCat • This subclasses Cat • The HappyCatsound method also calls the sound method in the Cat superclass, by using super.sound(), appending extra happy cat sounds! • Let’s look at the overall object diagram on the whiteboard...

Polymorphism • Means many forms • One of the four key aspects of OO: • abstraction, inheritance, encapsulation and polymorphism • Within OO means: • Allows a variable or method parameter to be written to take an object of type T, but will also be able to take an object of type S, where S is a subtype of T

Examples -myPets {abstract} Pet * S <: T (S is a subtype of T) T >: S (T is a supertype of S) S v1; T v2 = v1; // Is allowed Q: Given the UML class diagram is this allowed? Pet p = new Fish(); Cat c = p; Cat Fish Dog HappyCat

Other ways of saying it • We can always assign from a variable of the same type or a subtype to a variable of the same type or super-type • Left hand side of assignment must be the same type or a super-type of the right hand side of the assignment

Liskov Substitution Principle • Polymorphism as shown above is also called the Liskov Substitution Principle after Barbara Liskov in her 1993 paper: • Family values: A behavioural notion of subtyping, MIT Technical Report, TR-562b

Polymorphism and overriding • Subclasses can override a superclass’s non-final methods • At runtime the JVM works out which version of the method to run, and it’s always the most specialised method • E.g. Pet pet = new HappyCat(...); pet.sound(); // Which sound method is called (next slide)?

Study the following code and class diagram:Pet pet = new HappyCat(...);pet.sound();Which sound method is called? (choose one) <<abstract>> Pet --------------------- +sound(): String ... • Pet.sound • Cat.sound • HappyCat.sound • I don’t know Cat --------------------- +sound(): String +getBreed(): String ... HappyCat --------------------- +sound(): String ... • Let’s draw an object diagram on the whiteboard to illustrate

Yes or NoPet pet = new HappyCat(...); {abstract} Pet --------------------- +sound(): String ... Can I now call pet.getBreed()? Cat --------------------- +sound(): String +getBreed(): String ... HappyCat --------------------- +sound(): String ...

Polymorphism and overriding • What do we need to do to access the getBreed() method? • We use a cast operator Pet pet; // pet = ... Assume some code here Cat c = (Cat)pet; c.getBreed(); • What could go wrong here at runtime? (next slide)...

{abstract} Pet --------------------- +sound(): String ... Given the code fragment and UML class diagram:1. Pet pet;2. // 3. Cat c = (Cat)pet;4. c.getBreed();What could go wrong at runtime?(choose one) Fish --------------------- +sound(): String +getKind(): String ... Cat --------------------- +sound(): String +getBreed(): String ... • Nothing can go wrong • The variable pet might actually be a reference to a Fish causing an exception to be thrown at line 4 • The variable pet might actually be a reference to a Fish causing an exception to be thrown at line 3 • I don’t know HappyCat --------------------- +sound(): String ...

More on casting • What if I did the following? Object var = new String(“Hello”); Cat c = (Cat)var; • If the JVM allowed this assignment what could go wrong?... • More on casting in a later talk...

Java Interfaces and Abstract Classes (3)