Understanding Java Applets and Applications: Creating and Displaying Graphics

CSE 1340Class 5

Objectives • Differentiate between an application and an applet • Create an applet from Java source code • Write code to display a graphic, text, color, and the date in an applet • Create an HTML host document

Class 3 Objectives • Get an elementary understand object-orientation • Write a JAVA program with a Graphical User Interface (GUI) using the JOptionPane class • Differentiate between a Java Applet and a Java Application • Write a Java Applet

Import Packages • Use the import statement to access classes in the SDK • The java.lang package is automatically imported • Place the import statement before the class header • Use an asterisk (*) after the package name and period delimiter to import all necessary classes in the package

Call a System Date Constructor • Use the Date class in the java.util package to access the system date • Store the Date in an object variable • Declare the object variable by calling the Date constructor • The constructor is a method denoted by the new keyword followed by the object type and parentheses • Declaration syntax: objectType variableName = new objectType();

java applet public class WelcomeApplet2 extends JApplet{ public void paint( Graphics g) { g.drawString( “Welcome to”, 25, 25 ) g.drawString(“Java Programming”, 25, 40); } } 1. Name two methods in the above code. paint drawString 2. What kind of data is being passed to the paint method? A Graphics object

java applet public class WelcomeApplet2 extends JApplet { public void paint( Graphics g) { g.drawString( “Welcome to”, 25, 25 ) g.drawString(“Java Programming”, 25, 40); } } 3. What is the name of the Graphics object g 4. What kind of data is being passed to the drawString method? A string and two integers

import javax.swing.JOptionPane; The import statement includes particular classes from a particular package (folder) in the java library of classes

import javax.swing.JOptionPane; public class Welcome4 All code in java must be inside a class definition; the above line begins the definition of a class called Welcome4. The rules for class names are the same as the rules for any user-defined Java identifier (variables, constants, class names, etc.)

import javax.swing.JOptionPane; public class Welcome4 { public static void main(String args[ ]) Every Java application (one run on a desktop, not over the web through an html file) must have a method called main; that is beginning/entry point of execution of your Java application.

import javax.swing.JOptionPane; public class Welcome4 { public static void main(String args[ ]) { JOptionPane.showMessageDialog (null,”Welcome\nto\nJava\n” + “Programming!”); System.exit(0); } }

import javax.swing.JApplet; The import statement includes particular classes from a particular package (folder) in the java library of classes

import javax.swing.JApplet; The import statement includes particular classes from a particular package (folder) in the java library of classes import java.awt.Graphics;

import javax.swing.JApplet; import java.awt.Graphics; public class WelcomeApplet2 extends JApplet Applets (programs that run over the web through an html file) don’t have a method main; instead we want our class to be an applet type so we use the word extends to make our class a JApplet Keyword extends gives us inheritance in Java

import javax.swing.JApplet; import java.awt.Graphics; public class WelcomeApplet2 extends JApplet { public void paint(Graphics g) { g.drawString(”Welcome to ” ,25,25); g.drawString(“Java Programming!”, 25,40); } }

Every applet must have an associated html file that contains the name of the class created with the .java file; the following code would be entered into a text editor and saved as WelcomeApplet2.html <html> <applet code=“WelcomeApplet2.class” width = 300 height =30> </applet> </html>

import javax.swing.JOptionPane; public class Addition { public static void main(String argvs[ ]) {String firstNumber, secondNumber; int number1, number2; int sum;

firstNumber = JOptionPane.showInputDialog( “Enter first integer”); secondNumber= JOptionPane.showInputDialog( “Enter second integer”); number1= Integer.parseInt(firstNumber); number2=Integer.parseInt(secondNumber); sum = number1 + number2;

JoptionPane.showMessageDialog( null, “The sum is “ + sum,”Results”, JOptionPane.PLAIN_MESSAGE); System.exit(0); } }

Class Addition written as an Applet

import java.awt.Graphics; import javax.swing.*; // make available all classes // (not all packages BUT all classes in the // swing package (folder)

import java.awt.Graphics; import javax.swing.*; public class AdditionJApplet extends JApplet { double sum; public void init() {String firstNumber, secondNumber; double number1, number2;

firstNumber = JOptionPane.showInputDialog( “Enter first floating-point value”); secondNumber= JOptionPane.showInputDialog( “Enter second floating-point value”); number1= Double.parseDouble(firstNumber); number2= Double.parseDouble(secondNumber); sum = number1 + number2; }

public void paint(Graphics g) { g.drawRect(15, 10, 270, 20); g.drawString(“The sum is “ + sum,25,25); } }

html file associate with AdditionApplet.java file <html> <applet code = “AdditionApplet.class” width = 300 height = 50> </applet> </html>

Moving to the Web • Characteristics of an applet • Applets run within a browser/viewer and are usually delivered to the client machine via the Web • Applets cannot use system resources or files on the client machine • Convert the application into an applet • Import two packages • Change the class name and extend the Applet class • Include a paint method to draw text and display color and a graphic

Import Applet Packages • Applet package (java.applet.*) • Allows applets to inherit attributes and methods • AWT package (java.awt.*) • Provides access to color, draw methods, and GUI elements

Change the Class Name and Extend the Applet Class • Change the class name and file name to create a new applet file • Edit the comment header in the applet file • Add “extends Applet” in the class header to inherit from the superclass, Applet • Provides the init() method to load the applet in the browser window

The paint() Method • Accepts a Graphics object as a parameter • The Graphics object is commonly referred to by the variable name g • The variable g is created and initialized in the init() method • The variable g is a reference variable, or a specific instance of an object • The return type is void

The drawString() Method • Displays text in the applet window • Accepts three arguments • The String data • If the data is not a String object, convert it to a String object using the toString() method • The horizontal and vertical coordinates of the String • The coordinates are measured in pixels • Called by the Graphics object, g

Draw an Image • Declare an Image object • Use the getImage() method to load the image • The getImage() method calls the getDocumentBase() method to pull the image from the current folder • Use the drawImage() method to set the coordinates of the image

Set the Background Color • Use the setBackground() method to change the background color of the applet window • The setBackground() method does not need to be called from a reference variable

Creating an HTML Host Document • A host program, such as a Web page executes the applet

Creating an HTML Host Document • The Web page contains HTML tags to define a section or format • A tag consists of a start tag, denoted by <> and an end tag, denoted by </> • The tag, <APPLET>…</APPLET>, informs the browser of the applet • The applet tag encloses the name of the bytecode applet file and the width and height of the applet window

Running an Applet • An applet is run by opening the HTML host document • In TextPad, use the Run Java Applet command • At the command prompt, type appletviewer followed by the name of the host document • Use Applet Viewer to test the applet • Ignores irrelevant HTML code • Uses less memory than a browser • Does not have to be Java-enabled

Understanding Java Applets and Applications: Creating and Displaying Graphics

Understanding Java Applets and Applications: Creating and Displaying Graphics

Presentation Transcript

Law 1340 of 2009

Class 5

Geoffrey Chaucer 1340 1400

Geoffrey Chaucer 1340? - 1400

CSE 3 Class Overview

Chaucer (1340 -1400)

Class 5

CSE 598/494 Class 20

Class 5

Class 5

Class 5

Class 5

Class 5

Placing CSE Class On Line

CSE 260 – Class #2

Class 5

Class 5

Class #5

Class 5

Class 5

Class #5

Class 5