Java

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Java Writing it!
Programming in Java a recap in to the
implementation of Java OO Programming.

• John Murray john.murray_at_sunderland.ac.uk

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A good coding idea

• Java was developed by Sun Systems
  http//java.sun.com
• Language developed to help with the idea of code
  reusability
• Multi platform
• Been around for over 10 years

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Contents of a Java Class

• Constructor inputting parameters
• Attributes Class Variables
• Methods Code functions

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Class content descriptions

• Constructor
• Used when creating an instance of a class so that
  parameters / variables can be initialised.
• Constructor can be empty or can accept inputs.
• Constructor can call other methods of the class
  or any class that it has access to (providing
  methods are public)

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Class contents description cont.

• Constructor
• public class Tree
• public Tree() //Empty Constructor
• public Tree(String theName)
• treeName theName

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Class contents description cont.

• Attributes
• Used to store variables / parameters used in the
  class.
• Can be either public, private or protected
• Can use any of the Java primitive types.

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Class contents description cont.

• Attributes
• public class Tree
• public String treeName
• public int ageOfTree
• public double widthOfTrunk

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Class contents description cont.

• Methods
• Are used to execute / call a specific section of
  code.
• Think of like a C or C function.
• Can receive parameters or return them

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Class contents description cont.

• Methods
• private void setTreeName(String theName)
• treeName theName
• public int getAgeOfTree()
• return ageOfTree

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Complete Class

• public class Tree
• public String treeName
• public int ageOfTree
• public double widthOfTrunk
• public Tree(String theName)
• treeName theName
• private void setTreeName(String theName)
• treeName theName
• public int getAgeOfTree()
• return ageOfTree

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Exceptions

• Java has a very powerful exception handling
  system.
• Program may compile however invalid runtime
  errors may occur due to user input.
• These need to be handled!
• Use of try, catch and finally.
• Method must specify if it is to throw an
  exception

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The Try Block

• The try block contains any code that may be
  able to throw an exception
• BufferedReader read new BufferedReader(new
• InputStreamReader(System.in))
• public void getAge()
• ageOfTree read.readLine()

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Using the Try block

• public void getAge() throws IOException
• try
• int age read.readLine()

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The Catch block

• The catch block is used to process the exception
  thrown.
• Can have multiple catch blocks each processing a
  different exception.
• catch (InterruptedIOException ie)
• System.out.println(Stream was interupted
  ie)
• catch (IOException ioe)
• System.out.println(High Level Exception
  ioe)

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The Finally block

• finally is used to execute any code that is
  needed if none of the catch blocks resolve the
  problem.
• finally
• int ageOfTree 20 // Specify a default age

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Try Catch Finally

• try
• int ageOfTree read.readLine() // Get from
  user
• catch (InterruptedIOException ie)
• System.out.println(Stream was interrupted
  ie)
• catch (IOException ioe)
• System.out.println(High Level Exception
  ioe)
• finally
• int age 20 // Set default age

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Creating class instances

• We want to be able to create multiple instances
  of our Tree class, each having their own name and
  other properties.
• To do this we have to think about efficient code
  and code reusability.

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Consider the Tree class

• public class Tree
• int ageOfTree
• String treeName
• double widthOfTrunk
• public Tree(String theName, int theAge, double
  theWidth)
• treeName theName
• ageOfTree theAge
• widthOfTrunk theWidth

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Creating an instance of Tree

• An instance of a class creates a copy of that
  object in memory initialised with the set
  parameters.
• public class Forrest
• public static void main(String args)
• // This line creates an instance of Tree
• Tree oak new Tree(Oak,100,1.2)

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Creating a whole Forrest

• You can create arrays of the Tree class as
  follows
• public class Forrest
• public static void main (String args)
• Tree myTrees new Tree500
• myTrees0 new Tree(Oak,,)
• myTrees1 new Tree(Fair,,)
• . . . . .

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Abstract classes

• abstract classes are designed in the same was as
  we think about the word abstract, we abstract
  away from the inner workings of the world.
• Take a car, if we abstract away all we concern
  our self with is a car is simply an automobile,
  such as a motorbike, tractor or truck, i.e. a
  form of transport.

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Abstracting

• What properties does an automobile have?
• Wheels
• Registration number
• Colour
• Plus others
• Therefore, our abstract class should contain
  these as EVERY subclass of Automobile will also
  have them.

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Our Abstract Class

• If we use the Automobile example
• public abstract class Automobile
• public abstract void numOFWheels(int theWheels)
• public abstract void regNum(String theReg)
• public abstract void setColour(String
  theColour)
• This class ensures that any sub-class MUST have
  these methods.

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Sub-Classes

• We now create any sub classes of Automobile using
  extends
• public class Car extends Automobile //Class
  derives from Automobile
• // From super class
• public void numOfWheels(int theWheels)
• wheels theWheels
• // Methods only appropriate to this class
• public void setEngineSize(double theEngine)
• engine theEngine

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Creating instances of sub-class

• Now when we want to create instances of Car we
  have to ensure we override all abstract methods
  in Automobile.
• Car.java1 Car is not abstract and does not
  override abstract method setColour(java.lang.Strin
  g) in Automobile
• public class Car extends Automobile

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Summary

• Creating class
• Passing parameters between classes
• Understand Exceptions
• Be able to handle exceptions
• Abstract Classes
• Create
• Extend

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Questions

• ?