Polymorphism via Interfaces

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Polymorphism via Interfaces

• By Rick Mercer with help from many sources

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Polymorphism from Wepodia http//www.webopedia.com
/TERM/p/polymorphism.html

• In general, polymorphism is 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
• Polymorphism is considered to be a requirement of
  any true object-oriented programming language
  (OOPL)

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Polymorphism from mercer

• To understand polymorphism, take an example of a
  typical workday in an office. Kim 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 a
  book. Stephanie went back to figure out some
  setting in her word-processing program. Ian
  finished the company catalog.

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Polymorphism

• 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. Marty
  went back to work on the index of his new book.
  And Kim cleaned up the pastries.

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Polymorphic Messages

• 9 different behaviors with the same message!
• The message Get back to work is a polymorphic
  message
• a message that is understood by many different
  types of object (or employees in this case)
• but responded to with different behaviors based
  on the type of the employee Editor, Production,
  Marketing,

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Polymorphism

• Polymorphism allows the same message to be sent
  to different types to get different behavior
• In Java, polymorphism is possible through
• inheritance
• Overide toString to return different values
• interfaces
• Collections.sort sends compareTo messages to
  elements that understand Comparable

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Polymorphism

• The runtime message finds the correct method
• same message can invoke different methods
• the reference variable knows the type
• aString.compareTo(anotherString)
• anInteger.compareTo(anotherInteger)
• aButton.actionPerformed(anEvent)
• aTextField.actionPerformed(anEvent)
• aHashSet.add(anObject)
• aTreeSet.add(anObject)

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Polymorphism through Interfaces

• An interface describes a set of methods
• NOT allowed constructors, instance variables
• Interfaces are implemented by classes
• 646 java classes implement 1 or more interfaces
• Multiple classes implement the same interface
• are then guaranteed to have the same methods

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Implementing Comparable

• You can have types implement Comparable
• Need to do this to be sorted by Collections.sort
• public interface Comparable
• / Return 0 if two objects are equal less
  than
• zero if this object is smaller greater
  than
• zero if this object is larger. /
• public int compareTo(Object rhs)
• Try to sort BankAccounts without Comparable
• Implement Comparable and sort
• Team Activity Change compareTo to sort by balance

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Let BankAccount also be a Comparable

• public class BankAccount implements Comparable
• private String my_ID
• private double my_balance
• public BankAccount(String ID... // stuff
  deleted
• public int compareTo(Object argument)
• // Must complete this method or else it's an
  error
• // You will always see cast from Object to
  this class
• BankAccount right (BankAccount)argument
• return my_ID.compareTo(right.getID())

promise that this class has int
compareTo(Object)
Add compareTo method
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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"))

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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
• Such as ImageIcon