Transcript chap4InterfacesAndPo..

Chapter 4 (Horstmann’s Book)
Interface Types and Polymorphism:
Graphics, Timer, Animation
Hwajung Lee
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Interfaces
 A class defines a set of operations (the interface) and
statements (implementation).
 Separating the interface concept from that of a class can
help in the development of “generic” reusable code.
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Polymorphism
 Polymorphism: Ability to select different methods
according to actual object type.
 Multiple classes can implement the same interface type
so that it is possible to operate on a mixture of objects
from any of these classes.

public interface Icon
{
int getIconWidth();
int getIconHeight();
void paintIcon(Component c,
Graphics g,
int x,
int y);
}
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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;
...
}
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Can draw any object that implements Shape interface
Shape s = . . .;
g2.draw(s);

http://java.sun.com/j2se/1.3/docs/api/java/awt/Shape.html
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Rectangle2D.Double constructed with
 top left corner
 width
 height
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g2.draw(new Rectangle2D.Double(x, y, width, height));
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For Ellipse2D.Double, specify bounding box
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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);
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g2.drawString(text, x, y);
x, y are base point coordinates
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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

No need to name objects that are used only once.
Collections.sort(students,
new StudentComparatorByName());

No need to name classes that are used only once.
Comparator<Student> comp = new
Comparator<Student>()
{
public int compare(Student student1, Student student2)
{
return student1.getName().compareTo(student2.getName());
}
};

Anonymous class is commonly used in factory methods:
public class Student
{
public static Comparator<Student> comparatorByName()
{
return new Comparator<Student>()
{
public int compare(Student s1, Student s2) { . . . }
};
}
}

Neat arrangement if multiple comparators make sense
(by name, by grade, ...)
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Ch4/action1/ActionTest.java
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Method of Inner classes can access variables that are visible
in the enclosing scope
 e.g. actionPerformed method accesses the textField variable of the
enclosing main method
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If an inner class accesses a local variable from an enclosing
scope, the variable must be declared as final.
final JTextField textField = new TextField(Field_WIDTH);
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To construct multiple objects of the same anonymous class,
you must instantiate the anonymous class in a helper method
(a factory method) and then call it multiple times.
Note: you should declare parameters final
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Debug the following Code
public static ActionListener createGreetingButtonListener(String message)
{
return new
ActionListener()
{
public void actionPerformed(ActionEvent event)
{
textField.setText(message);
}
};
}

Answer:
public static ActionListener createGreetingButtonListener(final String message)
{
return new
ActionListener()
{
public void actionPerformed(ActionEvent event)
{
textField.setText(message);
}
};
}
Public class ActionTest
{
Public static void main(String[] args)
{
…
textField = new JTextField(FIELD_SIZE);
helloButton.addActionListener(
creatGreetingButtonListener(“Hello, World !”));
goodbyeButton. addActionListener(
creatGreetingButtonListener(“Goodbye, World !”));
…
}
// the helper method in the previous slide places here
}
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Javax.swing package
Timer generates a sequence of action events spaced apart at
equal time interval, and notifies a designated action listener.
ActionListener listener = ...;
final int DELAY = 1000; // 1000 millisec = 1 sec
Timer t = new Timer(DELAY, listener);
t.start();
 The start method returns immediately. A new thread of
execution is started that issues action events in the specified
frequency.

Ch4/timer/TimerTester.java
Public class ShapeIcon implements Icon
{
Public void paintIcon(Component c, Graphics g, int x, int y)
{
Paint the shape
}
…
}
ShapeIcon icon = new ShapeIcon(…);
JLabel label = new JLabel(icon);
ActionListener listener = new
ActionListener()
{
public void actionPerformed(ActionEvent event)
{
move the shape
label.repaint();
}
};
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Use timer to
 Move car shapes
 Draw car with CarShape
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Two responsibilities:
 Draw shape
 Move shape
 Define new interface type MoveableShape
: we can decoupled the animation from the car shape.
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Name the methods to conform to standard library
public interface MoveableShape
{
void draw(Graphics2D g2);
void translate(int dx, int dy);
}
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CarShape class implements MoveableShape
public class CarShape implements MoveableShape
{
public void translate(int dx, int dy)
{ x += dx; y += dy; }
...
}
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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
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Ch4/animation/MoveableShape.java
Ch4/animation/ShapeIcon.java
Ch4/animation/AnimationTester.java
Ch4/animation/CarShape.java