Presentation 6 Polymorphism through Interfaces

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Presentation 6PolymorphismthroughInterfaces

• C Sc 335
• Rick Mercer

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From Wepodia http//www.webopedia.com/TERM/p/polym
orphism.html

• polymorphism Generally, the ability to appear in
  many forms. In object-oriented programming,
  polymorphism refers to a programming language's
  ability to process objects differently depending
  on their data type or class. More specifically,
  it is the ability to redefine methods for derived
  classes. For example, given a base class shape,
  polymorphism enables the programmer to define
  different circumference methods for any number of
  derived classes, such as circles, rectangles and
  triangles. No matter what shape an object is,
  applying the circumference method to it will
  return the correct results. Polymorphism is
  considered to be a requirement of any true
  object-oriented programming language (OOPL).

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From Computing Fundamentals, Mercer

• To understand polymorphism, take an example of
  a typical workday in an office. Someone brought
  in pastries and everyone stood around chatting.
  When the food was mostly devoured, Jim, the
  president of the company, invited everyone to
  Get back to work. Sue went back to read a new
  section of a book she was editing. Tom continued
  laying out the new edition of a book. Stephanie
  went back to figure out some setting in her
  word-processing program. Ian finished the company
  catalog.
• Jeni met with Jim to discuss a new project.
  Chris began contacting professors to review a new
  manuscript. And Krista continued her Web search
  on whether colleges are using C or Java. Rick
  went back to work on the index of his new book.
  Eight different behaviors from the same message!
  The message Get back to work is a polymorphic
  messagea message that is understood by many
  objects (or employees), but responded to with
  different behaviors by different objects (or
  employees).

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How to get polymorphism in Java

• Polymorphism A single name can represent
  different behaviors
• toString add get iterator compareTo
• In Java, polymorphism is possible through
• inheritance
• interfaces
• JButton and JTextField objects send
  actionPerformed messages to different types of
  listeners that implement ActionListener
• Collections.sort sends compareTo messages to many
  different types that implement Comparable

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Polymorphism

• Polymorphism (many forms) allows the same message
  to be sent to different types
• The runtime message finds the correct method
• same message can invoke different methods
• the reference variable knows the type
• anAccount.toString()
• aString.toString()
• anArrayList.toString()
• aButton.addActionListener(buListener)
• aTextField.addActionListener(tfListener)
• aMenuItem.addActionListener(meListener)

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Polymorphism through Interfaces

• An interface describes a set of methods
• no constructors
• no instance variables
• The interface must be implemented by some class
• 646 java classes implement one or more interfaces
• Multiple classes implement the same interface
• each class is guaranteed to have the same methods
• Let's begin with a simple interface

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Old MacDonald had a farm

• Interfaces, a Java type, have public method
  headings followed by semicolons.
• no
• No methods are implemented
• the implementing class will do it
• public interface BarnyardAnimal
• public String sound()

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Multiple classes implement the same interface

• To implement an interface, you must have all
  method specified exactly as written in the
  interface.
• public class Cow implements BarnyardAnimal
• public String sound()
• return "moo"
• public class Chicken implements BarnyardAnimal
• public String sound()
• return "cluck"

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What is the output?

• BarnyardAnimal animalOne new Cow()
• BarnyardAnimal animalTwo new Chicken()
• System.out.println(animalOne.sound() " "
• animalOne.sound() " here,
  and a")
• System.out.println(animalTwo.sound() " "
• animalTwo.sound() "
  there.")
• System.out.println("Here a " animalOne.sound())
  
• System.out.println("there a "
  animalTwo.sound())
• System.out.println("everywhere a "
• animalTwo.sound() " "
• animalTwo.sound() ".")

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Interfaces

• Interfaces are used by Java in many ways
• You class must implement the Comparable interface
  to sort and binarySearch elements in a collection
• Several classes implement java.util.List
• Can have a List parameter take and ArrayList,
  Vector, or LinkedList argument
• Must implement an interface to handle events
• Here are three required interfaces
• ActionListener MouseListener KeyListener

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Polymorphism through interfaces

• Can have many different classes of ActionListener
  objects
• have dozens of classes implement ActionListener
• may need a different method for every button,
  text field, or menu item in a GUI for correct
  behavior
• They all can be treated as ActionListener objects
• They can be passed as arguments to this method
• public void addActionListener(ActionListener ae)
• Adds the specified action listener to receive
  action events from JButtons, JTextFields,
• Parameters aListener - an instance of a class
  that implements the ActionListener interface

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Polymorphism through interfaces

• Each ActionListener object has its own
  actionPerformed method
• actionPerformed is a polymorphic message
• Graphical components JButton, JTextField,
  JMenutItem send the polymorphic message
• we don't see them
• Run Polymorphism.java
• which is squeezed into to slide that follows

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• // The three listener clases from page 226 are
  included below with public removed so this file
  compiles
• // Construct a window and add some other
  graphical components to it
• import java.awt.event.ActionEvent // Need one as
  an argument for actionPerformed
• import java.awt.event.
• import javax.swing.JButton
• import javax.swing.JOptionPane
• public class Polymorphism
• public static void main(String args)
• // Construct instances of three classes that
• // can understand the actionPerformed
  messages
• ActionListener one new ListenerOne()
• ActionListener two new ListenerTwo()
• ActionListener three new ListenerThree()
• // The actionPerformed message needs an
  ActionEvent

Run this program to see the polymorphic
actionPerformed message sent to instances of
three different classes that implement
ActionListener. The output shows three different
methods executed with the same message
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Assignment Compatible

• Can assign an instance of a class that implements
  and interface to a variable of the interface type
• Comparable str new String("abc")
• Comparable anInt new Integer(123)
• Comparable acct new BankAccount("B", 1)
• Comparable day new Date()
• All Known Implementing Classes for Comparable
• BigDecimal BigInteger Byte ByteBuffer
  Character CharBuffer Charset CollationKey
  Date Double DoubleBuffer File Float
  FloatBuffer IntBuffer Integer Long LongBuffer
  ObjectStreamField Short ShortBuffer String
  URI

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Assignment Compatible

• Can pass instances of a class that implement an
  interface to the interface type parameter
• addActionListener(ActionListener anyListener)
• addActionListener(new ButtonListener())
• addActionListener(new TextFieldListener())
• Both classes must implement the ActionListener
  interface
• This design shows a preference to compile time
  errors rather than the runtime errors that occur
  later when the button sends a message the object
  does not understand

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Displaying an Image

• Use JOptionPane to display message
• JOptionPane.showMessageDialog(null, "Hello,
  World!")
• Note icon to the left

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ImageIcon implements Icon

• JOptionPane.showMessageDialog(
• null, "Hello, World!", "Message",
  
• JOptionPane.INFORMATION_MESSAGE, new
  ImageIcon("home34.jpg")) // could be.gif

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Here is the method heading

• public static void showMessageDialog(Component par
  entComponent,
• Object message,
• String title,
• int messageType,
• Icon icon)
• The 5th parameter is Icon, an interface
• The argument must be an instance of a class that
  implements the Icon interface

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Create your own Icon

• See pages 140..145 of Horstmann
• Shows how to paint
• We'll do that Thursday