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Given by Nancy McCracken (foils by Tom Scavo) at NPAC/ECS Java Academy on February to April 1998 and 1999. Foils prepared 6 Feb 99
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Summary of Material
| Introduction |
| Java Applets |
| Understanding Applet Methods |
| Elementary Java Graphics |
| Java Geometry |
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Java Academy
Part I: Elementary Java
Prerequisites
Online Java Resources
Software
Notation
Applications Programming Interface
Variable Conventions
Java Applets
Applets and Applications
My First Applet
Class Definition
Methods and Arguments
Importing Java Classes
HelloWorld.java
The Java Compiler
HelloWorld.html
The <APPLET> Tag
The <OBJECT> Tag
Viewing the Applet
Applet Development Cycle
Understanding Applet Methods
Applet Methods
Overriding Applet Methods
Applet Life Cycle
The paint Method
Overriding the paint Method
Revised Life Cycle
The update Method
The repaint Method
Clearing the Applet
Definition of update
Elementary Java Graphics
Graphics Concepts
Coordinate System
Applet Dimensions
Graphics Methods
The drawLine Method
The drawRect Method
The drawRoundRect Method
The draw3DRect Method
The drawOval Method
The drawArc Method
Fill Methods
Java Geometry
The Polygon Class
The addPoint Method
TriangleTest Applet
TriangleTest Applet (cont'd)
An Equilateral Triangle
QuadrilateralTest Applet
Another Polygon Constructor
OctagonTest.java
A Regular Octagon
A Surprise!
Outside Index
Summary of Material
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| Tom Scavo |
| <trscavo@npac.syr.edu> |
| NPAC @ SU |
| 111 College Place |
| Syracuse, NY 13244-4100 |
| Syracuse University |
| College of Engineering and Computer Science |
| Northeast Parallel Architectures Center |
| present |
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| Introduction |
| Java Applets |
| Understanding Applet Methods |
| Elementary Java Graphics |
| Java Geometry |
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| A good working knowledge of HTML and the World Wide Web |
| A high-end PC running Windows 95/NT |
| A reasonably good network or dialup connection |
The following are helpful, but not necessary:
|
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| Java Academy Home Page http://www.npac.syr.edu/projects/k12javaspring98/ |
| NPAC Java Resources http://www.npac.syr.edu/projects/tutorials/Java/ |
| The Java Tutorial http://java.sun.com/tutorial/ |
Other web sites of interest:
|
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| Sun's Java Development Kit (JDK): http://java.sun.com/products/jdk/ |
| Netscape Communicator with Java 1.1 AWT: http://developer.netscape.com/software/jdk/download.html |
| Microsoft Internet Explorer: http://www.microsoft.com/ie/download/ |
| JavaEdit by Dick Chase (freeware): http://www.tiac.net/users/dchase/javaedit.htm |
| TextPad by Helios Software (shareware): http://www.textpad.com/ |
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| Code is written in a fixed-width, bold yellow font |
| Within a code segment, an italicized word denotes a name to be chosen by the programmer |
| In the text, we often give a method's signature, while omitting the method body. Thus we write: public void paint( Graphics g ); which tells you everything you need to know to use method paint(...), but (intentionally) says nothing about how it is implemented |
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| A method's signature along with a short description is sometimes referred to as an applications programming interface (API) |
| Much of the official Java documentation and textbooks use APIs to communicate the Java language |
| We adopt the same conventions here |
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| The following variable names are used consistently throughout this document: String s; Graphics g; Font f; FontMetrics fm; Color c; Component c; |
| In different |
| sections! |
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| There are two types of Java programs: applets and applications |
| A Java applet lives on the web and runs inside an ordinary web browser window |
| A Java application runs on any machine with a Java interpreter installed |
| In this course, we concentrate on applets... |
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| Our first applet is as simple as can be: // File: HelloWorld.java import java.applet.Applet; import java.awt.Graphics; public class HelloWorld extends Applet { public void paint( Graphics g ) { g.drawString( "Hello World!", 25, 25 ); } } |
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| public class HelloWorld extends Applet { ... } |
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| public class HelloWorld extends Applet { public void paint( Graphics g ) { g.drawString( "Hello World!", 25, 25 ); } } |
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| // File: HelloWorld.java import java.applet.Applet; import java.awt.Graphics; |
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| The complete Java source file: // File: HelloWorld.java import java.applet.Applet; import java.awt.Graphics; public class HelloWorld extends Applet { public void paint( Graphics g ) { g.drawString( "Hello World!", 25, 25 ); } } |
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| The next step is to compile the source file: > javac HelloWorld.java |
| The Java compiler (javac) compiles the high-level source code into bytecodes |
| The file of bytecodes created by the compiler is called "HelloWorld.class" |
| Note carefully the ".class" extension |
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| <!-- File: HelloWorld.html --> <HTML> <HEAD> <TITLE>HelloWorld applet</TITLE> </HEAD> <BODY BGCOLOR="#FFFFFF"> <APPLET CODE="HelloWorld.class" WIDTH=150 HEIGHT=50> </APPLET> </BODY> </HTML> |
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| The most important part of the HTML file is the <APPLET> tag: <APPLET CODE="HelloWorld.class" WIDTH=150 HEIGHT=50> </APPLET> which loads the bytecodes into the browser |
| Note that the "applet window" is specified to be 150 pixels wide and 50 pixels high |
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| In HTML 4.0, the <APPLET> tag has been deprecated in favor of the more general <OBJECT> tag : <OBJECT CLASSID="HelloWorld.class" CODETYPE="application/java" WIDTH=150 HEIGHT=50> </OBJECT> |
| The above <OBJECT> tag is equivalent to the <APPLET> tag on the previous foil |
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| Once the source file has been compiled and the HTML file has been created, the applet may be viewed: > appletviewer HelloWorld.html |
| Alternatively, you may view the applet with a Java-enabled browser such as Netscape Communicator or Microsoft Internet Explorer |
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| The following methods are invoked automatically by the Java interpreter: public void init(); public void start(); public void stop(); public void destroy(); |
| Initially, these methods of the Applet class all have empty bodies, but... |
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| An applet may override any or all of these methods, like this: import java.applet.Applet; public class AppletDemo extends Applet { public void init() { /* insert code */ } public void start() { /* insert code */ } public void stop() { /* insert code */ } public void destroy() { /* insert code */ } } |
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| init() and destroy() are called once and only once during the life of the applet |
| start() and stop() are called repeatedly |
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| Other methods are called automatically by the Java interpreter, for instance: public void paint( Graphics g ); |
| The paint(...) method is called immediately after start() and any time the applet window must be redrawn |
| Like the others, paint(...) can (and should) be overridden by the programmer |
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| Override the paint(...) method like this: import java.applet.Applet; import java.awt.Graphics; public class AppletDemo extends Applet { public void init() { /* insert code */ } public void start() { /* insert code */ } public void stop() { /* insert code */ } public void destroy() { /* insert code */ } public void paint( Graphics g ) { // insert code here } } |
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| Like start() and stop(), the paint(...) method will be called by Java many times during the life of an applet |
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| All of the methods in the previous diagram are called automatically by the Java VM |
| What if the programmer wants to repaint the applet explicitly? |
| In that case, call the update(...) method: public void update( Graphics g ); |
| The update(...) method simply clears the applet window and calls paint(...) |
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| Note, however, that update(...) takes a Graphics argument, which is inconvenient |
| Instead, call the repaint() method: public void repaint(); which in turn calls update(...) |
| Note: The repaint() method is almost never overridden by the programmer |
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| Let's examine update(...) more closely... |
| Given a Graphics object g, the following code fragment clears the applet window: int w = getSize().width; int h = getSize().height; g.setColor( getBackground() ); g.fillRect( 0, 0, w, h ); g.setColor( getForeground() ); |
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| So here is update(...) in its entirety: public void update( Graphics g ) { // clear the applet and call paint: int w = getSize().width; int h = getSize().height; g.setColor( getBackground() ); g.fillRect( 0, 0, w, h ); g.setColor( getForeground() ); paint( g ); } |
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| Think of a Java applet as a graphics window on which to draw text and other objects |
| The graphics window has a particular width and height as specified in the <APPLET> tag |
| The origin is in the top left-hand corner of the graphics window: the x-coordinate increases from left to right, while the y-coordinate increases from top to bottom |
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| In Java, the width and height of the applet are obtained as follows: int width = getSize().width; int height = getSize().height; |
| These are the same width and height given in the <APPLET> tag: <APPLET ... WIDTH=... HEIGHT=... > |
| Note: All measurements are in pixels |
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| drawLine(...) |
| drawRect(...) |
| drawRoundRect(...) |
| draw3DRect(...) |
| drawOval(...) |
| drawArc(...) |
| drawString(...) |
| fillRect(...) |
| fillRoundRect(...) |
| fill3DRect(...) |
| fillOval(...) |
| fillArc(...) |
| Important methods from the Graphics class: |
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| The drawLine(...) method draws a line segment with endpoints (x1,y1) and (x2,y2): public void drawLine( int x1, int y1, int x2, int y2 ); |
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| The drawRect(...) method draws a rectangle anchored at point (x,y): public void drawRect( int x, int y, int w, int h ); |
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| The drawRoundRect(...) method draws a rectangle with rounded corners: public void drawRoundRect( int x, int y, int w, int h, int W, int H ); |
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| The draw3DRect(...) method draws a 3-d rectangle anchored at point (x,y): public void draw3DRect( int x, int y, int w, int h, boolean raised ); |
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| The drawOval(...) method draws an ellipse anchored at point (x,y): public void drawOval( int x, int y, int w, int h ); |
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| The drawArc(...) method draws an arc anchored at point (x,y): public void drawArc( int x, int y, int w, int h, int t1, int t2 ); |
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| Each "draw" method (except drawLine and drawString) has a corresponding "fill" method |
| The syntax is the same except that the word "draw" is replaced by "fill" (e.g., fillRect) |
| An object is filled with the current color, chosen as follows: g.setColor( c ); where c is a Color object |
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| The java.awt.Polygon class is used to represent arbitrary polygons: Polygon p = new Polygon(); p.addPoint( 50, 25 ); p.addPoint( 75, 100 ); p.addPoint( 25, 100 ); |
| The above triangle is drawn like this: g.drawPolygon( p ); |
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| Each call to the addPoint(...) method adds a vertex to the polygon |
| When the drawPolygon(...) method is called, the last vertex is automatically joined with the first (except in Java 1.0, which has a permanent bug) |
| There is a corresponding fillPolygon(...) method, of course |
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| Our test applet creates two triangles: p1 = new Polygon(); p1.addPoint( 50, 25 ); p1.addPoint( 75, 100 ); p1.addPoint( 25, 100 ); p2 = new Polygon(); p2.addPoint( 125, 25 ); p2.addPoint( 175, 100 ); p2.addPoint( 125, 100 ); |
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| The test applet draws one triangle and fills the other: public void paint( Graphics g ) { g.drawPolygon( p1 ); g.fillPolygon( p2 ); } |
| In the exercises, you are asked to draw an equilateral triangle... |
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| An equilateral triangle has three sides of equal length |
| To draw an equilateral triangle precisely, we must determine its height h |
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| Our next test applet draws two quadrilaterals, for example: p = new Polygon(); p.addPoint( 25, 25 ); p.addPoint( 125, 25 ); p.addPoint( 125, 75 ); p.addPoint( 25, 75 ); |
| There's another way to do it, however... |
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| Here's another way to instantiate a polygon: int xValues = { 25, 125, 125, 25 }; int yValues = { 25, 25, 75, 75 }; Polygon p = new Polygon( xValues, yValues, 4 ); |
| To fill this quadrilateral, type: g.fillPolygon( p ); |
| Note: The number of vertices is required in this version of the Polygon constructor |
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| Our next test applet draws an octagon inside a square |
| It does this by trisecting each side of the square |
| This octagon is not regular, however... |
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| A regular octagon has all sides equal |
| Solve for y in terms of x: |
| x |
| x |
| y |
| y |
| x |
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| Our final test applet draws a hexagon inside a square |
| All sides of the hexagon are equal, but this hexagon is not regular! |
| We leave it to you to figure this out... |