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Introduction to Java 2 Programming

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1 Introduction to Java 2 Programming
Lecture 1 Java, Principles of OO, UML

2 Overview Introducing Java Principles of Object Oriented Programming
Key features of the language Principles of Object Oriented Programming What is OOP? Why is it important? Basic principles and advantages The Unified Modelling Language UML Class Diagrams

3 Some History Developed and maintained by Sun Microsystems
Originally called Oak Aimed at producing an operating environment for networked devices and embedded systems …but has been much more successful Design objectives for the language Simple, object-oriented, Distributed, multi-threaded, and platform neutral Robust, secure, scaleable When Java was produced as an independent language, it was envisaged that it would help revolutionise the web, providing more dynamic features to web pages, e.g. animation, more complex user interfaces, etc. However its actually had more success as a server-side development language rather than for applets or desktop application development. That’s not to say it can’t do those things, and do them well, but it’s a niche that its occupies very well due to a number of features. Dynamic web pages are now typically the realm of Macromedia Flash and similar tools.

4 The Virtual Machine Java is both compiled and interpreted
Source code is compiled into Java bytecode Which is then interpreted by the Java Virtual Machine (JVM) Therefore bytecode is machine code for the JVM Java bytecode can run on any JVM, on any platform …including mobile phones and other hand-held devices Networking and distribution are core features In other languages these are additional APIs Makes Java very good for building networked applications, server side components, etc.

5 Features of the JVM The Garbage Collector
Java manages memory for you, the developer has no control over the allocation of memory (unlike in C/C++). This is much simpler and more robust (no chance of memory leaks or corruption) Runs in the background and cleans up memory while application is running The Just In Time compiler (JIT) Also known as “Hot Spot” Continually optimises running code to improve performance Can approach the speed of C++ even though its interpreted These two features provide some big advantages. Memory management can be error prone. Memory leaks are often difficult to diagnose properly (or even identify). Anyone who has worked with C/C++ is likely to have encountered these problems. The JIT provides a task that the developer would otherwise have to do manually: profile the code to determine the performance critical sections. While this is still worthwhile, the basic JIT compiler in the JVM provides a good head-start. Again profiling is a tricky task as performance bottlenecks are never where you expect. Because the JVM has complete knowledge of what an application does, its in a position to adjust the code to provide some basic tuning. This means performance testing a Java application needs to allow time for the JIT to kick in. Typically see slower performance early on, which then improves before reaching a plateau.

6 Features of the JVM Security Class Loading
Java offers very fine control over what an application is allowed to do E.g. Read/write files, open sockets to remote machines, discover information about the users environment, etc Used in Java Applets to create a “sandbox”. Stops a rogue applet attacking your machine. Makes Java very safe, an important feature in distributed systems Class Loading Loading of bytecode into the virtual machine for execution Code can be read from a local disk, over a network, or the Internet Allows downloading of applications and applets on the fly …and even ‘mobile code’

7 Versions of Java Java Language vs Java Platform
Current version of the language is 1.4.1 Core language plus additional APIs is called the Java 2 platform Three versions of the Java 2 Platform, targetted at different uses Java 2 Micro Edition (J2ME) Very small Java environment for smart cards, pages, phones, and set-top boxes Subset of the standard Java libraries aimed at limited size and processing power Java 2 Standard Edition (J2SE) The basic platform, which this course will cover Java 2 Enterprise Edition (J2EE) For business applications, web services, mission-critical systems Transaction processing, databases, distribution, replication

8 The Java APIs Sun are constantly adding new features and APIs
The Core Java API is now very large Often difficult to keep up with every change Separate set of extension APIs for specific purposes E.g. Telephony, Web applications, Game programming All new developments reviewed through Java Community Process (http://www.jcp.org) Chance for developers to provide feedback on emerging standards and APIs Useful to keep an eye on what's coming through Also a wide range of “open source“ APIs available E.g. through the Jakarta project (http://jakarta.apache.org)

9 Useful Resources Useful resources on the web
Java home (http://java.sun.com) Articles, Software and document downloads, Tutorials Java Developer Services Early access downloads, forums, newsletters, bug database Javaworld (http://www.javaworld.com) Java magazine site, good set of articles and tutorials IBM developerWorks (http://www.ibm.com/developerWorks) Technology articles and tutorials

10 Principles of Object Oriented Programming
Introducing Java Key features of the language Principles of Object Oriented Programming What is OOP? Why is it important? The Unified Modelling Language UML Class Diagrams

11 Object-Oriented Programming
Understanding OOP is fundamental to writing good Java applications Improves design of your code Improves understanding of the Java APIs There are several concepts underlying OOP: Abstract Types (Classes) Encapsulation (or Information Hiding) Aggregation Inheritance Polymorphism There are two ways to approach Java. Either as just another language, I.e. begin with the basic syntax and show how procedural programs can be created, and then teach OO concepts later on. Or you begin with the OO concepts and introduce the syntax along the way. I prefer the latter approach, which this course will take, because understanding OOP is critical to working well with Java. You can only get the most from the language by understanding these features. OOP has several key concepts which we’ll review now, but will investigate in more detail as we learn more about the Java language. At this point we’re just introducing some of the basic terminology.

12 What is OOP? Modelling real-world objects in software
Why design applications in this way? We naturally classify objects into different types. By attempting to do this with software aim to make it more maintainable, understandable and easier to reuse In a conventional application we typically: decompose it into a series of functions, define data structures that those functions act upon there is no relationship between the two other than the functions act on the data OO modelling isn’t only about representing real-world objects, but it’s a useful working definition. Contrast functional decomposition versus OOP. Note that many languages have added “object based” Features so these are extreme views, but should illustrate the main differences.

13 What is OOP? How is OOP different to conventional programming?
Decompose the application into abstract data types by identifying some useful entities/abstractions An abstract type is made up of a series of behaviours and the data that those behaviours use. Similar to database modelling, only the types have both behaviour and state (data) Note that this is reminiscent of database modelling: we identify the entities that we want to model and then define their attributes. Here we go a step further and identify the behaviours that those entities should have so we can use that data.

14 Abstract Data Types Identifying abstract types is part of the modelling/design process The types that are useful to model may vary according to the individual application For example a payroll system might need to know about Departments, Employees, Managers, Salaries, etc An E-Commerce application may need to know about Users, Shopping Carts, Products, etc Object-oriented languages provide a way to define abstract data types, and then create objects from them It’s a template (or ‘cookie cutter’) from which we can create new objects For example, a Car class might have attributes of speed, colour, and behaviours of accelerate, brake, etc An individual Car object will have the same behaviours but its own values assigned to the attributes (e.g. 30mph, Red, etc) In Java an abstract type is known as a class.

15 Note that there’s no relationship between data and functions
Note that there’s no relationship between data and functions. Data may be reused in several places, so each function has to have knowledge of that data, e.g. what types it contains, where its stored, etc, etc. In an OO model each object maintains its own data. The functions and data are “fused” into an abstract data type. Note that this is more manageable as I can change the structure of any piece of data, e.g. integer to floating point, add/remove new pieces of information without impacting any of the rest of the application. It’s what helps make OO systems more reusable.

16 Encapsulation The data (state) of an object is private – it cannot be accessed directly. The state can only be changed through its behaviour, otherwise known as its public interface or contract This is called encapsulation

17 Encapsulation Main benefit of encapsulation
Internal state and processes can be changed independently of the public interface Limits the amount of large-scale changes required to a system

18 What is an OO program? What does an OO program consist of?
A series of objects that use each others behaviours in order to carry out some desired functionality When one object invokes some behaviour of another it sends it a message In Java terms it invokes a method of the other object A method is the implementation of a given behaviour. OO programs are intrinsically modular Objects are only related by their public behaviour (methods) Therefore objects can be swapped in and out as required (e.g. for a more efficient version) This is another advantage of OO systems OO programs are highly modular, which can make them very easy to customise and extend. For example I could switch in an entirely different object into the application so long as it has the same behaviour (public interface) as the previous object. The rest of the system is oblivious to the change. That’s why the public interface of an class/object is referred to as its “contract”. Just like a legal document, it can be seen as an agreement between an object and the others that use its behaviour, that if they send it a particular message (or in Java terms call its method) then some operation will be carried out. How that operation is achieved is of no interest to the calling object.

19 Aggregation Aggregation is the ability to create new classes out of existing classes Treating them as building blocks or components Aggregation allows reuse of existing code “Holy Grail” of software engineering Two forms of aggregation Whole-Part relationships Car is made of Engine, Chassis, Wheels Containment relationships A Shopping Cart contains several Products A List contains several Items Aggregation should be familiar to anyone who has used Visual Basic: forms are made up from other components (controls). In engineering terms its like taking pre-fabricated components (e.g. resistors, transistors, etc) and building something out of those parts (e.g. a radio). Should be obvious why this is the “holy grail” – could significantly Reduce costs of developing software. While OO doesn’t get us there, its currently the best way we know of building maintainable applications.

20 Inheritance Inheritance is the ability to define a new class in terms of an existing class The existing class is the parent, base or superclass The new class is the child, derived or subclass The child class inherits all of the attributes and behaviour of its parent class It can then add new attributes or behaviour Or even alter the implementation of existing behaviour Inheritance is therefore another form of code reuse Define one type in terms of another type. Both motorbikes and cars are types of vehicles.

21 Polymorphism Means ‘many forms’
Difficult to describe, easier to show, so we’ll look at this one in a later lesson In brief though, polymorphism allows two different classes to respond to the same message in different ways E.g. both a Plane and a Car could respond to a ‘turnLeft’ message, however the means of responding to that message (turning wheels, or banking wings) is very different for each. Allows objects to be treated as if they’re identical

22 Summary! In OO programming we Benefits of OO programming
Define classes Create objects from them Combine those objects together to create an application Benefits of OO programming Easier to understand (closer to how we view the world) Easier to maintain (localised changes) Modular (classes and objects) Good level of code reuse (aggregation and inheritance)

23 Overview Introducing Java Principles of Object Oriented Programming
Key features of the language Principles of Object Oriented Programming What is OOP? Why is it important? The Unified Modelling Language UML Class Diagrams

24 Unified Modelling Language
UML is a diagramming tool for describing and documenting object oriented applications Programming language independent Used for modelling an application before its engineered Twelve different diagrams in all, with many complex details Generally though only two of these are used regularly Class diagrams Sequence diagrams The reason I’m introducing a few UML basics is because it makes talking about applications much easier, as we can produce a diagram describing the classes and their relationships, rather than just jumping into the code. It’s also a useful skill in its own right, and is generally encountered in OO design documents.

25 Unified Modelling Language
Class Diagrams Describe classes and interfaces …their properties …their public interface …and their relationships (e.g. inheritance, aggregation) Sequence Diagrams Describe how objects send messages to one another Useful for describing how a particular part of an application works We’ll be covering just class diagrams Very useful for describing APIs and discussing OO applications

26 UML -- Classes Box with 3 sections The top contains the class name
The middle lists the classes attributes The bottom lists the classes methods Can indicate parameters and return types to methods, as well as their visibility

27 UML -- Association A line between two classes indicates a relationship
Extra information can be added to describe the relationship Including Its name The roles that the classes play The cardinality of the relationship (how many objects are involved) E.g. a Person worksFor a Company, which has many employees

28 UML -- Comments Useful for adding text for the readers of your diagram
The symbol looks like a little post-it note, with a dotted line joining it to the class or relationship that its describing

29 UML -- Aggregation Aggregation (a whole-part relationship) is shown by a line with clear diamond. As aggregation is a form of relationship you can also add the usual extra information I.e. Name Roles Cardinality

30 UML -- Inheritance Inheritance is shown by a solid arrow from the sub-class to the super-class The sub-class doesn’t list its super-class attributes or methods, unless its providing its own alternate version (I.e. is extending the behaviour of the base class)

31 UML -- Interfaces Interfaces are a way to specify behaviour (a public contract) without data or implementation. Interfaces are classed with an extra label next to their name: <<Interface>> A dotted arrow from a class to an interface explains that the class fulfills the contract specified by that interface

32 Example #1

33 Example #2

34 Example #3


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