Introduction to Classes and Objects

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Introduction to Classes and Objects

• Dr. Tim Margush
• University of Akron

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Goals

• Differentiate between classes and objects
• Enumerate the basic components of a class
• Define and use a custom class
• Distinguish between primitive and reference types

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What is a Class?

• A description of the capabilities and attributes
  of an object
• attributes, properties, fields, data members
• methods, actions, messages
• In Java, all classes are subordinate to the class
  Object
• In Java, a class is sometimes called an object
  factory

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What is an Object?

• A block of memory holding an encoded
  representation of an instance of a class
• In Java, objects are usually referred to by a
  reference variable
• Reference variables contain address information
  specifying the location of the encoded data
  representing the object

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Class Essentials

• Access type
• Class name
• Parent name
• Attributes
• Methods
• Documentation

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Defining the Human Class

• //A pretty useless class
• public class Human
• private String name
• private char sex
• public String toString()
• return name " " sex

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Creating a Human

• //Using the class Human
• public class TestHuman
• public static void main(String args)
• Human me new Human()
• System.out.println(me.toString())

Error!
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Initialization

• //A pretty useless class
• public class Human
• private String name
• private char sex
• These instance variables are not initialized
  (properly)
• Default initialization is usually not sufficient

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Constructor

• //A pretty useless class
• public class Human
• private String name
• private char sex
• public Human()
• name "Adam"
• sex 'M'

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Creating a Human

• //Using the class Human
• public class TestHuman
• public static void main(String args)
• Human me new Human()
• System.out.println(me.toString())

Adam M
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Terminology

• Declare a variable
• String name
• Human me
• Initialize a variable
• name "Adam"
• me new Human()
• Instantiate an object

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Terminology

• Method header / body
• public String toString()
• Method call
• me.toString()
• Constructor
• public Human()

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Parameter

• //A more flexible, but still useless class
• public class Human
• private String name
• private char sex
• public Human(String aName)
• name aName
• sex 'M'

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Argument

• //Using the class Human
• public class TestHuman
• public static void main(String args)
• Human me new Human("Tim Margush")
• System.out.println(me.toString())

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Parameters

• Human me new Human("Tim Margush")
• public class Human
• private String name
• public Human(String aName)
• name aName
• sex 'M'

Tim Margush
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Terminology

• Parameter List
• public Human(String aName, char aSex)
• Packages
• default package (no import needed)
• class TestHuman uses class Human
• import java.util.Scanner
• class TestHuman might use a Scanner
• java.lang.String
• java.lang. is implicitly imported

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Terminology

• Return Type
• public String toString()
• public static void main(String args)
• Local variable
• Human me
• Instance variable (field)
• private String name
• Holds an attribute of the object

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Getters and Setters

• Use public methods to set or get instance
  variables
• public void setSex(char s)
• if (s'M') sex 'M'
• if (s'F') sex 'F'
• public char getSex()
• return sex

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Terminology

• Primitive types
• Data types directly supported by the language
• int a long b boolean done char initial
• These memory locations contain data encoded
  according to the type of the variable
• Reference Types
• Data refering to an object a pointer to the
  object
• String x Human me
• These variables contain a reference to an object

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Default Constructor

• An explicit constructor with no parameters
• Performs "customized default" initialization
• The implicit constructor provided by the compiler
  
• Performs "Java default" initialization
• zero, '\0' (nul), false, null
• Exists ONLY if there are no explicit constructors

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Constructor Invocation

• new Human()
• public Human()
• new Human('F')
• public Human(char s)
• new Human("Harold")
• public Human(String n)
• new Human("Fred", 'M')
• public Human(String n, char s)

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Floating Point vs Integer

• byte, short, int, long
• 1/8, 2/16, 4/32, 8/64 bytes/bits
• 2bits distinct values
• Range is -2bits-1 through 2bits-1-1
• float, double
• 4/32, 8/64 bytes/bits
• Fewer than 2bits distinct values
• Range includes selected rational numbers

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Precision

• Floating point data is sometimes precise
• 1.0 is stored exactly
• 0.1 is never exact the closest rational in the
  set of double data values is chosen
• Floating point operations often magnify errors
• println(0.16f 10) displays 1.5999999

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Precision

• Conditions can be misleading
• float x
• //some computation to determine x
• if (x ! 0)
• x may be 0.0000000000000000012 due to floating
  point error
• if (Math.abs(x) lt 1e-6)

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Floating Point IO

• double x in.nextDouble()
• println(x)
• printf(".3fn", x)
• printf("7.0f", x)

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Literals

• Data values specified directly in the source
• 32 46.7 true '\n' "Joe"
• Literals imply a data type
• int double boolean char String
• Special notation
• 32.7f 1f 9876543210L 037 0xFF 3e2

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GUI Dialogs

• Used to display messages or acquire input via a
  GUI container
• Standard dialogs are available in
  javax.swing.JOptionPane
• MessageDialog
• InputDialog

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A Message Dialog

• import javax.swing.JOptionPane
• public class TestDialogs
• public static void main(String args)
• JOptionPane.showMessageDialog(
• null, "Greetings Earthling!")
• System.out.println("Continuing...")

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Modal Dialog

• Pauses execution until the dialog is dismissed
• Dialog is displayed, execution pauses

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Modal Dialog

• Pauses execution until the dialog is dismissed
• Dialog is displayed, execution pauses
• Dialog is dismissed, execution resumes

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An Input Dialog

• import javax.swing.JOptionPane
• public class TestDialogs
• public static void main(String args)
• String sAge JOptionPane.showInputDialog(
• null, "How old are you?")
• int age Integer.parseInt(sAge)
• JOptionPane.showMessageDialog(null, "You
  are " age " years old. ")

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Summary

• A class is a factory for the instantiation of an
  object
• Each object has its own memory, and consequently
  its own characteristics.
• Each object shares methods with all other objects
  of the same class
• Methods can accept arguments and return a value

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Summary

• The new operator allocates memory for an object,
  invokes a constructor to initialize the object,
  and evaluates to a reference to the new object
• Local variables are allocated when their method
  is called and must be initialized before use

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Summary

• Instance variables
• Are generally private
• Are allocated when an object is created
• Have a default initialization
• Often initialized in constructors
• Get and Set methods are used to access and modify
  instance members

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Summary

• Variables can be a primitive or reference type
• Floating point types are not precise, but useful
  to represent some rational numbers