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Features of Java
CS 3331
Sections 4.4.7 and 5.5
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Outline
Abstract class Interface Application --- animation applets
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Motivation --- Drawing Board
+ init(): void
+ paint(g: Graphics) : void
DrawingBoard
Triangle
+ getX(): int
+ getY(): int
+ getColor(): Color
+ draw(g: Graphics): void
- x: int
- y: int
- color: Color
Shape
0..*
shapes
Circle Rectangle
java.applet.Applet
for (Shape s: shapes) { s.draw(g);}
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Class Shape
public class Shape { private int x, y; private Color c; public Shape(int x, int y, Color c) { this.x = x; this.y = y; this.c = c; } public int getX() { return x; } public int getY() { return y; } public Color getColor { return c; } public void draw(Graphics g) { /* … */ }}
Q: What can be wrong with this implementation?
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At Least Two Problems
Instantiation of the Shape class, e.g.,Shape s = new Shape(10, 10, Color.RED);
Definition of the draw method Null implementation, i.e.,
public void draw(Graphics g) { } But, what if subclasses forget to override it?
=> not detected by Java compiler
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Solution: Abstract Class
public abstract class Shape { private int x, y; private Color c;
protected Shape(int x, int y, Color c) { this.x = x; this.y = y; this.c = c; } public abstract void draw(Graphics g); // no body here! public int getX() { return x; } public int getY() { return y; } public Color getColor { return c; }}
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Abstract Classes?
Classes that can’t be instantiated Used to define common properties that
are to be inherited by subclasses Often provide partial implementations May include abstract methods, methods
that have no body
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How Abstract Classes Solve the Problems?
Instantiation of abstraction classes is prohibited by Java compilers, e.g.,
new Shape(10, 10, Color.RED); // compilation error
No implementation for abstract method Compilation error if concrete subclasses
provide no implementations for inherited abstract methods
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In Sum, Abstract Classes …
Provide partial implementations to be inherited by subclasses
May include abstract methods Are good for factoring out common
properties among classes
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Outline
Abstract classes Interfaces Application --- animation applets
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Interfaces
Declare features to be supported by classes Provide no implementation (except for default
method in Java 8) Only allow public abstract methods and
constants (public static final fields)
public interface ActionListener { void actionPerformed(ActionEvent event);}
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Why Interfaces?
To draw automobiles …
DrawingBoard
Circle Rectangle Triangle
Vehicle
Automobile
0..* Shape{abstract}
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How to Draw Automobiles?
<<interface>>
Drawable
+ draw(g: Graphics) : void
By programming to the interface.
DrawingBoard
Circle Rectangle Triangle
0..*
Shape{abstract}
+ draw(g: Graphics) : void
DrawableAutomobile
Vehicle
Automobile
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In Sum, Interfaces …
Good for establishing a well-defined boundary between modules (subsystems)
Thus, make programs more reusable and maintainable
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Abstract Classes vs. Interfaces
Partial code vs. no code at all (except for default method in Java 8)
Class vs. interface
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Exercise Separate the display of DigitalClock to support various
ways of displaying time, e.g., digital, analog, customized background, etc. Explain your design by drawing a UML class diagram.
DigitalClock
# timer: Timer
# font: Font
# color: Color
+ DigitalClock(): void
+ start(): void
+ stop(): void
+ paint(g: Graphics): void
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Applications --- Animation Applets
Enhanced digital clock applet Scrolling banner applet
Initial version Double-buffered version
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Enhanced Digital Clock Applet
Setting applet parameters in the Web page
<html>…<applet code=“DigitalClock2.class” width=“250” height=“80”> <param name=“color” value=“blue”></applet>…</html>
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Getting Applet Parameters
import java.awt.Color;public class DigitalClock2 extends DigitalClock { public void init() { String param = getParameter(“color”); if (“red”.equals(param)) { color = Color.RED; } else if (“blue”.equals(param)) { color = Color.BLUE; } else if (“yellow”.equals(param)) { color = Color.YELLOW; } /* … */ else { color = Color.GREEN; } }}
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Animation Applets
Enhanced digital clock applet Scrolling banner applet
Initial version Double-buffered version
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The java.awt.Graphics Class
Class Graphics Represents graphics context, an abstraction of
various drawing surfaces, e.g., screen, printer, off-screen image (an image stored in memory).
Provide a rich set of graphics methods.
drawString() drawLine() drawArc() fillArc() drawOval() fillOval()drawPolygon() fillPolygon()drawRect() fillRect() drawRoundRect() fillRoundRect()
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Graphics Class (Cont.)
Other methods
setColor(color) set the current colorsetFont(font) set the current font setPaintMode() set the paint, or overwrite mode setXORMode(color) set the XOR mode getColor() get the current color getFont() get the current font getFontMetrics() get the font metrics of the current font getFontMetrics(font) get the font metrics for the specified font
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The java.awt.FontMetrics Class
getAscent() getDescent() getHeight() getLeading() stringWidth(s)
Upwidth
height
leading
ascent
descentbaseline
Information about the rendering of a particular font on a particular screen.
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Scrolling Banner Applet
import java.awt.*;import java.awt.event.*;import javax.swing.*;
public class ScrollingBanner extends java.applet.Applet { <field declarations> public void init() { ... } public void paint(Graphics g) { ... } public void start() { ... } public void stop() { ... }}
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Field Declarations
protected String text; protected Font font = new java.awt.Font("Sans-serif", Font.BOLD, 24); protected Dimension dim; protected int x, y; protected int delay = 100; protected int offset = 1;protected Timer timer; // animation timer
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Initialization
public void init() { // get parameters "delay" and "text" String att = getParameter("delay"); if (att != null) { delay = Integer.parseInt(att); } att = getParameter("text"); if (att != null) { text = att; } else { text = “Go Miners!”; } // set initial position of the text dim = getSize(); x = dim.width; y = font.getSize();
// initialize animation timertimer = new Timer(delay, e -> repaint());
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Painting the Current Frame
Go Miners!Go Miners! Go Miners!
length viewing area
(dim.width, y)(dim.width-1, y)(-length, y) (x, y)
(0, 0)
leftmost position rightmost positioncurrent position
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Painting the Current Frame (Cont.)public void paint(Graphics g) { // get the font metrics to determine the length of the text g.setFont(font); FontMetrics fm = g.getFontMetrics(); int length = fm.stringWidth(text); // adjust the position of text from the previous frame x = x - offset; // if the text is completely off to the left end // move the position back to the right end if (x < -length) { x = dim.width; } // set the pen color and draw the background g.setColor(Color.BLACK); g.fillRect(0, 0, dim.width, dim.height); // set the pen color, then draw the text g.setColor(Color.GREEN); g.drawString(text, x, y); }
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The start() and stop() Methodspublic void start() { timer.start();} public void stop() { timer.stop(); }
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Exercise: Vertical Scrolling
Define a subclass of ScrollingBanner, called ScrollingBanner3, that scrolls the banner vertically. Reuse code as much as possible and minimize code duplication.
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How to Avoid Flickering?
Flickering is caused by repaint() repaint() calls the update() method. The default update() method does the following:
paint the whole area with the background color; set the foreground color; call the paint() method.
The update() method is also called by the system to update windows.
Solution: override the update() method use an off-screen image
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Using Off-Screen Image
Double buffering
import java.awt.*; public class ScrollingBanner2 extends ScrollingBanner { protected Image image; // off-screen image protected Graphics offscreen; // off-screen graphics
public update(Graphics g) { ... } public paint(Graphics g) { ... }}
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Using Off-Screen Image (Cont.)
public void update(Graphics g) { // create the offscreen image if it is the first time if (image == null) { image = createImage(dim.width, dim.height); offscreen = image.getGraphics(); } // draw the current frame into the off-screen image // using the paint method of the superclass super.paint(offscreen); // copy the off-screen image to the screen g.drawImage(image, 0, 0, this); }
public void paint(Graphics g) { update(g); }
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Animation Applet Idiom Category
Behavioral implementation idiom Intent
For an applet to continuously update its appearance without user input or intervention
Also known asActive Applet
ApplicabilityUse the Animation Applet Idiom to animate dynamic processes
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Animation Applet Idiom (Cont.)
import java.awt.*;import java.awt.event.*;import javax.swing.*;
public class AnimationApplet extends java.applet.Applet {
protected Timer timer = null; protected int delay;
public void init() { timer = new Timer(delay, e -> repaint());}
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Animation Applet Idiom (Cont.)
public void start() { timer.start(); }
public void stop() { timer.stop(); } public void paint(Graphics g) { <paint the current frame> }
<other methods and fields>}
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Exercise: Multiple Bouncing Balls
Enhance the bouncing ball applet in Example 8.12 of the textbook to support multiple balls. Bounce several balls of different sizes. Change directions of balls when they collide with each
other. Use an applet parameter, say numOfBalls, to specify
the number of balls to bounce. Optionally, play a beep sound when a ball collides with
another ball.
