Starting Chapter 4 Starting

1 Course Outline* Covered in first half until Dr. Li takes over. JAVA and OO: Review what is Object Oriented Programming How it is implemented in Java. Why we use Java some strengths of the language. some history. How can I write and run Java Programmes. editor, SDK, compiler, java tools Java is a very simple language: course requirement is some familiarity with an OO language. Will review the grammar and syntax – a handful of slides – you should be able to get what you need from a suitable text book Concentrate on Java core libraries How to structure code solutions in Java Patterns and anti-Patterns Java collections and generics But not in detail Assume familiar with an OO language. Except for …. Not so trivial and worth giving you an introduction What they are an why you need to be familiar with them. Starting

2 Course Outcomes You will be able to write, compile and run a simple helloworld programme. You will have the intellectual tool kit to turn yourself into a first rate programmer. If you are already a first rate programmer. well 1 st rate programmers are always looking to improve their skills and develop their toolkits I hope I will introduce you to some new ideas, which you can use Starting

3 Java & OO* JAVA and OO: Objects are a software method of modelling an object in the problem domain. The object may be concrete and relate a real object car, person, book It may be abstract and relate to a construct in the solution domain. It will typically have a state : a description of the object, which may include the properties which distinguish the object from others of the same type It will have methods : these change state and/or perform operations and calculations. Methods are accessed by sending messages to the objects. Once created objects have an existence independent of the other objects in the programme. Objects are created by the programmer defining a class and instantiating objects of that class. Object State Methods Messages Class : Instantiation Starting

4 Classes Class: A class is a recipe or blueprint for an object A class may be used to create many objects An object is a coherent bundle of methods and data. It is the dynamic realisation of a class. Only exists while the programme is running An object is easy to think about and talk about Well designed classes are good for implementation good for design and communicating ideas The set of methods is called the interface A message is sent to the method to perform a sequence of operations ( normally on the data) How these operations are achieved is of interest only to the author. Irrelevant to the user – called encapsulation Eases use – no need to understand implementation Eases development – implementation can be altered as often as required with no impact on the user. Implementation is flexible, interface should be fixed Class Persistance Useful throughout the process Encapsulation core part of OO Other meanings Persistance Design to the Interfacce 3 Principles Starting

5 OO description 3 Principles Starting

6 … 3 Principles Starting

7 Object creation* Objects are created. Once created they have an independent existance. And the objects themselves may create further objects or groups of objects Instantiated from classes Starting

8 Object communication(i) They send messages to each other The messages may simply change the state of another class, or a response may be elicited. They may return data or output data. Starting

9 Java & OO(i)* JAVA and OO : Design A java object should exhibit high coherence and weak coupling. All the information about that “object” should reside in one place : coherence One object should be (as far as possible) completely independent of all others. Independence promotes development and reuse. Use the object in another context. Develop the object with no reference to users. Encapsulation ( data hiding) the internals of an object should not be visible to the outside world. Inheritance all the methods and state of an class can be used as the basis of a new class. Inheritance allows us to use an old class simply without disturbing the operation of the base class benefitting from improvements in the base class Need to make clear these concepts and we will start by using BlueJ Object isolation Encapsulation Inheritance Starting

10 BlueJ* JAVA and OO : Programme creation and execution A number of ways to create, develop and execute java code. Simple editor and the command line SDK – eclipse, netbeans, ….. BlueJ BlueJ a beginners development environment. Graphical representation of the programme. Environment is free to download Quite sophisticated development is possible Simple debugger Standalone running of the programmes which have been developed Easy migration to netbeans. Starting

11 Creating Objects* Classes and Objects A class may contain almost nothing. Javadoc may be used to create documentation To create an object a constructor method is used. Provided by the JVM if not present public class car() { } Constructor public car(){ } Add some state Always put a default constructor in explicitly. Minimum class JAVADOC – tool shipped as part of the SDK Starting

12 Documentation* Using JAVADOC Comments in a java file are //single line /* everything between */ Comments with special format are picked up by javadoc and put in a suitable place in the documentation. Javadoc comments are about the interface – the only thing the user of a class cares about. Comments internal to the code are ignored. Constructors return only a pointer to the object created. One primitive data type is double Let us add a method. double Starting

13 Outputting* output Can be done using System.out.println():puts out strings Lets look at creating a object inside another object When a object is created if you want to refer to it you need to create a pointer of a suitable type to refer to it. System.out.println() of an object actually prints out the string from toString(). For useful output we need to override the toString method. The ability to replace the method of a superclass with a method of the subclass is crucial to OO programming. Relevant to sub class without messing up superclass JVM takes care of turning doubles to strings Here it is the same type Overriding Starting