Review of Objects

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Review of Objects
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• // header
• public class Lab1MainClass
• public int y
• public static void main(String args )
• int x
• supportMethod( )
• // end main method
• public static supportMethod( )
• // end supportMethod
• // end Lab1MainClass

class definition shared variables main
method not-shared method call support
method note end brackets
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Computer Memory
Memory space
Your program main
support method
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• // header
• import java.util.Random
• public class Lab1MainClass
• public int y
• public Random myGen
• public static void main(String args )
• int x
• myGen new Random()
• // end Lab1MainClass

library shared variables
? ?
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Instantiation

• You or someone else writes a class
• public class someClass
• ..
• You use the class in your program
• You must make a local copy
• someClass myObject new someClass( )
• Or, two-step
• public someClass myObject
• myObject new someClass()

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What is a class?

• A program, with no main
• Contains useful methods and variables, that you
  might want to use in your program, that HAS a
  main.

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• import javax.swing.JOptionPane
• import javax.swing.JButton
• public class test
• public static void main (String args )
• JOptionPane.showMessageDialog(null, Hello)
• JButton myButton new JButton( )

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Computer Memory
Memory space
The Java class libraries
Your program main
JOptionPane, JFrame, Random, Graphics, System,
100s of them
showMessageDialog
new
JButton methods
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Writing Programs
Memory space
The Java libraries
Your program main
10,000s of classes to choose from
millions of methods
new
objects
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What if Libraries arent sufficient?
Memory space
The Java libraries
Your program main
10,000s to choose from
millions of methods
Write your own class
new
objects
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What a Class does

• A collection of methods and variables.
• Every Object created from a class, operates as an
  independent, freely running program.
• Only one main allowed
• main spawns useable objects, sometimes many
  objects of the same class.

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What we know about Objects

• Classes are of no value until
• They are instantiated

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Classes

• Collections of methods and variables
• You write the classes to do real work

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Objects

• They are instantiated from Classes
• An object, is a Class that has been put to
  work.
• Since Classes can be used many times in the same
  program, each instance of a Class is a
  separate, usable copy.

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Copies of Classes

• Copy is an oversimplification, but the concept
  has run-time value
• A copy is created using the new statement.
• A copy of a Class is an Object.
• non-Static Classes can never be used directly.
• They must be copied to Objects
• The copy step is called instantiation.
• Your program owns and controls the copy.

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Instantiation

• Once a class is defined, making it your own for
  use, using the new keyword
• myClass myObject new myClass( )
• Creates an instance of the Class.
• the Class solves a Class of problems.

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Applications - The main Method

• The computer runs it, you dont.
• The computer calls the main method
• The main method uses methods, contained in
  Objects, derived from Classes that you write.

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• public static void main(String args)
• ShowColors app new ShowColors( )
• // any method in ShowColors is now useable
• // as app.MethodName( )

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The Random class

• See the demo