Object-Oriented Design & Patterns

Cay S. Horstmann

Chapter 4

Interfaces and Polymorphism

Chapter Topics

• Displaying an Image 
  
• Polymorphism
  
• Drawing Shapes  
• The Comparable Interface  
• The Comparator Interface  
• Anonymous Classes  
• Frames and User Interface Components  
• User Interface Actions 
  
• Timers
  
• 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 paintComponent(Component c, Graphics g, int x, int y)
}

Interface Types

• No implementation
• Implementing class must supply implementation of all methods
• Ch4/icon2/MarsIcon.java
• showMessageDialog expects Icon object
• Ok to pass MarsIcon
• Ch4/icon2/IconTest.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

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
  Ch4/icon3/CarIcon.java

The Comparable Interface Type

• Collections  has static sort method:
  

  ArrayList a = . . .
  Collections.sort(a);

• Objects in list must implement the Comparable interface type
  

  public interface Comparable
  {
     int compareTo(Object other);
  }
  

• object1.compareTo(object2) returns
• Negative number if object1 less than object2
• 0 if objects identical
• Positive number if object1 greater than object2

The Comparable Interface Type

• sort method compares and rearranges elements
  if (object1.compareTo(object2) > 0) . . .
• String class implements Comparable interface type: lexicographic (dictionary) order
• Country class: compare countries by area
  Ch4/sort1/Country.java
  Ch4/sort1/CountrySortTest.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
  {
     int compare(Object object1, Object object2);
  }

• Pass comparator object to sort:
  

  Collections.sort(list, comp);
  

The Comparator interface type

• Ch4/sort2/CountryComparatorByName.java
  Ch4/sort2/ComparatorTest.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

Anonymous Classes

• No need to name objects that are used only once
  

  Collections.sort(countries,
     new CountryComparatorByName());

• No need to name classes that are used only once
  

  Comparator comp = new
     Comparator()
     {
        public int compare(Object obj1, Object obj2)
        {
           Country country1 = (Country)obj1;
           Country country2 = (Country)obj2;
           return country1.getName().compareTo(country2.getName());
        }
     };
  

Anonymous Classes

• anonymous new expression:
• defines anonymous class that implements Comparator
• defines compare method of that class
• constructs one object of that class
• Cryptic syntax for very useful feature

Anonymous Classes

• Commonly used in factory methods:
  public static Comparator comparatorByName()
  {
     return new Comparator()
     {
        public int compare(Object o1, Object o2) { . . . }
     };
  }
• Collections.sort(a, Country.comparatorByName());
• Neat arrangement if multiple comparators make sense
  (by name, by area, ...)
  

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.show();

Adding Components

• Construct components
  

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

• Set content pane layout
  

  Container contentPane = frame.getContentPane();
  container.setLayout(new FlowLayout());
  

• Add components to content pane
  

  contentPane.add(helloButton);

• Ch4/frame/FrameTest.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

• Add action code into  actionPerformed method
  
• Gloss over routine code
  

helloButton.addActionListener(new
   ActionListener()
   {
      public void actionPerformed(ActionEvent event)
      {
          textField.setText("Hello, World");
      }
   });

• When button is clicked, text field is set
  

Accessing Variables from Enclosing Scope

• Remarkable: Inner class can access variables from enclosing scope
  e.g. textField
• Can access enclosing instance fields, local variables
  
• Local variables must be marked final
  final JTextField textField = ...;
  

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!");

• Ch4/action1/ActionTest.java

Constructing Related Actions

• Write helper method that constructs objects
• Pass variable information as parameters
• Declare parameters final

public static ActionListener createGreetingButtonListener(
   final String message)
{
   return new
      ActionListener()
      {
         public void actionPerformed(ActionEvent event)
         {
            textField.setText(message);
         }
      };
}

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
• Ch4/timer/TimerTest.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 translate(int dx, int dy);
  }
• CarShape class implements MoveableShape
  public class CarShape implements MoveableShape
  {
     public void translate(int dx, int dy)
     { x += dx; y += dy; }
     . . .
  }
  

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

• Ch4/animation/MoveableShape.java
• Ch4/animation/ShapeIcon.java
• Ch4/animation/AnimationTest.java
• Ch4/animation/CarShape.java

Implementing the Animation