Classes and Objects in Java

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Classes and Objects in Java
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Classes and Objects

• A Java program consists of one or more classes
• A class is an abstract description of objects
• Here is an example class
• class Dog ...description of a dog goes here...
  
• Here are some objects of that class

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Methods may contain temporary data

• Data described in a class exists in all objects
  of that class
• Example Every Dog has its own name and age
• A method may contain local temporary data that
  exists only until the method finishes
• Example
• void wakeTheNeighbors( ) int i 50
  // i is a temporary variable while (i gt 0)
  bark( ) i i 1

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Classes always contain constructors

• A constructor is a piece of code that
  constructs, or creates, a new object of that
  class
• If you dont write a constructor, Java defines
  one for you (behind the scenes)
• You can write your own constructors
• Example
• class Dog String name int age
  Dog(String n, int age) name n
  this.age age

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Diagram of program structure

• A program consists of one or more classes
• Typically, each class is in a separate .java file

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A bad program

• public class Dog String name int age
  Dog(String name, int age)
  this.name name this.age age
  public static void main(String args)
  bark() void bark()
  System.out.println("Woof!")

--------- new Dog("Fido", 5).bark()

• non-static method bark() cannot be referenced
  from a static context

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A complete program

• class Dog String name int age
  Dog(String n, int age) name n
  this.age age void bark()
  System.out.println("Woof!")

• void wakeTheNeighbors( ) int i 50
  while (i gt 0) bark( ) i i
  1 public static void main(String
  args) Dog fido new Dog("Fido", 5)
  fido.wakeTheNeighbors()
• // ends the class

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Overview

• In this presentation we will discuss
• Classes and objects
• Methods for objects
• Printing results

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Classes and objects

• A class is the type of an object
• int classSize
• Color myColor
• variable classSize has type int
• variable myColor has type Color
• classSize 5
• myColor Color.RED
• 5 is a literal value of type int
• Color.RED is a literal value of type Color
• There are exactly eight primitive types
• There are thousands of classes, and you can
  create more

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Declarations

• You declare variables to hold primitive values
  like this
• int classSize
• double area
• You declare variables to hold objects like this
• Color uglyBrown
• String myName

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Assignment statements

• An assignment statement has the form
• variable expression
• Examples
• classSize 57
• area pi radius radius
• uglyBrown new Color(175, 175, 30)
• myName "David Matuszek"

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Methods

• Primitives have operations, classes have methods
• You cannot define new operations, but you can
  define new methods for a class

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Data in classes and objects

• A class is the type of an object
• A class describes
• How to make a new object of that class
• Example new Color(175, 175, 30)
• What kind of data is in an object
• Example a Color object contains three numbers
  representing the amount of red, green, and blue
• The methods of an object (the actions it can
  perform)
• Example a Color object can tell you how much red
  it contains
• int redAmount myColor.getRed()

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Sending messages to objects

• We dont perform operations on objects, we talk
  to them
• This is called sending a message to the object
• A message looks like this
• object.method(extra information)
• The object is the thing we are talking to
• The method is a name of the action we want the
  object to take
• The extra information is anything required by the
  method in order to do its job
• Examples
• g.setColor(Color.pink)
• amountOfRed Color.pink.getRed( )

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Messages to a Graphics

• If you have a Graphics, and its name is g, here
  are some things you can do with g
• Tell it to use a particular color
• g.setColor(Color.ORANGE)
• Ask it what color it is using
• Color currentColor g.getColor()
• Tell it to draw a line
• g.drawLine(14, 23, 87, 5)

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Messages to a Color

• Once you make a Color, you cannot change it you
  can only ask it for information
• // Make a new purplish colorColor myColor
  new Color(100, 0, 255)
• // Ask how much blue is in itint amountOfBlue
  myColor.getBlue()
• // Ask the color for a brighter version of
  itselfColor brightColor myColor.brighter()
• The last method doesnt change the color it
  makes a new color

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String

• A String is an object, but...
• ...because Strings are used so much, Java gives
  them some special syntax
• There are String literals "This is a String"
• There is an operation, concatenation, on Strings
• Pat" Palmer" gives PatPalmer"
• In other respects, Strings are just objects

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String methods

• If s is the name of the string "Hello", then
• s.length() tells you the number of characters in
  String s (returns 5)
• s.toUpperCase() returns the new String "HELLO"(s
  itself is not changed)
• But you can say s s.toUpperCase()

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New vocabulary

• class the type, or description, of an object
• object an instance, or member, of a class
• message something that you say to a class,
  either telling it something or asking it for
  information
• a parameter, or argument, used in calling a
  method of a class or object

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The End
I invented the term Object-Oriented, and I can
tell you I did not have C in mind.
--Alan Kay, creator of
Smalltalk.