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Programming
James King
12 August 2003
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Aims
Give overview of concepts addressed in
Web based programming module
Teach you enough Java to write simple
web and network applications
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Warning
THIS IS NOT A WEB AUTHORING COURSE

We will not teach HTML, PSP, etc…
We will show you how to write Java
applications that use the web and internet
We will (hopefully) improve your
programming ability
We will teach Object orientated concepts,
good programming practice and how to
debug/document your code
It is vital to bring the code examples
with you to the lecture
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The Course Topics
Object orientation and Java
programming
Applets and GUI programming
File handling
Input and Output Streams
Multiple threads of execution
Network programming
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Useful Resources
These slides are the course book
hopelive.hope.ac.uk/imc/kingj
The BlueJ development environment is available
free from www.bluej.org
The blueJ book is useful if you want additional
practical exercises. Barnes, D and Kolling, M.
Objects first with Java – A practical introduction
using BlueJ. Prentice Hall. 2003
http://java.sun.com
This site contains a freely
downloadable version of Java for most operating
systems and computer hardware. It also contains
the very useful Java tutorial and Java language
documentation.
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Advice
You can’t learn to program just by
understanding the theory
Programming requires practice and
more practice.
Don’t give up because it is not easy to start
with once you grasp the concepts it will get
easier
Don’t be surprised when you make mistakes learn from them
No programming language or environment is
perfect sometimes the error messages are
difficult to understand or don’t point you to
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the cause of the problem
A few Words about the Labs
The labs are designed for you to learn how Java
program behave and experiment and have fun
You will learn faster and have more fun if you take the
programming examples and modify them to see what
happens.
 You will get used to the error messages
Try to run them in your head or on paper before
you run them on the computer
 you will get a feel for what the language can and can
not do
All the examples are carefully chosen to illustrate a
point. Don’t worry that they are not all useful programs
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This Weeks Topics
Object Orientated Concepts

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Objects
Methods and Attributes
Messages, parameters and protocols
Classes
Instances
Inheritance and overriding
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Objects
Basic Java Language Section
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Object oriented concepts
Objects
Objects model real world entities such as a
person, food, car, house, umbrella
Objects only need to model the “interesting”
information and behaviours


For a personnel database person may contain
name, address, age, current wage etc. and
behaviours such as change address, promotion
In a computer game health, current weapon and
direction may be important and behaviours such
as shoot, walk, run and change direction
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Anatomy of an object Terminology
The individual pieces of information
such as the health of a monster or the
number of bullets are called attributes
The entire information stored inside an
object can be referred to en masse as
its state
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Anatomy of an object
The behaviours of on object such as change
address are implemented inside methods
Methods are bundles of code
When invoked the code in a method is
executed line by line
Methods may examine and modify any of the
attributes inside the object they are part of
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Java Language Structure
Basic Java Language Section
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Java Structure – Based around
blocks
Blocks are defined between a { and a }
{ must be balanced by a }
A Method is a block with a name
Attributes also require names
Names cannot contain spaces or start
with a number such as 2
A block may be nested totally inside a
block but no block may be partially
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inside another block
Java Class Example
Name of class
class Hero
Name of attribute
Indicates
start
of
class
{
Name of method
int bullets;
void shoot()
{
Indicates start of method
bullets=bullets-1;
Indicates end of method
}
}
Indicates end of class
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Java Class Example2
class Monster
{
int health;
void take_damage()
{
health=health-10;
}
}
This is the body of
the class it contains
one method and
one attribute
This is the body of
method it contains
one line of code
and is inside the
body of the class
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Java Structure – Based around
blocks
One block
{
}
{
}
{
{
}
followed by
another
block is fine
{
{
}
One block
inside by
another
block is fine
}
One partially inside by
another block is an
error
}
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Object oriented concepts
What is a program?
A program is a collection of objects
(possible many different types) that
interact together by calling each other’s
methods.
For example in a computer game if the
hero shoots a monster several methods
are called:
The hero's gun uses one bullet (shoot
method)
The monster loses health (take
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damage method)
Object oriented concepts
Terminology
Invoking a behavior by calling a
method is referred to as sending the
object a message
The set of messages an object
understands is called its protocol
Sending an object a message it does
not understand usually results in a error
and your program stopping/crashing
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Collaborating Objects
user hits the
mouse button
Shoot message
bullets =
bullets -1
Take damage message
Hero object
receives a shoot
message
health =
health - 10
Monster object receives
a take_damage
message
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Objects Summary
The state of the object is stored inside the
object in attributes e.g. the age of the person
Responses to messages are stored as
executable code (the code associated with a
message is called a method).
The set of messages an object can
understand is its protocol
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Messages and Objects
Basic Java Language Section
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Simple Messages
Sometimes receiving a message is
enough information to perform your
behaviour
For instance receiving a message that it
is your birthday is enough to increment
your age
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Object oriented concepts
Objects – Java Example
Name of method
class Monster
{
int health;
void take_damage()
{
health=health-10;
}
}
Indicates it’s a simple
message
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Messages with Parameters
Sometimes you need additional
information to perform a behaviour.

For instance if the monster is hit by a nonstandard weapon we need to know how
much damage the weapon did.
This additional information is presented
as one or more parameters (also
called arguments in some older
books)
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Objects – More complex
Type of parameter
Messages (more about types later)
Name of
parameter
class Monster
{
int health;
void take_unusual_damage(int damage)
{
health = health - damage;
}
The attribute health is modified to take the
same value as the parameter minus its old
}
value. When modified its overwritten its
old value is then lost forever
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