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Java Applets Road Map Introduction to Java Applets
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tarix | 07.11.2018 | ölçüsü | 162,5 Kb. | | #78552 |
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Java Applets
Road Map Review applets that ship with JDK Make our own simple applets - Introduce inheritance
- Introduce the applet environment
html needed for applets Reading: The applet coverage in the book is different from the coverage for today. The best review for this material is this PowerPoint presentation.
Applet Viewer and browsers An applet is a program that suns inside an applet viewer. The applet viewer takes care of providing the environment and calling several of the applet’s methods. Modern browsers come with applet capabilities. The JDK also comes with an applet viewer. - The JDK applet viewer is really just a minimum browser. It only understands the applet tag.
3.3 Simple Java Applet: Drawing a String Now, create applets of our own Upcoming program - Create an applet to display
- "Welcome to Java!!"
- Show applet and HTML file, then discuss them line by line
3.3 Simple Java Applet: Drawing a String - Import predefined classes grouped into packages
- When you create applets, import Applet class (in the package java.applet)
- import the Graphics class (package java.awt) to draw graphics
- import specifies directory structure
3.3 Simple Java Applet: Drawing a String - Applets have at least one class declaration (like applications)
- Rarely create classes from scratch
- Use pieces of existing classes
- Inheritance - create new classes from old ones (this will be the next major topic)
- Begins class declaration for class WelcomeApplet
- Keyword class then class name
- extends followed by class name
- Indicates class to extend (Applet)
- Applet : superclass (base class)
- WelcomeApplet : subclass (derived class)
- WelcomeApplet now has methods and data of Applet
3.3 Simple Java Applet: Drawing a String - Class Applet defined for us
- Someone else defined "what it means to be an applet"
- Applets require over 200 methods!
- extends Applet
- Inherit methods, do not have to declare them all
- Do not need to know every detail of class Applet
3.3 Simple Java Applet: Drawing a String - Class WelcomeApplet is a blueprint
- appletviewer or browser creates an object of class WelcomeApplet
- Keyword public required
- File can only have one public class
- public class name must be file name
3.3 Simple Java Applet: Drawing a String - Our class inherits method paint from Applet
- By default, paint has empty body
- Override (redefine) paint in our class
- Methods init, start and paint.
- Guaranteed to be called automatically
- Our applet gets "free" version of these by inheriting from Applet
- Free versions have empty body (do nothing)
- Every applet does not need all three methods
- Override the ones you need
- Applet container “draws itself” by calling method paint
3.3 Simple Java Applet: Drawing a String - Method paint
- Draws graphics on screen
- void indicates paint returns nothing when finishes task
- Parenthesis define parameter list - where methods receive data to perform tasks
- Normally, data passed by programmer, as in JOptionPane.showMessageDialog
- paint gets parameters automatically
- Graphics object used by paint
- Mimic paint's first line
3.3 Simple Java Applet: Drawing a String - Body of paint
- Method drawString (of class Graphics)
- Called using Graphics object g and dot (.)
- Method name, then parenthesis with arguments
- First argument: String to draw
- Second: x coordinate (in pixels) location
- Third: y coordinate (in pixels) location
- Java coordinate system
- Measured in pixels (picture elements)
- Upper left is (0,0)
3.3 Simple Java Applet: Drawing a String Running the applet - Compile
- As you would a regular file in JCreator
- javac WelcomeApplet.java (from the command line)
- If no errors, bytecodes stored in WelcomeApplet.class
- Create an HTML file
- Loads the applet into appletviewer or a browser
- Ends in .htm or .html
- To execute an applet
3.3 Simple Java Applet: Drawing a String - Simple HTML file (WelcomeApplet.html)
- Usually in same directory as .class file
- Remember, .class file created after compilation
- HTML codes (tags)
- Usually come in pairs
- Begin with < and end with >
- Lines 1 and 4 - begin and end the HTML tags
- Line 2 - begins
- Specifies code to use for applet
- Specifies width and height of display area in pixels
- Line 3 - ends
3.3 Simple Java Applet: Drawing a String - appletviewer only understands
- Ignores everything else
- Minimal browser
- Executing the applet
- appletviewer WelcomeApplet.html
- Perform in directory containing .class file
3.3 Simple Java Applet: Drawing a String Running the applet in a Web browser
3.4 Drawing Strings and Lines More applets - First example
- Display two lines of text
- Use drawString to simulate a new line with two drawString statements
- Second example
- Method g.drawLine(x1, y1, x2, y2 )
- Draws a line from (x1, y1) to (x2, y2)
- Remember that (0, 0) is upper left
- Use drawLine to draw a line beneath and above a string
import java.awt.*; import java.applet.*; public class WelcomeApplet2 extends Applet { public void init() { } public void paint(Graphics g) { g.drawString( "Welcome to", 25, 25 ); g.drawString( "Java Programming!", 25, 40 ); } }
WelcomeLines.java 2. Class WelcomeLines (extends Applet) 3. paint 3.1 drawLine 3.2 drawLine 3.3 drawString Program Output import java.awt.*; import java.applet.*; public class WelcomeApplet3 extends Applet { public void init() { } public void paint(Graphics g) { g.drawString("Welcome to Java Programming!", 25, 25 ); g.drawLine (15, 10, 210, 10); g.drawLine (15, 30, 210, 30); } }
HTML file
3.4 Drawing Strings and Lines Method drawLine of class Graphics - Takes as arguments Graphics object and line’s end points
- X and y coordinate of first endpoint
- X and y coordinate of second endpoint
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