SE450: Horstmann Chapter 4

Contents [0/41]

Object-Oriented Design & Patterns [1/41]

Chapter Topics [2/41]

Displaying an Image [3/41]

Displaying an Image [4/41]

Displaying an Image [5/41]

The Icon Interface Type [6/41]

Interface Types [7/41]

The Icon Interface Type and Implementing Classes [8/41]

Polymorphism [9/41]

Polymorphism [10/41]

A Variable of Interface Type [11/41]

Polymorphism [12/41]

Benefits of Polymorphism [13/41]

The Comparable Interface Type [14/41]

The Comparable Interface Type [15/41]

The Comparator interface type [16/41]

The Comparator interface type [17/41]

Anonymous Classes [18/41]

Anonymous Classes [19/41]

Anonymous Classes [20/41]

Frames [21/41]

Adding Components [22/41]

User Interface Actions [23/41]

User Interface Actions [24/41]

Accessing Variables from Enclosing Scope [25/41]

User Interface Actions [26/41]

Constructing Related Actions [27/41]

Timers [28/41]

Drawing Shapes [29/41]

Drawing Rectangles and Ellipses [30/41]

Drawing Ellipses [31/41]

Drawing Line Segments [32/41]

Relationship Between Shape Classes [33/41]

Drawing Text [34/41]

Filling Shapes [35/41]

Defining a New Interface Type [36/41]

CRC Card for the MoveableShape Interface Type [37/41]

Defining a New Interface Type [38/41]

Implementing the Animation [39/41]

Implementing the Animation [40/41]

Implementing the Animation [41/41]

Object-Oriented Design & Patterns [1/41]

Cay S. Horstmann

Chapter 4 Interfaces and Polymorphism

Chapter Topics [2/41]

Displaying an Image
Polymorphism
The Comparable Interface
The Comparator Interface
Anonymous Classes
Frames and User Interface Components
User Interface Actions
Timers
Drawing Shapes
Designing an Interface

Displaying an Image [3/41]

Use JOptionPane to display message:

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

Note icon to the left

Displaying an Image [4/41]

Can specify arbitrary image file

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

Displaying an Image [5/41]

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 [6/41]

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

Interface Types [7/41]

No implementation
Implementing class must supply implementation of all methods
showMessageDialog expects Icon object
Ok to pass MarsIcon

file:horstmann/ch04_icon2/MarsIcon.java [source] [doc-public] [doc-private]

package horstmann.ch04_icon2;
import java.awt.Color;
import java.awt.Component;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Ellipse2D;

import javax.swing.Icon;

/**
   An icon that has the shape of the planet Mars.
 */
public class MarsIcon implements Icon
{
  /**
      Constructs a Mars icon of a given size.
      @param aSize the size of the icon
   */
  public MarsIcon(int aSize)
  {
    size = aSize;
  }

  public int getIconWidth()
  {
    return size;
  }

  public int getIconHeight()
  {
    return size;
  }

  public void paintIcon(Component c, Graphics g, int x, int y)
  {
    Graphics2D g2 = (Graphics2D) g;
    Ellipse2D.Double planet = new Ellipse2D.Double(x, y,
        size, size);
    g2.setColor(Color.RED);
    g2.fill(planet);
  }

  private int size;
}

file:horstmann/ch04_icon2/IconTester.java [source] [doc-public] [doc-private]

package horstmann.ch04_icon2;
import javax.swing.JOptionPane;

public class IconTester
{
  public static void main(String[] args)
  {
    JOptionPane.showMessageDialog(
        null,
        "Hello, Mars!",
        "Message",
        JOptionPane.INFORMATION_MESSAGE,
        new MarsIcon(50));
    System.exit(0);
  }
}

The Icon Interface Type and Implementing Classes [8/41]

Polymorphism [9/41]

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 [10/41]

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 [11/41]

Polymorphism [12/41]

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 [13/41]

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 [14/41]

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 [15/41]

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

file:horstmann/ch04_sort1/Country.java [source] [doc-public] [doc-private]

package horstmann.ch04_sort1;
/**
   A country with a name and area.
 */
public class Country implements Comparable<Country>
{
  /**
      Constructs a country.
      @param aName the name of the country
      @param anArea the area of the country
   */
  public Country(String aName, double anArea)
  {
    name = aName;
    area = anArea;
  }

  /**
      Gets the name of the country.
      @return the name
   */
  public String getName()
  {
    return name;
  }

  /**
      Gets the area of the country.
      @return the area
   */
  public double getArea()
  {
    return area;
  }


  /**
      Compares two countries by area.
      @param other the other country
      @return a negative number if this country has a smaller
      area than otherCountry, 0 if the areas are the same,
      a positive number otherwise
   */
  public int compareTo(Country other)
  {
    if (area < other.area) return -1;
    if (area > other.area) return 1;
    return 0;
  }

  private String name;
  private double area;
}

file:horstmann/ch04_sort1/CountrySortTester.java [source] [doc-public] [doc-private]

package horstmann.ch04_sort1;
import java.util.ArrayList;
import java.util.Collections;

public class CountrySortTester
{
  public static void main(String[] args)
  {
    ArrayList<Country> countries = new ArrayList<Country>();
    countries.add(new Country("Uruguay", 176220));
    countries.add(new Country("Thailand", 514000));
    countries.add(new Country("Belgium", 30510));

    Collections.sort(countries);
    // Now the array list is sorted by area
    for (Country c : countries)
      System.out.println(c.getName() + " " + c.getArea());
  }
}

The Comparator interface type [16/41]

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 [17/41]

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

file:horstmann/ch04_sort2/CountryComparatorByName.java [source] [doc-public] [doc-private]

package horstmann.ch04_sort2;
import java.util.Comparator;

public class CountryComparatorByName implements Comparator<Country>
{
  public int compare(Country country1, Country country2)
  {
    return country1.getName().compareTo(country2.getName());
  }

}

file:horstmann/ch04_sort2/ComparatorTester.java [source] [doc-public] [doc-private]

package horstmann.ch04_sort2;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

public class ComparatorTester
{
  public static void main(String[] args)
  {
    ArrayList<Country> countries = new ArrayList<Country>();
    countries.add(new Country("Uruguay", 176220));
    countries.add(new Country("Thailand", 514000));
    countries.add(new Country("Belgium", 30510));
    Comparator<Country>  comp = new CountryComparatorByName();
    Collections.sort(countries, comp);
    // Now the array list is sorted by area
    for (Country c : countries)
      System.out.println(c.getName() + " " + c.getArea());
  }
}

Anonymous Classes [18/41]

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

Anonymous Classes [19/41]

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 [20/41]

Commonly used in factory methods:

public static Comparator<Country> comparatorByName()
{
   return new Comparator<Country>()
   {
      public int compare(Country country1, Country country2)

      { . . . }
   };
}

Collections.sort(a, Country.comparatorByName());
Neat arrangement if multiple comparators make sense
(by name, by area, ...)

Frames [21/41]

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 [22/41]

Construct components

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

Set frame layout
```
frame.setLayout(new FlowLayout());
```
Add components to frame
```
frame.add(helloButton);
```

file:horstmann/ch04_frame/FrameTester.java [source] [doc-public] [doc-private]

package horstmann.ch04_frame;
import java.awt.FlowLayout;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JTextField;

public class FrameTester
{
  public static void main(String[] args)
  {
    JFrame frame = new JFrame();

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

    final int FIELD_WIDTH = 20;
    JTextField textField = new JTextField(FIELD_WIDTH);
    textField.setText("Click a button!");

    frame.setLayout(new FlowLayout());

    frame.add(helloButton);
    frame.add(goodbyeButton);
    frame.add(textField);

    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setVisible(true);
  }
}

User Interface Actions [23/41]

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 [24/41]

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 [25/41]

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 [26/41]

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!");
```

file:horstmann/ch04_action1/ActionTester.java [source] [doc-public] [doc-private]

package horstmann.ch04_action1;
import java.awt.FlowLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JTextField;

public class ActionTester
{
  public static void main(String[] args)
  {
    JFrame frame = new JFrame();

    final int FIELD_WIDTH = 20;
    final JTextField textField = new JTextField(FIELD_WIDTH);
    textField.setText("Click a button!");

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

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

    JButton goodbyeButton = new JButton("Say Goodbye");

    goodbyeButton.addActionListener(event -> textField.setText("Goodbye, World!"));

    frame.setLayout(new FlowLayout());

    frame.add(helloButton);
    frame.add(goodbyeButton);
    frame.add(textField);

    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setVisible(true);
  }
}

Constructing Related Actions [27/41]

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 [28/41]

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

file:horstmann/ch04_timer/TimerTester.java [source] [doc-public] [doc-private]

package horstmann.ch04_timer;
import java.awt.FlowLayout;
import java.awt.event.ActionListener;
import java.util.Date;

import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.Timer;

/**
   This program shows a clock that is updated once per second.
 */
public class TimerTester
{
  public static void main(String[] args)
  {
    JFrame frame = new JFrame();

    final int FIELD_WIDTH = 20;
    final JTextField textField = new JTextField(FIELD_WIDTH);

    frame.setLayout(new FlowLayout());
    frame.add(textField);

    ActionListener listener = event -> {
      Date now = new Date();
      textField.setText(now.toString());
    };
    final int DELAY = 1000;
    // Milliseconds between timer ticks
    Timer t = new Timer(DELAY, listener);
    t.start();

    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setVisible(true);
  }
}

Drawing Shapes [29/41]

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 [30/41]

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 [31/41]

Drawing Line Segments [32/41]

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 [33/41]

Drawing Text [34/41]

g2.drawString(text, x, y);
x, y are base point coordinates

Filling Shapes [35/41]

Fill interior of shape
```
g2.fill(shape);
```
Set color for fills or strokes:
g2.setColor(Color.red);
Program that draws car

file:horstmann/ch04_icon3/CarIcon.java [source] [doc-public] [doc-private]


package horstmann.ch04_icon3;
import java.awt.Color;
import java.awt.Component;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Line2D;
import java.awt.geom.Point2D;
import java.awt.geom.Rectangle2D;

import javax.swing.Icon;

/**
   An icon that has the shape of a car.
 */
public class CarIcon implements Icon
{
  /**
      Constructs a car of a given width.
      @param aWidth the width of the car
   */
  public CarIcon(int aWidth)
  {
    width = aWidth;
  }

  public int getIconWidth()
  {
    return width;
  }

  public int getIconHeight()
  {
    return width / 2;
  }

  public void paintIcon(Component c, Graphics g, int x, int y)
  {
    Graphics2D g2 = (Graphics2D) g;
    Rectangle2D.Double body
    = new Rectangle2D.Double(x, y + width / 6,
        width - 1, width / 6);
    Ellipse2D.Double frontTire
    = new Ellipse2D.Double(x + width / 6, y + width / 3,
        width / 6, width / 6);
    Ellipse2D.Double rearTire
    = new Ellipse2D.Double(x + width * 2 / 3, y + width / 3,
        width / 6, width / 6);

    // The bottom of the front windshield
    Point2D.Double r1
    = new Point2D.Double(x + width / 6, y + width / 6);
    // The front of the roof
    Point2D.Double r2
    = new Point2D.Double(x + width / 3, y);
    // The rear of the roof
    Point2D.Double r3
    = new Point2D.Double(x + width * 2 / 3, y);
    // The bottom of the rear windshield
    Point2D.Double r4
    = new Point2D.Double(x + width * 5 / 6, y + width / 6);

    Line2D.Double frontWindshield
    = new Line2D.Double(r1, r2);
    Line2D.Double roofTop
    = new Line2D.Double(r2, r3);
    Line2D.Double rearWindshield
    = new Line2D.Double(r3, r4);

    g2.fill(frontTire);
    g2.fill(rearTire);
    g2.setColor(Color.red);
    g2.fill(body);
    g2.draw(frontWindshield);
    g2.draw(roofTop);
    g2.draw(rearWindshield);
  }

  private int width;
}

Defining a New Interface Type [36/41]

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 [37/41]

Defining a New Interface Type [38/41]

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 [39/41]

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 [40/41]

file:horstmann/ch04_animation/MoveableShape.java [source] [doc-public] [doc-private]

package horstmann.ch04_animation;
import java.awt.Graphics2D;

/**
   A shape that can be moved around.
 */
public interface MoveableShape
{
  /**
      Draws the shape.
      @param g2 the graphics context
   */
  void draw(Graphics2D g2);
  /**
      Moves the shape by a given amount.
      @param dx the amount to translate in x-direction
      @param dy the amount to translate in y-direction
   */
  void translate(int dx, int dy);
}

file:horstmann/ch04_animation/ShapeIcon.java [source] [doc-public] [doc-private]

package horstmann.ch04_animation;
import java.awt.Component;
import java.awt.Graphics;
import java.awt.Graphics2D;

import javax.swing.Icon;

/**
   An icon that contains a moveable shape.
 */
public class ShapeIcon implements Icon
{
  public ShapeIcon(MoveableShape shape,
      int width, int height)
  {
    this.shape = shape;
    this.width = width;
    this.height = height;
  }

  public int getIconWidth()
  {
    return width;
  }

  public int getIconHeight()
  {
    return height;
  }

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

  private int width;
  private int height;
  private MoveableShape shape;
}

file:horstmann/ch04_animation/AnimationTester.java [source] [doc-public] [doc-private]

package horstmann.ch04_animation;
import java.awt.FlowLayout;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.Timer;

/**
   This program implements an animation that moves
   a car shape.
 */
public class AnimationTester
{
  public static void main(String[] args)
  {
    JFrame frame = new JFrame();

    final MoveableShape shape
    = new CarShape(0, 0, CAR_WIDTH);

    ShapeIcon icon = new ShapeIcon(shape,
        ICON_WIDTH, ICON_HEIGHT);

    final JLabel label = new JLabel(icon);
    frame.setLayout(new FlowLayout());
    frame.add(label);

    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setVisible(true);

    final int DELAY = 100;
    // Milliseconds between timer ticks
    Timer t = new Timer(DELAY, event -> {
      shape.translate(1, 0);
      label.repaint();
    });
    t.start();
  }

  private static final int ICON_WIDTH = 400;
  private static final int ICON_HEIGHT = 100;
  private static final int CAR_WIDTH = 100;
}

file:horstmann/ch04_animation/CarShape.java [source] [doc-public] [doc-private]

package horstmann.ch04_animation;
import java.awt.Graphics2D;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Line2D;
import java.awt.geom.Point2D;
import java.awt.geom.Rectangle2D;

/**
   A car that can be moved around.
 */
public class CarShape implements MoveableShape
{
  /**
      Constructs a car item.
      @param x the left of the bounding rectangle
      @param y the top of the bounding rectangle
      @param width the width of the bounding rectangle
   */
  public CarShape(int x, int y, int width)
  {
    this.x = x;
    this.y = y;
    this.width = width;
  }

  public void translate(int dx, int dy)
  {
    x += dx;
    y += dy;
  }

  public void draw(Graphics2D g2)
  {
    Rectangle2D.Double body
    = new Rectangle2D.Double(x, y + width / 6,
        width - 1, width / 6);
    Ellipse2D.Double frontTire
    = new Ellipse2D.Double(x + width / 6, y + width / 3,
        width / 6, width / 6);
    Ellipse2D.Double rearTire
    = new Ellipse2D.Double(x + width * 2 / 3, y + width / 3,
        width / 6, width / 6);

    // The bottom of the front windshield
    Point2D.Double r1
    = new Point2D.Double(x + width / 6, y + width / 6);
    // The front of the roof
    Point2D.Double r2
    = new Point2D.Double(x + width / 3, y);
    // The rear of the roof
    Point2D.Double r3
    = new Point2D.Double(x + width * 2 / 3, y);
    // The bottom of the rear windshield
    Point2D.Double r4
    = new Point2D.Double(x + width * 5 / 6, y + width / 6);
    Line2D.Double frontWindshield
    = new Line2D.Double(r1, r2);
    Line2D.Double roofTop
    = new Line2D.Double(r2, r3);
    Line2D.Double rearWindshield
    = new Line2D.Double(r3, r4);

    g2.draw(body);
    g2.draw(frontTire);
    g2.draw(rearTire);
    g2.draw(frontWindshield);
    g2.draw(roofTop);
    g2.draw(rearWindshield);
  }

  private int x;
  private int y;
  private int width;
}

Implementing the Animation [41/41]

Revised: 2007/09/11 16:22