Chapter 4Interface Types and

Polymorphism: Graphics, Timer, Animation

Why? 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.

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.

The Icon Interface Type public interface Icon

{int getIconWidth();int getIconHeight();void paintIcon(Component c,

Graphics g, int x, int y);

}

Shape Interface Type & Polymorphism (1)

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);

http://java.sun.com/j2se/1.3/docs/api/java/awt/Shape.html

Shape Interface Type & Polymorphism (2)

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

Shape Interface Type & Polymorphism (3)

Drawing Ellipses

Shape Interface Type & Polymorphism (4) 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);

Shape Interface & Polymorphism (5) Relationship

Between Shape Interface and Classes

Shape Interface Type & Polymorphism (6)

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

Shape Interface Type & Polymorphism (6)

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

Anonymous Classes 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()); } };

Factory Methods 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, ...)

User Interface Action (1)

Ch4/action1/ActionTest.java

User Interface Action (2)Accessing Variables from Enclosing Scope

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

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);

User Interface Action (3) 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

Debug the following Codepublic static ActionListener createGreetingButtonListener(String

message){ return new

ActionListener() { public void actionPerformed(ActionEvent event) { textField.setText(message); } };

}

User Interface Action (4) 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

}

User Interface Action (5)

Timers 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 secTimer 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

Designing an Interface Type (cont.)

Public class ShapeIcon implements Icon{

Public void paintIcon(Component c, Graphics g, int x, int y){

Paint the shape}…

}

Designing an Interface Type (cont.)

ShapeIcon icon = new ShapeIcon(…);JLabel label = new JLabel(icon);

ActionListener listener = newActionListener(){

public void actionPerformed(ActionEvent event){

move the shapelabel.repaint();

}};

Designing an Interface Type (cont.)

Use timer to move car shapes Draw car with CarShape Two responsibilities:

Draw shape Move shape

Define new interface type MoveableShape : we can decoupled the animation from the car shape.

CRC Card for the MoveableShape Interface Type

Designing an 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 MoveableShapepublic class CarShape implements MoveableShape{   public void translate(int dx, int dy)   { x += dx; y += dy; }   . . .}

Implementing the Animation Label contains icon that draws shapeTimer 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

Implementing the Animation

Ch4/animation/MoveableShape.java Ch4/animation/ShapeIcon.java Ch4/animation/AnimationTester.java Ch4/animation/CarShape.java