CCP111N Java Reminder Week 1

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CCP111N Java Reminder Week 1

• James King
• 2008/9

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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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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 cant learn to program just by understanding
  the theory
• Programming requires practice and more practice.
• Dont give up because it is not easy to start
  with once you grasp the concepts it will get
  easier
• Dont 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 dont point you to the cause of the
  problem

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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. Dont worry that they are not
  all useful programs

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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 inside another block

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Java Class Example
Name of class
class Hero int bullets void shoot()
bulletsbullets-1
Name of attribute
Indicates start of class
Name of method
Indicates start of method
Indicates end of method
Indicates end of class
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Java Class Example2
This is the body of the class it contains one
method and one attribute
class Monster int health void
take_damage() healthhealth-10
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 conceptsWhat is a program?

• A program is a collection of objects (possible
  many different types) that interact together by
  calling each others 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 damage method)

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Object oriented conceptsTerminology

• 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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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 conceptsObjects Java Example
Name of method
class Monster int health void
take_damage() healthhealth-10
Indicates its 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
  non-standard 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 Messages
Type of parameter (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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Creating Instances of Objects
Basic Java Language Section
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Modelling multiple People/entities/things

• Often you need to model lots of monsters such as
  in a castle
• It would be inefficient to separately write code
  for each Monster you will use
• We can manufacture multiple instances of a class
  such as Monster easily
• The class is a template for manufacturing
  instances

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Object oriented conceptsManufacturing Instances
Name of class to create an instance of
Name of object to store the instance

• Monster snotty new Monster()
• Monster grotty new Monster()

The type of this instance (should be same as Name
of class on the right of the new)
Note Java is case sensitive. By convention
instances of classes start with a lower case
letter and classes with an upper case letter
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Object oriented conceptsInstances

• An instance is an object of a particular class
• e.g grotty and snotty are the names for the
  instances of the Monster class we created
• Each instance has the same behavior
• Each instance has its own independent copy of the
  attributes
• When we create an instance of a class the
  attributes can be given starting values (about
  this more later)

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Sending messages to an instance

• In Java code we send messages to the names of the
  instances like this
• snotty.take_damage()
• grotty.take_unusual_damage(200)
• These messages will change the state of the
  object snotty by decreasing its health by 10 and
  grotty by decreasing its health by 200 OUCH!

Calls snottys take damage with no parameters
Calls grottys take unusual damage with a
parameter value 200
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How Parameters Work
Damage is initialised to 200 I.e. the value of
the parameter take_unusual_damage was called with

• class Monster
• int health
• void take_unusual_damage(int damage)
• health health - damage
• snotty.take_unusual_damage(200)
• // More code

Then the code inside the method is run
When this line is executed control jumps to the
take_unusual_damage method inside the snotty
instance of Monster
When the method has been completed the next line
after the call to the method is run
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Initialising Instances during creation
Basic Java Language Section
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Initialising Instances

• When new is used to create an object the
  attributes are created.
• We can initialise its attributes. This is done by
  creating a method with the same name as the class
• This special method is called the constructor

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Initialising Objects

• class Hero
• int bullets// attribute (state)
• Hero() // constructor called during new Hero()
• bullets 10
• h new Hero()
• // more code

Calls the constructor in Hero to initialise the
instance before storing it in h