CST252 Network Software Design

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CST252 Network Software Design

• Lecture 3 Objects and Classes

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First Assignment

• Now available on the website
• Simple Videostore program
• Following a fairly explicit recipe
• Due 11th March 2002
• May need code from website as examples
• EmployeeTest.java as basis
• GregorianCalendar.add(..,..)
• Searching for a String in an array of Strings

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Resources

• Java Tutorial (java.sun.com)
• Learning the Java Language/OO prog concepts
• Course Website
• users.wmin.ac.uk/lancasd/CCTM91
• Book
• First chapters of any Java book
• or C. Horstmann and G. Cornell. Core Java
• Volume 1 chapter 4

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Lecture 2 Review

• Fundamental Programming Structures
• Control Flow
• Reading input - two choices
• InputTest - using popup window
• ConsoleInputTest - using console
• Arrays
• Strings (documentation)
• Elementary Objects and Classes

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If/Else conditional

• if (YourSales gt Target)
• performance OK
• bonus 100
• else
• performance bad
• bonus 0

NO
YES
Sales gt target
performanceOK
performancebad
bonus 100
bonus 0
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Elementary Classes

• Modified HelloWorld classes
• Test class to create and invoke methods
• Class methods and how to call them
• Instance fields, private
• Today in more detail

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Quiz

• What (if anything) is wrong with the following
  lines?
• A) String inputstr ...//get input from user
• int age integer.parseInt(inputstr)
• B) long a new long256
• boolean b new boolean10
• for(int i0 ilta.length i)
• ai ii
• bi false

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Lecture 3 Overview

• About Objects and Classes
• O-O style of programming
• Using standard classes
• Date and Calendar classes
• Creating your own classes
• Employee class

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Object Oriented

• For modern, large non-scientific programs, helps
  organisation and management
• Classes are software Packages
• Data and methods together
• Clear and precise interface to use the class
• Objects
• Instances of a class
• More than one instance possible

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Classes - Examples

• Represent a single concept
• Point, Rectangle, Ellipse
• Data location (x,y), size
• Methods translate, rescale
• BankAccount
• Data account balance
• Methods credit, debit, getbalance
• Employee
• Data name, salary, phonenumber, idnumber
• Methods raisesalary, setphonenumber
• classes - nouns
• methods - verbs

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Objects

• An object is a particular example of a class
• An ellipse of a given size at a particular
  position
• A bankaccount with a given balance
• A specific employee
• There can be many objects from the same class
  differing in their internal data

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Terminology

• Encapsulation (data hiding)
• combining data and behaviour (methods) in a
  package, and hiding the implementation
• The data in an object are known as instance
  fields
• The functions and procedures that operate on the
  data are called methods

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Terminology cont.

• A specific object that is an instance of a class
  has a particular set of instance fields that
  together define its state
• Classes can be built up from other classes by
  extending them - this process is known as
  inheritance and is the subject of L4

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Using Existing Classes

• Java comes with large class libraries
• Have already used
• System.out
• Math
• These are special - you can use their methods
  without creating an object of the class (the
  methods are static)
• HelloWorld and other Test classes that contain
  the static method main() are similarly special

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Date class

• Date is a typical class from the Java library
• To create an instance of the class use a
  constructor
• This is a special method in the class with the
  same name as the class
• Use the new operator with the constructor
• new Date()
• This initialises the new object to the current
  date and time - so now can call a method

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A Date program

• import java.util.
• public class DateTest
• public static void main(String args)
• Date now new Date()
• String s now.toString()
• System.out.println(s)

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Syntax

• Use new to create an object of the class
• Call methods on that object using a dot
• object.method(parameters)
• The method toString() is a common method that is
  often found in Java library classes, and you
  should also implement it in your own classes

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Object Variable (reference)

• Date birthday new Date()

Date
birthday

• The object variable refers to the newly created
  object
• Different from variables for primitive types

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Object Declaration

• Date deadline //declaration
• deadline new Date() //OK
• deadline birthday //both refer to same object

Date
birthday
deadline
If deadline is now changed, birthday will also
change. This is not the same as happens for
primitive data types.
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Date Class

• The Date class is actually more like a time class
  - it contains the number of milliseconds since
  Jan 1st 1970.
• This is not very useful for manipulating dates in
  terms of years, months, and days - but is good
  for comparing dates (before/after)
• In order to allow internationalisation, methods
  of Date such as getYear() have been deprecated
  (see documentation) in favour of Calendar classes

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GregorianCalendar Class

• The GregorianCalendar class expresses dates in
  familiar calendar notation
• new GregorianCalendar()
• new GregorianCalendar(1999,11,31)
• new GregorianCalendar (1999,Calendar.DECEMBER,31)
  
• An add() method to change the date
• Calendar deadline new GregorianCalendar()
• deadline.add(Calendar.DAY_OF_MONTH, 3)

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Relate Dates/Calendars

• Conversion from Calendar to Date class
• GregorianCalendar hirecal new
  GregorianCalendar( year, month, day)
• Date hireday hirecal.getTime()
• From Date to Calendar
• Calendar cal new GregorianCalendar()
• Date now new Date()
• cal.setTime(now)

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A Date/Calendar program

• import java.util.
• public class DCTest
• public static void main(String args)
• Calendar due new GregorianCalendar()
• Date now due.getTime()
• System.out.println(now.toString())
• due.add(Calendar.DAY_OF_MONTH, 20)
• Date duedate due.getTime()
• System.out.println(duedate.toString())

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Dates/Calendar Example Code

• Examples used in this lecture
• DateTest.java
• DCTest.java
• To demonstrate DateFormat (java.text)
• DateFormatTest.java
• Larger code prints out the calendar for this
  month
• CalendarTest.java

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Building Classes Yourself

• So far HelloWorld type classes which just contain
  a single main() method and never need to be
  instantiated
• Usually need several classes, one of which is
  like a class tester containing a main method
• Last week modified HelloWorld class, today
  something more useful

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Employee Class

• Simple class representing employees, with
  instance fields for
• name
• salary
• start date of employment
• Methods for raising salary, and some get/set
  methods to access the private data
• Another EmployeeTest class to test it

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Files

• Two files, one for each class
• Can compile each separately, or just the test
  class (javac will compile the other classes
  referred to there)
• Or one file called by the same name as the public
  class (EmployeeTest.java). The other class is not
  labelled public
• In either case, two .class files are generated

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Public/Private Keywords

• One constructor and 4 methods, all public so any
  method in any class can call them
• 3 instance fields, all private so the only
  methods that can access them are methods of the
  employee class itself
• This is encapsulation

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Constructor

• Initialises the instance fields
• Same name as that of class
• No return value
• Class can have more than one constructor
• May take several parameters
• Only ever use with the new operator
• new Employee(James Bond,100000,1950,1,1)
• Cannot use to reset the instance fields

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Methods

• Can access private instance fields of their class
  - eg raisesalary()
• Get methods
• getName()
• getSalary()
• getHireDay()
• Sometimes make methods private to help break up
  the work of the public methods

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Naming Convention

• By convention (followed in all the code for the
  Java libraries)
• Class names begin with a capital letter
• Method and instance field names begin with a
  small letter (though they may have capitals later
  in the name)
• Follow this in your own code
• An example of quality of code for grading the
  assignment

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Get/Set methods vs public Instance fields

• If the instance fields are public (break
  encapsulation) then they can be accessed
  directly e.name (no brackets)
• This is generally a bad thing, it is better to
  use get/set (mutator/accessor) methods for the
  fields you want to be able to access
• This allows a change of internal rep and error
  checking without affecting other classes

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Static Keyword

• Static Fields
• Are the same for each object of the class
• Available without instantiating the class
  (Math.PI)
• Static Methods
• Do not operate on objects
• No need to instantiate the class
  main(),Math.exp(x)
• Cannot access other non-static methods or
  instance fields
• If you find yourself using lots of static
  keywords - you are doing something wrong

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Parameters to Methods

• Call by reference
• pass an address, so the method can modify whats
  there
• Call by value
• just pass a copy of the value, the original isnt
  changed

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Java is always call by value

• Straightforward for primitive types
• double percent 10
• harry.raiseSalary(percent)
• For object references
• The object reference is copied by value
• So can modify the object (eg instance fields)
• But cannot modify the original reference (eg swap)

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More About Constructors

• Often there are several constructors depending on
  what you want to initialise
• new GregorianCalendar()
• new GregorianCalendar(1999,11,31)
• Default constructor without parameters does not
  need to be specified (but better if it is)

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Packages

• To organise files for large projects
• Using packages
• import java.util.
• import java.sql.
• import java.util.Date

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Making Packages Yourself

• Before all the code for classes
• package uk.ac.wmin.lancasd.test
• The files for this package go into a directory
  corresponding to this path
• Classpath, and how the VM locates classes

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Design of Classes

• For large projects this is important
• Also important to be able to rapidly explain the
  overall features of designs to other people
  without describing code details
• Use pictures
• UML is a widely accepted standard for such
  pictures showing the relationship between classes
  - not covered in this class

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Class Design Hints

• Keep data private
• Initialise data
• Not all fields need get/set methods
• Break up large classes with too many
  responsibilities

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Summary - OO vs structured

• C programming
• structured programming
• common data and many functions
• Java programing
• Classes that encapsulate data and methods
• Objects instantiating the classes
• Reusability, safety, organisational/management
  advantages