Object Oriented Programming

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Object Oriented Programming

• mr. Hanley

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Programs built from smaller components

• Todays software programs are generally built
  from pieces of code known as classes
• These classes serve all kinds of different
  purposes, from displaying visual elements on the
  screen to keeping track of files, dates and
  times, words and customers, aliens and heroes
  (including Master Chief of course)

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What is a class?

• But what is a class? Arent I in a class? Im
  confused!!!
• A class is similar to a blue print for a house.
  It includes a set of instructions that describe
  how a house is made, what kind of elements a
  house will contain as well as what kinds of
  things a the house will be able to do once its
  constructed.

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What classes written by other people have we used
so far?

• Scanner
• System
• Random
• Math
• String

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How are these classes?

• These classes have been provided by various
  programmers and offered to us for free (all
  right, got to love a freebie!!!)
• Each of these is a blue print in java for how a
  specific software component (object) will behave
• Now Im really confused, can you explain further?

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Ok, so what is an object?

• An object is like the constructed house, while a
  ____________ is like the blue print.
• Ok, I think Im starting to get it, can you give
  me an example?
• JTextField is a class, radiusTF is an object
• Inside of a frame, radiusTF is instanciated as a
  JTextField with the following command
• JTextField radiusTF new JTextField()

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An example Object
radiusTF
text
bounds 120, 80, 50, 20
background color Color.grey

• So, what happens when we create an object?
• Our computer allocates memory for the object

foreground color Color.black
editable true
visible true
public String getText()
public JTextField()
public void setText(String txt)
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Objects have their own data
radiusTF
text
bounds 120, 80, 50, 20
background color Color.grey

• Each object has its own instance variables or
  state
• This object, for example has a background color
  of grey and is editable

foreground color Color.black
editable true
visible true
public String getText()
public JTextField()
public void setText(String txt)
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Objects communicate via messages
radiusTF
text Radius will go here
bounds 120, 80, 50, 20
background color Color.grey

• To control objects, we send them messages
• To set the text for a JTextField, we send the
  message setText with a String inside the ()
• radiusTF.setText(Radius will go here!)

foreground color Color.black
editable true
visible true
public String getText()
public JTextField()
public void setText(String txt)
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You can also examine the state
radiusTF
text Radius will go here
bounds 120, 80, 50, 20
background color Color.grey

• An object can report its state via a message
  typically name get----()
• Example, getText() will send the caller whatever
  phrase is currently in the JTextField
• String temp radiusTF.getText()

foreground color Color.black
editable true
visible true
public String getText()
public JTextField()
public void setText(String txt)
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Constructors are cool
radiusTF
text Radius will go here
bounds 120, 80, 50, 20
background color Color.grey

• Constructors are special methods
• They give an object its initial state by setting
  up all of its variables
• Constructors ALWAYS have the same name as the
  class!!!

foreground color Color.black
editable true
visible true
public String getText()
public JTextField()
public void setText(String txt)
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What happens when multiple objects are created?
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Cool, what can objects do?

• As we mentioned before, a blueprint lays out how
  a house is put together (constructors), what
  things will be in the house (instance variables)
  and what kinds of things a house can do (methods)
• Lets look at another class to see how these
  three parts of a blueprint work

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• //From page 162 of your text
• public class Student
• //instance variables
• public String name
• private int test1
• private int test2
• private int test3
• //Constructor method
• //Initialize a student's name to the empty
  string and his test scores to zero
• public Student()
• name ""
• test1 0
• test2 0
• test3 0

//Other methods public Student(String nm)
name nm test1 0
test2 0 test3 0 //set a
students name public void setName(String nm)
name nm /Get a student's name public
String getName() return name
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• ////Compute and return a student's average
• public int getAverage()
• int average //will hold the average of the
  3 quizzes
• average (int)Math.round((test1test2test3)/
  3.0)
• return average
• Compute and return a student's highest score
• public int getHighScore()
• int highScore
• highScore test1
• if (test2 gt highScore) highScore test2
• if (test3 gt highScore) highScore test3
• return highScore

• public void setScore(int i, int score)
• if (i 1) test1 score
• else if (i 2) test2 score
• else test3 score
• //Get the score on the indicated test
• public int getScore(int i)
• if (i 1)
• return test1
• else if (i 2)
• return test2
• else
• return test3

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• //Return a string representation of a student's
  name, test scores and average
• public String toString()
• String str
• str "Name " name "\n"
• "Test 1 " test1 "\n"
• "Test 2 " test2 "\n"
• "Test 3 " test3 "\n"
• "Average " getAverage()
• return str
• //end of class

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How is the student class used?

• An application can create instances of a class by
  declaring a variable of that class and by using
  the new command
• For example, to create 4 student objects
• Student s1 new Student()
• Student s2 new Student()
• Student s3 new Student()
• Student s4 new Student()

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First, the house construction

• Whenever an object is instanciated, a constructor
  is activated
• The zero arg constructor simply sets the name to
  be blank and the test scores to 0
• A constructor is called for each object
• The constructor is called 4 times
• In the StudentFrame, the one arg constructor is
  used to pass the Student Name in upon creation

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What happens when we instantiate 4 students?
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What kinds of things appear in class files?

• Import statements
• Used to identify where classes are coming from
• Extends relationships
• Inheritance relationship
• Builds upon the class it extends
• All methods and variables from ancestors are
  inherited

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What kinds of things appear in class files?

• Implements relationships (one or more)
• Means that certain methods MUST be present
• These methods have specific names and parameters
• You can leave them blank at first but must at
  least stub code them
• Global variable declarations
• Global variables exist for the life of the object

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What kinds of things appear in class files?

• Methods
• Methods are sub commands that exist in the class
• They must be called or invoked in order to begin
  executing
• Public methods are callable by client programs
• Private methods can only be called by this class
• When a method is called, any parameters that are
  in the () must be supplied

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What kinds of things appear in class files?

• Constructors
• Constructors are special methods that get called
  whenever a new command is issued for this class
• Constructors MUST have the same name as the class
• A class may have multiple constructors with
  different parameters
• The job of the constructor is to give the object
  an initial state

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What kinds of things appear in class files?

• Constants
• Constants are cool, they allow a programmer to
  define a value that wont change throughout the
  program
• PI, speed of light in a vaccum, number of onces
  in a liter
• final double OZSPERLITER 33.8140227
• Capital letters are used by tradition by C and
  C programmers for constants, please stick with
  this tradition

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What kinds of things appear in class files?

• Program Comments
• //Used to document the program for other
  programmers
• / This method works for multiline comments/
• Inner classes and other classes
• Used when they are only needed by this particular
  class

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Summary

• Classes are used to provide reusable software
  blueprints to other programmers
• This concept is incredibly powerful
• Upon creating instances of these classes, we gain
  access to all of the data and logic that these
  blueprints contain