polymorphism and interfaces horstmann ch 4. outline interface polymorphism function object anonymous...
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Polymorphism and interfaces
Horstmann ch 4
Outline
• Interface• Polymorphism
• Function object
• Anonymous class
• User Interface Action
• Scope of variables
• (Large) example
• Graphics in Java
Displaying an image
JOptionPane.showMessageDialog(null, ”Welcome!”);
Displaying an image
Use image from file
JOptionPane.showMessageDialog(
null,
”Welcome!”,
”Message”,
JOptionPane.INFORMATION_MESSAGE,
new ImageIcon(”au.jpg”));
Displaying an image
Draw image
JOptionPane.showMessageDialog(
null,
”Welcome!”,
”Message”,
JOptionPane.INFORMATION_MESSAGE,
new CarIcon(100));
Interface• How can an image from a file be used in the same way as
an image specified by a drawing algorithm?• By using an interface!• Interface Icon abstracts the relevant properties
public interface Icon {
int getIconWidth();
int getIconHeight(); void paintIcon(Component c, Graphics g, int x, int y);}
Interface contain no implementation of methods
Interface
JOptionPane.showMessageDialog(
null,
”Welcome!”,
”Message”,
JOptionPane.INFORMATION_MESSAGE,
new CarIcon(100));
public static void showMessageDialog(
Component parentComponent,
Objekt message,
String title,
int messageType,
Icon icon)
showMessageDialog knows about au-logo and car picture ONLY as something implementing interface Icon
Interface
• Implementing class must supply implementation of all methods
public class CarIcon implements Icon {
public int getIconWidth() { . . . }
public int getIconHeight() { . . . }
public void paintIcon(Component c, Graphics g,
int x, int y) { . . . }
}
QUIZ
Which assertions are true?
1. none
2. A
3. B
4. C
5. A, B
6. A, C
7. B, C
8. A, B, C
9. I don’t know
A. An interface specifies method headers
B. An interface specifies method bodies
C. Comparable<String> implements interface String
Outline
• Interface
• Polymorphism• Function object• Anonymous class• User Interface Action• Scope of variables• (Large) example• Graphics in Java
Decoupling
• showMessageDialog expects an Icon object• showMessageDialog need not know whether the actual
type of the object is CarIcon or ImageIcon or …
Polymorphic variable
public static void showMessageDialog( … , Icon anIcon)
• 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 methods getIconWidth, getIconHeight, etc.
Method Polymorphism
public static void showMessageDialog( … , Icon anIcon)
• showMessageDialog must compute size of dialog • width = icon width + message size + blank size • How do we know the icon width?
int width = anIcon.getIconWidth();• 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
QUIZ
Which lines result in errors?
1.none
2.a
3.b
4.a+b
5. I don’t know Sad s;
s = new Choleric();
a) s.cry();
b) s.laugh();
QUIZ
Which lines result in errors?
1.none
2.a
3.b
4.c
5.a+b
6.a+c
7.b+c
8.a+b+c
9. I don’t know
Sad s;
Choleric c;
c = new Choleric();
a) s = c;
b) c = s;
c) c = (Choleric) s;
Outline
• Interface• Polymorphism
• Function object• Anonymous class• User Interface Action• Scope of variables• (Large) example• Graphics in Java
Example: sorting
• Sort by name– AntiChrist, 108 min– Gran Torino, 116 min– Mænd der hader kvinder, 155 min– Slumdog Millionaire, 120 min
• Sort by duration– AntiChrist, 108 min– Gran Torino, 116 min– Slumdog Millionaire, 120 min– Mænd der hader kvinder, 155 min
Comparable interface type• Collections has static sort method.
For a being an array list Collections.sort(a); • Objects in list must implement the Comparable interface type
public interface Comparable<T> { int compareTo(T other);}
• object1.compareTo(object2) returns – Negative number if object1 less than object2 – 0 if objects identical – Positive number if object1 greater than object2
• How can we sort Films by both name and length? • Can't implement Comparable twice!
dIntProg
Comparator interface type
• Comparator interface type gives added flexibility public interface Comparator<T> { int compare(T object1, T object2);
}
• Comparator object is a function object
• Pass comparator object to sort:Collections.sort(list, comp);
Strategy
ArrayList<Film> films = . . . ;
Comparator<Film> comp1 = new NameComp();
Collections.sort(films, comp1);
Comparator<Film> comp2 = new LenghtComp();
Collections.sort(films, comp2);
Outline
• Interface• Polymorphism• Function object
• Anonymous class• User Interface Action• Scope of variables• (Large) example• Graphics in Java
Anonymous Classes
• No need to name classes that are used only once
Comparator<Film> comp = newComparator<Film>() { public int compare(Film f1, Film f2) {
return f1.getName().compareTo(f2.getName()); } };
• Same asComparator<Film> comp = new NameComp();
public class NameComp implements Comparator<Film> { public int compare(Film f1, Film f2) { return f1.getName().compareTo(f2.getName()); }}
Anonymous Classes• Commonly used in factory methods:
public static Comparator<Film> comparatorByName() { return new Comparator<Film>() { public int compare(Film c1, Film c2) {...} };}
• Collections.sort(a, Film.comparatorByName()); • Neat arrangement if multiple comparators make sense
(by name, by length, ...)
QUIZ
Which lines result in errors?
1.None (or missing code)
2.a
3.b
4.a+b
5. I don’t know Sad s;
a) s = new Sad() {
b) public void laugh() {. . .}
};
Outline
• Interface• Polymorphism• Function object• Anonymous class
• User Interface Action• Scope of variables• (Large) example• Graphics in Java
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 componentsJButton helloButton = new JButton("Say Hello");
• Set frame layoutframe.setLayout(new FlowLayout());
• Add components to framesframe.add(helloButton);
User Interface Actions
• Add listener object(s) to button
public interface ActionListener {
void actionPerformed(ActionEvent event);
}
• Listeners are notified when button is clicked
User Interface Actions
helloButton.addActionListener(newActionListener() {
public void actionPerformed(ActionEvent event) { textField.setText("Hello, World"); } });
• When button is clicked, text field is set
User Interface Actions
• Constructor attaches listener:helloButton.addActionListener(listener);
• Button remembers all listeners
• When button clicked, button notifies listenerslistener.actionPerformed(event);
• Listener sets text of text fieldtextField.setText("Hello, World!");
Outline
• Interface• Polymorphism• Function object• Anonymous class• User Interface Action
• Scope of variables• (Large) example• Graphics in Java
Accessing Variables from Enclosing Scope
• Remarkable: Inner class can access variables from enclosing scopee.g. textField
• Can access enclosing instance fields, local variables
• Local variables must be marked final
final JTextField textField = ...;
. . .
helloButton.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent event) { textField.setText("Hello, World"); } });
final
QUIZ
public class Quiz {
private final TextField display = …;
public ActionListener createButtonListener( final String message) {
final String prefix = “NOTE: “; return new ActionListener() { public void actionPerformed(ActionEvent event) { display.setText(prefix + message); } }; }}
1. None
2. a
3. b
4. c
5. a, b
6. a, c
7. b, c
8. a, b, c
9. I don’t know
a
bc
Which of the 3 ”final” can be removed without errors?
Outline
• Interface• Polymorphism• Function object• Anonymous class• User Interface Action• Scope of variables
• (Large) example• Graphics in Java
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
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
Implementing the Animation
Outline
• Interface• Polymorphism• Function object• Anonymous class• User Interface Action• Scope of variables• (Large) example
• Graphics in Java
Graphics in Java…• How is the car Icon drawn?
• 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 interfaceShape s = . . .;g2.draw(s);
Drawing Rectangles
• Rectangle2D.Double constructed with – top left corner – width – height
g2.draw(
new Rectangle2D.Double(x, y, width, height)
);
Drawing Ellipses
• For Ellipse2D.Double, specify bounding box
Drawing Line Segments
• Point2D.Double is a point in the plane • Line2D.Double joins two 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("Message", 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 (uses class CarIcon)