Applets Murray Cole Applets 1 Portability and Security JVM - - PowerPoint PPT Presentation

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Applets Murray Cole Applets 1 Portability and Security JVM - - PowerPoint PPT Presentation

Sep 29, 2022 239 likes •433 views

I V N E U R S E I H T Y T O H F G R E U D I B N Applets Murray Cole Applets 1 Portability and Security JVM and bytecode make portability easy facilitates sharing of programs, which can be good and

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T H E U N I V E R S I T Y O F E D I N B U R G H

Applets

Murray Cole

Applets

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1

Portability and Security

  • JVM and bytecode make portability “easy”
  • facilitates sharing of programs, which can be good and bad
  • can we make sharing safer?

Applets

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2

Applets

A sub-class of Java programs which

  • run from within web pages (byte code is provided by the web site

(server) but run in your browser (client))

  • are restricted in what they can do (no local file access, no

networking (eg e-mail), no “messing with the local system”). This is called sandboxing. The restrictions are enforced by the JVM through its security manager.

Applets

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  • n board illustration of client server nature of web activity, download

byte code & run on client, not server. Think of some little graphical

  • app. Also reduced load on server.
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java.lang.Object (equals, getClass, toString, . . . )

  • java.awt.Component

(addXListener, repaint . . . )

  • java.awt.Container

(add, remove, paint, setLayout, . . . )

  • java.awt.Panel

(Panel, addNotify)

  • java.applet.Applet

(init, stop, start, destroy . . . )

Applets

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Class java.applet.Applet

The applet interface (in the Java sense, not the GUI sense) specifies a number of new methods, including

  • void init(), invoked when the applet is first downloaded
  • void start(), invoked whenever the applet is started (or

re-started) Since it extends Component it also has a paint method.

Applets

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public class Clicker extends Applet { private int clicks; // Click counter public void paint(Graphics g) { g.setColor(Color.white); g.fillRect(0, 0, 400, 400); g.setColor(Color.black); g.setFont(new Font("TimesRoman", Font.BOLD, 64)); g.drawString("Clicks: " + clicks, 60, 240); } public void init() { addMouseListener(new MouseAdapter() { public void mouseClicked(MouseEvent e) { clicks++; repaint(); } }); }}

Applets

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The HTML file

HyperText Markup Language (HTML) describes web pages. <html> <head> <title>The Clicker Applet</title> </head> <body> <h1>The Clicker Applet</h1> <applet code="Clicker.class" width="400" height="400"> </applet> <hr> </body> </html>

Applets

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A Really Useful Applet

Applets

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8

Design Issues (1)

The game itself

  • board and current moves
  • choosing a computer move
  • winning

Applets

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public class OXO extends Applet { int movesmade; boolean player[] = new boolean[9]; boolean computer[] = new boolean[9]; boolean MoveOK (int m) { return !(computer[m] || player[m]); } void computerMove () { int m; m = (int) (Math.random()*9); while (!MoveOK(m)) { m = (int) (Math.random()*9); } movesmade++; computer[m] = true; }

Applets

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// Check all the possible ways of winning boolean won (boolean []moves) { return (moves[0] && moves[1] && moves[2]) || (moves[3] && moves[4] && moves[5]) || (moves[6] && moves[7] && moves[8]) || (moves[0] && moves[3] && moves[6]) || (moves[1] && moves[4] && moves[7]) || (moves[2] && moves[5] && moves[8]) || (moves[0] && moves[4] && moves[8]) || (moves[2] && moves[4] && moves[6]); }

Applets

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public void start() { movesmade = 0; for (int i=0; i<9; i++) { computer[i] = false; player[i] = false; }

Applets

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this.addMouseListener(new MouseAdapter() { public void mouseReleased(MouseEvent e) { int x = e.getX(); int y = e.getY(); Dimension d = getSize(); int r = (y * 3) / d.height; int c = (x * 3) / d.width; if (MoveOK(r*3 + c) && !won(player) && !won(computer)) player[r*3+c] = true; // Make the move movesmade++; repaint(); if (!won(player) && (movesmade < 9)) { computerMove(); repaint(); } } }});

Applets

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Design Issues (2)

Representing the game on screen

  • capturing the player’s move
  • drawing the grid
  • drawing moves

Applets

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public void paint(Graphics g) { Dimension d = getSize(); setBackground(Color.white); g.setColor(Color.black); int xoff = d.width / 3; int yoff = d.height / 3; g.fillRect(xoff-2, 0, 4, d.height); g.fillRect(2*xoff-2, 0, 4, d.height); g.fillRect(0, yoff-2, d.width, 4); g.fillRect(0, 2*yoff-2, d.width, 4); for (int r = 0 ; r < 3 ; r++) for (int c = 0 ; c < 3 ; c++) if (player[r*3+c]) drawX(g, r, c, d.width/3, d.height/3); else if (computer[r*3+c]) drawO(g, r, c, d.width/3, d.height/3); }

Applets

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void drawO (Graphics g, int r, int c, int boxw, int boxh) { g.setColor(Color.red); g.fillOval(c*boxw + boxw/4, r*boxh + boxh/4, boxw/2, boxh/2); g.setColor(Color.white); g.fillOval(c*boxw + boxw/4 + boxw/10, r*boxh + boxh/4 + boxw/10, boxw/2 - boxw/5, boxh/2 - boxw/5); }

Applets

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void drawX (Graphics g, int r, int c, int boxw, int boxh) { Polygon p1 = new Polygon(); Polygon p2 = new Polygon(); // Top left to bottom right swish p1.addPoint(c*boxw + boxw/4, r*boxh + boxh/4); p1.addPoint(c*boxw + boxw/4, r*boxh + boxh/4 + boxw/5); p1.addPoint((c+1)*boxw - boxw/4, (r+1)*boxh - boxh/4); p1.addPoint((c+1)*boxw - boxw/4, (r+1)*boxh - boxh/4

  • boxw/5);

// Similarly bottom left to top right swish g.setColor(Color.blue); g.fillPolygon(p1); g.fillPolygon(p2); }

Applets