24 First Applet

7/31/2019 24 First Applet

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A Simple Applet


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Applets and applications

An applet is a Java program that runs on a web page

Applets can be run within any modern browser

To run modern Java applets, old browsers need an up-to-date

Java plugin appletviewer is a program that can run

An application is a Java program that runs all by itself


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The Applet class

To create an applet, you must import the Applet class

This class is in the java.applet package

TheAppletclass contains code that works with a

browser to create a display window Capitalization matters!

applet and Applet are different names


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Importing the Applet class

Here is the directive that you need:

import java.applet.Applet;

import is a keyword

java.applet is the name of the package

A dot ( .) separates the package from the class

Applet is the name of the class

There is a semicolon ( ; ) at the end


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Thejava.awtpackage

awt stands for Abstract Window Toolkit

The java.awt package includes classes for:

Drawing lines and shapes

Drawing letters

Setting colors

Choosing fonts

If its drawn on the screen, then java.awt isprobably involved!


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Importing the java.awt package

Since you may want to use many classes from the

java.awt package, simply import them all:

import java.awt.*;

The asterisk, or star (*), means all classes

The import directives can go in any order, but must be

the first lines in your program


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C and C++ programmers only

C and C++ have an #include directive that copies a

library function into your program

This makes your program bigger

Javas import gives you access to the library

It does notmake your program bigger

Its OK to use lots ofinclude directives!


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The applet so far


import java.awt.*;


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Your applet class

public class Drawing extends Applet { }

Drawing is the name of your class

Class names should always be capitalized extends Applet says that our Drawing is a kind ofApplet, but with added capabilities Javas Applet just makes an empty window

We are going to draw in that window The only way to make an applet is to extend Applet


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The applet so far


import java.awt.*;

// CIT 591 example

public class Drawing extends Applet {

we still need to put some code in here...

}


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The paint method, part 2

public void paint(Graphics g) { } public says that anyone can use this method

void says that it does not return a result

A Graphics(short for Graphics context) is an

object that holds information about a painting

It remembers what color you are using

It remembers what font you are using

You can paint on it (but it doesnt remember what youhave painted)


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Colors

The java.awtpackage defines a class named Color

There are 13 predefined colorshere are their fully-qualified names:

For compatibility with older programs (before the naming

conventions were established), Java also allows color

names in lowercase: Color.black, Color.darkGray, etc.

Color.BLACK Color.PINK Color.GREENColor.DARK_GRAY Color.RED Color.CYAN

Color.GRAY Color.ORANGE Color.BLUE

Color.LIGHT_GRAY Color.YELLOW

Color.WHITE Color.MAGENTA


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New colors

Every color is a mix of red, green, and blue

You can make your own colors:

new Color(red,green,blue)

Amounts range from 0 to 255 Black is (0, 0, 0), white is (255, 255, 255)

We are mixing lights, not pigments

Yellow is red + green, or (255, 255, 0)


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Setting a color

To use a color, we tell our Graphicsgwhat color we

want:

g.setColor(Color.RED);

g will remember this color and use it for everythinguntil we tell it some different color


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The paint method so far

public void paint(Graphics g) {

g.setColor(Color.BLUE);

draw a rectangle

g.setColor(Color.RED);draw another rectangle

}

}


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Pixels

A pixel is a picture (pix) element

one pixel is one dot on your screen

there are typically 72 to 90 pixels per inch

java.awt measures everything in pixels


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Javas coordinate system

Java uses an (x, y) coordinate system

(0, 0) is the top left corner

(50, 0) is 50 pixels to the right of (0, 0) (0, 20) is 20 pixels down from (0, 0)

(w - 1, h - 1) is just inside the bottom right corner, where w

is the width of the window and h is its height

(0, 0)

(0, 20)

(50, 0)

(50, 20)

(w-1, h-1)


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Drawing rectangles

There are two ways to draw rectangles:

g.drawRect(left,top,width,height);

g.fillRect(left,top,width,height);


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The complete applet


import java.awt.*;

// CIT 591 example

public class Drawing extends Applet {

public void paint(Graphics g) {

g.setColor(Color.BLUE);

g.fillRect(20, 20, 50, 30);g.setColor(Color.RED);

g.fillRect(50, 30, 50, 30);

}

}


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Some more java.awt methods

g.drawLine(x1 ,y1,x2 ,y2 );

g.drawOval( left,top,width,height );

g.fillOval( left,top,width,height);

g.drawRoundRect( left,top,width,height );

g.fillRoundRect( left,top ,width,height);

g.drawArc( left,top,width,height,

startAngle,arcAngle ); g.drawString(string,x,y);


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The HTML page

You can only run an applet in an HTML page The HTML looks something like this:

DrawingApplet Applet

BlueJ will create this HTML for you


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24 First Applet

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