CC1007NI: Further Programming

Week 3 - 4

Dhruba SenModule Leader (Islington College)

Abstract Classes

What is Abstration?An abstract class is one that cannot be

instantiated.

All other functionality of the class still exists, and its fields, methods, and constructors are all accessed in the same manner.

You just cannot create an instance of the abstract class.

What is Abstration?If a class is abstract and cannot be instantiated,

the class does not have much use unless it is subclassed (Inheritance).

This is typically how abstract classes come about during the design phase.

A parent class contains the common functionality of a collection of child classes, but the parent class itself is too abstract to be used on its own.

Why abstraction?If you want a class to contain a particular method

but you want the actual implementation of that method to be determined by child classes, you can declare the method in the parent class as abstract.

‘abstract’ KeywordUse ‘abstract’ keyword to declare a class or

method as abstract.E.g.

public abstract class Animal public abstract void eat()

Abstract methodsAbstract method would have no definition, and its

signature is followed by a semicolon, not curly braces as follows:

E.g.public abstract double computePay();

<no method body, no curly braces, semicolon at the end>

Abstract methodsDeclaring a method as abstract has two results:

The class must also be declared abstract. If a class contains an abstract method, the class must be abstract as well.

Any child class must either override the abstract method or declare itself abstract.

Abstract methodsA child class that inherits an abstract method

must override it. If they do not, they must be abstract, and any of their children must override it.

Eventually, a descendant class has to implement the abstract method; otherwise, you would have a hierarchy of abstract classes that cannot be instantiated.

Simulations

Programs regularly used to simulate real-world activities

•They are often only partial simulations

•They often involve simplifications.–Greater detail has the potential to provide greater accuracy

–Greater detail typically requires more resources

Predator-prey simulationsThere is often a delicate balance between

species.A lot of prey means a lot of food.A lot of food encourages higher predator numbers.More predators eat more prey.Less prey means less food.Less food means ...

The foxes-and-rabbits project

Main classes of interestFox

Simple model of a type of predator.Rabbit

Simple model of a type of prey.Simulator

Manages the overall simulation task.Holds a collection of foxes and rabbits.

Example of the visualization

Room for improvementFox and Rabbit have strong similarities but do

not have a common superclass.The update step involves similar-looking code.The Simulator is tightly coupled to specific

classes.It ‘knows’ a lot about the behavior of foxes and

rabbits.

The Animal superclassPlace common fields in Animal:

age, alive, locationMethod renaming to support information hiding:

run and hunt become act.Simulator can now be significantly decoupled.

The act method of AnimalStatic type checking requires an act method in Animal.

There is no obvious shared implementation.

Define act as abstract:abstract public void act(List<Animal> newAnimals);

Abstract classes and methodsAbstract methods have abstract in the

signature.Abstract methods have no body.The presence of at least one abstract method

makes the class abstract.Abstract classes cannot be instantiated.Concrete (i.e. non-abstract) subclasses complete

the implementation.

The Animal classpublic abstract class Animal{ fields omitted  /** * Make this animal act - that is: make it do * whatever it wants/needs to do. */ abstract public void act(List<Animal> newAnimals);   other methods omitted}

ReviewAbstract methods allow static type checking

without requiring implementation.Abstract classes function as incomplete

superclasses.No instances.

Abstract classes support polymorphism.

Interfaces

• Interfaces

•Multiple inheritance

Main concepts to be covered

Abstract classes and methodsAbstract methods have abstract in the

signature.Abstract methods have no body.The presence of at least one abstract method

makes the class abstract.Abstract classes cannot be instantiated.Concrete (i.e. non-abstract) subclasses complete

the implementation.

The Animal classpublic abstract class Animal{ fields omitted  /** * Make this animal act - that is: make it do * whatever it wants/needs to do. */ abstract public void act(List<Animal> newAnimals);  other methods omitted}

Further abstraction

Selective drawing (multiple inheritance)

Multiple inheritanceHaving a class inherit directly from multiple

ancestors.Java forbids it for classes.Java permits it for interfaces.

No competing implementation.

Interfaceinterface

A Java programming language keyword used to define a collection of method definitions and constant values. It can later be implemented by classes by means of the "implements" keyword.

Interfaces as method specifications Interfaces specify method signatures only

(like abstract methods.. The only difference is all the methods of an interface must be like this).

Each method signature is followed by a semi-colon (;)

An Actor interface public interface Actor{ fields omitted

/** * Perform the actor's daily behavior. */ void act(List<Actor> newActors);  other methods omitted}

Features of interfacesAll methods are abstract.There are no constructors.All methods are public.All fields are public, static and final.

Multiple interfacesBecause interfaces simply specify method

signatures, a single class can implement several different interfaces in order to ensure more methods can be implemented.

Classes implement an interfacepublic class Fox extends Animal implements Drawable{ any desired properties // implement required methods [modifiers] returnType methodName1(arguments) { executable code }

any other desired methods}

public class Hunter implements Actor, Drawable{ ...}

Implementing an Interface When a class implements an interface, it is

essentially signing a contract. Either the class must implement all the methods

declared in the interface and its superinterfaces, or the class must be declared abstract.

The method signature in the class must match the method signature as it appears in the interface. A class that implements the ActionListener interface must contain the method ActionPerformed

Interfaces as typesImplementing classes do not inherit code, but ...... implementing classes are subtypes of the

interface type.So, polymorphism is available with interfaces as

well as classes.

Example Interfaces

Interfaces are useful for the following:Capturing similarities among unrelated classes

without artificially forcing a class relationship Declaring methods that one or more classes are

expected to implement Modelling multiple inheritance, a feature of some

object-oriented languages that allows a class to have more than one superclass

THANK YOU.