Java Classes

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Java Classes
Using Java Classes Introduction to UML
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Java and object-orientation

• Java is an object-oriented language
• a Java program consists of one or more classes,
  each of which contain attributes and methods
• Java programs create and use instances of these
  classes called objects
• we can use UML (Unified Modelling language) to
  design and document our object-oriented designs
• UML class diagrams can easily be translated into
  Java code

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Why classes?

• convenient way of packaging together related data
  and functionality
• this is called encapsulation
• Example - Spike

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Spike's data

• colour of body and features
• position (x and y coordinates)
• height

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Spike's functionality

• construct a new Spike object and initialise its
  data
• draw
• move to a given position
• get its current position (x or y coordinate)
• change its height to a given value

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Java classes

• in Java we use the following terms
• attributes for the data
• methods for the functionality
• attributes are usually private
• other objects can't read or change them
• public methods are used to interact with the
  object
• typically methods could
• read an attribute
• set (change) an attribute
• do something (eg paint) using attribute values

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Why classes?

• once we have defined a class we can create one or
  more instances (objects) of that class
• class is the blueprint
• object is the actual instance
• each object has its own data
• can be different to that of other objects of the
  same class
• methods are used to interact with individual
  objects

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How to create and use objects

• declare a variable of the desired class
• Spike spike1
• create objects of that class using the
  constructor and the keyword new
• spike1 new Spike()
• use the name of the object to call its methods
• spike1.moveTo(50, 100)
• spike1.drawSpike()

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Other classes youve seen

• Console
• readString(), readInt(). methods
• System.out
• print() and println() methods
• You also used the String class
• Take a look at your first Java project again.

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UML class diagrams

• UML Universal Modelling Language
• standard notation for designing and documenting
  object-oriented systems
• a UML class diagram is a box with 3 compartments
• top class name
• middle attributes
• bottom methods

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More detailed class diagrams

• attribute visibility
• for public
• - for private
• type of attribute, return type of method
• after
• type and name of method parameters
• in brackets after method name

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Using classes

• very often in Object-Oriented programming we will
  use classes written by someone else
• part of Java language
• in libraries, like Console
• written by our colleagues (Spike)
• (hopefully) these classes will be well designed,
  tested and documented
• we can use them with confidence
• this is called software reuse
• allows rapid building of robust systems
• now we will go through another example of using a
  library (someone else's) class

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Day

• part of corejava library
• encapsulates all information about a date
• private attributes
• day
• month
• year
• all integers
• why not have a dayOfWeek attribute?

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Day methods

• what do you think the other methods do?
• hint it is good programming practice to give
  methods meaningful names!
• can refer to documentation for more information
• hopefully the class developer wrote meaningful
  comments!!

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Constructors

• all classes have a constructor method that is
  called when an object is first created
• all constructors must have the same name as the
  class and are public
• constructors never have a return type
• Java provides a default constructor
• creates space in memory for the object and all
  its attributes
• classes can provide their own version(s) to do
  any initialisation steps required
• which methods are the Day constructors and what
  do you think they do?

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Day class

• two constructor methods
• public Day()
• creates a Day object with todays date
• public Day(int yyyy, int m, int d)
• creates a Day object with given date

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get- methods

• often need a method to return the value of a
  private attribute
• way of reading the attribute without changing it
• conventional to use a get- method
• method getX( ) returns the attribute X
• public so other classes can use it
• return type is the same type as the attribute
• Day has three get- methods
• what do you think they do?

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set- methods

• set- methods are used to change the value of
  given attribute
• give write- access to a private attribute
• method setX(type a) sets the value of attribute X
  to the new value a
• public so other classes can use it
• parameter type is the same type as the attribute
• return type is usually void
• Day does not have any set- methods
• why do you think this is?

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toString() method

• all Java classes have a default toString() method
• returns a String containing information about the
  object
• the default is
• name of class _at_ hash code of the object
• for example Day_at_1a16869
• many classes override this method to return
  something more meaningful
• in this case Day2004,9,22
• name of class, then year, month, day in square
  brackets

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Day

• other methods of interest
• advance(int n) advances the date by n days
• not as easy as it looks have to consider end of
  months and years
• int weekday()
• returns the day of the week
• 0 for Sunday to 6 for Saturday
• int daysBetween(Day b)
• calculates and returns the number of days between
  2 Day objects

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Information hiding

• the logic within these methods is quite
  complicated
• fortunately we don't need to know how they are
  implemented
• as long as we know what they do, we can
  confidently use them without knowing how they do
  it
• in OO terms this is called information hiding

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Using the Day class

• we'd now like to write a program that uses some
  of the features of the Day class
• before we write our program we need to think
  about what it will do
• Scenario - a small application program is
  required which can
• ask the user for a number and tell them what date
  it will be in that many day's time

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How can we do this?

• could create a Day object
• it encapsulates all the information about a date
• what date should we represent?
• today's date
• use default constructor
• how can we get input from the user?
• use Console.readInt() method
• how can we advance the date?
• use the Day object's advance() method
• luckily we don't need to write it ourselves!!!

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How can we do this?

• how can we output the results?
• use the Day object's toString() method, and print
  the returned String to the console
• how should we structure the program?
• put code in one class
• DayInterface.java
• small, so put all code in main() method
• import corejava. to access Console and Day
  classes

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DayInterface.java

• import corejava.
• public class DayInterface
• public static void main(String args)
• Day theDate
• int amount
• theDate new Day()
• amount Console.readInt("How many days
  from now? ")
• theDate.advance(amount)
• System.out.println(amount " days from
  today is " theDate.toString())

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Output

• How many days from now? 250
• 250 days from today is Day2005,7,26

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Summary

• have looked at classes
• why we use them - encapsulation
• what they are made of (attributes and methods)
• how we can document them with UML class diagrams
• how we can use library classes in our own
  programs
• information hiding we don't need to know how
  they work!
• tutorial
• design some class diagrams
• write some more applications using other people's
  classes