Object-Oriented Design & Patterns

Cay S. Horstmann

Chapter 4

Interfaces and Polymorphism

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Chapter Topics

• Displaying an Image 
  
• Polymorphism
  
• The Comparable Interface
• The Comparator Interface
• Lambda Expressions
• Frames and User Interface Components
• User Interface Actions 
• Timers
• Drawing Shapes
• Designing an Interface

Displaying an Image

• Use JOptionPane to display message:
  

  JOptionPane.showMessageDialog(null, "Hello, World!");
  

• Note icon to the left

Displaying an Image

• Can specify arbitrary image file
  

  JOptionPane.showMessageDialog(
   null, 
   "Hello, World!",
   "Message",
   JOptionPane.INFORMATION_MESSAGE,
   new ImageIcon("globe.gif"));
  

Displaying an Image

• What if we don't want to generate an image file?
• Fortunately, can use any class that implements Icon interface type
  
• ImageIcon is one such class
• Easy to supply your own class

The Icon Interface Type

public interface Icon
{
   int getIconWidth();
   int getIconHeight();
   void paintIcon(Component c, Graphics g, int x, int y);
}

Interface Types

• No implementation
• Implementing class must supply implementation of all methods
• ch04/icon2/MarsIcon.java
• showMessageDialog expects Icon object
• Ok to pass MarsIcon
• ch04/icon2/IconTester.java

The Icon Interface Type and Implementing Classes

Polymorphism

• public static void showMessageDialog(...Icon anIcon)
• showMessageDialog shows
  • icon
  • message
  • OK button
• showMessageDialog must compute size of dialog
• width = icon width + message size + blank size
• How do we know the icon width?

  int width = anIcon.getIconWidth();
  

Polymorphism

• showMessageDialog doesn't know which icon is passed
  • ImageIcon?
  • MarsIcon?
  • . . .?
• The actual type of anIcon is not Icon
• There are no objects of type Icon
• anIcon belongs to a class that implements Icon
• That class defines a getIconWidth method

A Variable of Interface Type

Polymorphism

• Which getIconWidth method is called?
• Could be
  • MarsIcon.getIconWidth
  • ImageIcon.getIconWidth
  • . . .
• Depends on object to which anIcon reference points, e.g.

  showMessageDialog(..., new MarsIcon(50))
  

• Polymorphism: Select different methods according to actual object type

Benefits of Polymorphism

• Loose coupling
  • showMessageDialog decoupled from ImageIcon
  • Doesn't need to know about image processing
• Extensibility
  • Client can supply new icon types

The Comparable Interface Type

• Collections  has static sort method:

  ArrayList<E> a = . . .
  Collections.sort(a);
  

• Objects in list must implement the Comparable interface type

  public interface Comparable<T>
  {
   int compareTo(T other);
  }
  

• Interface is parameterized (like ArrayList)
• Type parameter is type of other

The Comparable Interface Type

• object1.compareTo(object2) returns
  • Negative number if object1 less than object2
  • 0 if objects identical
  • Positive number if object1 greater than object2
• sort method compares and rearranges elements if (object1.compareTo(object2) > 0) . . .
• String class implements Comparable<String> interface type: lexicographic (dictionary) order
• Country class: compare countries by area ch04/sort1/Country.java ch04/sort1/CountrySortTester.java

The Comparator interface type

• How can we sort countries by name?
• Can't implement Comparable twice!
• Comparator interface type gives added flexibility

  public interface Comparator<T>
  {
     int compare(T obj1, T obj2);
  }
  

• Pass comparator object to sort:

  Collections.sort(list, comp);
  

The Comparator interface type

• ch04/sort2/CountryComparatorByName.java ch04/sort2/ComparatorTester.java
• Comparator object is a function object
• This particular comparator object has no state
• State can be useful, e.g. flag to sort in ascending or descending order

Lambda Expressions

• Tedious to define a class that implements Comparator and to make just one instance thereof
• Lambda expression syntax provides shortcut

  Comparator<Country> comp = (first, second) ->
  first.getName().compareTo(second.getName());

• Same effect as
  • Defining a class implementing Comparator<Country>...
  • ...whose compare method has the given body...
  • ...and constructing an instance of that class.

Lambda Expression Syntax

• Simplest form:

  (arguments) -> expression

• Can only be assigned to a variable of interface type.
• That interface must have a single abstract method (SAM)
• Such an interface is called a functional interface
• No need to specify parameter or return types—inferred from SAM
• No need to specify local variable—can directly pass to method:

  Collections.sort(countries, (first, second) ->
     first.getName().compareTo(second.getName()));

Accessing Variables from the Enclosing Scope

• Ok to access variables from enclosing scope:

  public static Comparator<Country> byName(boolean ascending)
  {
     return (first, second) -> (ascending ? 1 : -1)
        * first.getName().compareTo(second.getName());
  }

• Can access local variables and instance variables
• Local variables must be effectively final
  • Actually declared final
  • Could be declared final (i.e. are never assigned to)
• If lambda body doesn't fit into an expression, use {...} and return

  public static Comparator<Country> byName(boolean ascending)
  {
     return (first, second) ->
     {
        int factor;
        if (ascending) factor = 1; else factor = -1;
        return factor * first.getName().compareTo(second.getName());
     }
  }

Frames

• Frame window has decorations
  • title bar
  • close box
  • provided by windowing system

• JFrame frame = new JFrame();
  frame.pack();
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  frame.setVisible(true);
  

Adding Components

• Construct components

  JButton helloButton = new JButton("Say Hello");
  

• Set frame layout

  frame.setLayout(new FlowLayout());
  

• Add components to frame

  frame.add(helloButton);
  

• ch04/frame/FrameTester.java

User Interface Actions

• Previous program's buttons don't have any effect
• Add listener object(s) to button
• Belong to class implementing ActionListener interface type

  public interface ActionListener
  {
     int actionPerformed(ActionEvent event);
  }
  

• Listeners are notified when button is clicked

User Interface Actions

• Use lambda expressions for action listeners

  helloButton.addActionListener(event -> Button action);

• Example:

  helloButton.addActionListener(event ->
     textField.setText("Hello, World!"));

• Must specify event even though we don't use it.

User Interface Actions

• Constructor attaches listener:

  helloButton.addActionListener(listener);
  

• Button remembers all listeners
• When button clicked, button notifies listeners

  listener.actionPerformed(event);
  

• Listener sets text of text field

  textField.setText("Hello, World!");
  

• ch04/action1/ActionTester.java

Constructing Related Actions

• Write helper method that constructs listeners

  public static ActionListener createGreetingButtonListener(
  String message)
  {
     return event -> textField.setText(message);
  }

• Pass variable information as parameters

  helloButton.addActionListener(
     createGreetingButtonListener("Hello, World!"));
  goodbyeButton.addActionListener(
     createGreetingButtonListener("Goodbye, World!"));

Timers

• Supply delay, action listener

  ActionListener listener = ...; 
  final int DELAY = 1000; // 1000 millisec = 1 sec 
  Timer t = new Timer(DELAY, listener); 
  t.start();

• Action listener called when delay elapsed
• ch04/timer/TimerTester.java

Drawing Shapes

• paintIcon method receives graphics context of type Graphics
• Actually a Graphics2D object in modern Java versions

  public void paintIcon(Component c, Graphics g, int x, int y)
  {
     Graphics2D g2 = (Graphics2D)g;
      . . .
  }
  

• Can draw any object that implements Shape interface

  Shape s = . . .;
  g2.draw(s);
  

Drawing Rectangles and Ellipses

• Rectangle2D.Double constructed with
  • top left corner
  • width
  • height

• g2.draw(new Rectangle2D.Double(x, y, width, height))

  ;

• For Ellipse2D.Double, specify bounding box

Drawing Ellipses

Drawing Line Segments

• Point2D.Double is a point in the plane
• Line2D.Double joins to points

  Point2D.Double start = new Point2D.Double(x1, y1);
  Point2D.Double end = new Point2D.Double(x2, y2);
  Shape segment = new Line2D.Double(start, end);
  g2.draw(segment);
  

Relationship Between Shape Classes

Drawing Text

• g2.drawString(text, x, y);

• x, y are base point coordinates

Filling Shapes

• Fill interior of shape

  g2.fill(shape);
  

• Set color for fills or strokes: g2.setColor(Color.red);
• Program that draws car ch04/icon3/CarIcon.java

Defining a New Interface Type

• Use timer to move car shapes
• Draw car with CarShape
• Two responsibilities:
  • Draw shape
  • Move shape
• Define new interface type MoveableShape

CRC Card for the MoveableShape Interface Type

Defining a New Interface Type

• Name the methods to conform to standard library

• public interface MoveableShape
  {
     void draw(Graphics2D g2);
     void move();
  }

• CarShape class implements MoveableShape

  public class CarShape implements MoveableShape
  {
     public void translate(int dx, int dy)
     { x++; }
     . . .
  }

Implementing the Animation

• Label contains icon that draws shape
• Timer action moves shape, calls repaint on label
• Label needs Icon, we have MoveableShape
• Supply ShapeIcon adapter class
• ShapeIcon.paintIcon calls MoveableShape.draw

Implementing the Animation

• ch04/animation/MoveableShape.java
• ch04/animation/ShapeIcon.java
• ch04/animation/AnimationTester.java
• ch04/animation/CarShape.java

Implementing the Animation