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8/2/2019 Applets n Swings-surabhi Prakash

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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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Packages and classes

Java supplies a huge library of pre-written “code,”

ready for you to use in your programs

Code is organized into classes

Classes are grouped into packages

One way to use this code is to import it

You can import a single class, or all the classes in a

package



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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

The Applet class 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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The java.awt package

“awt” stands for “Abstract Window Toolkit”

The java.awt package includes classes for: Drawing lines and shapes

Drawing letters

Setting colors

Choosing fonts

If it’s drawn on the screen, then java.awt is probably involved!



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



The applet so far


import java.awt.*;



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 of Applet, but with added capabilities Java’s 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



The applet so far


import java.awt.*;

public class Drawing extends Applet {

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

}



The paint method

Our applet is going to have a method to paint some

colored rectangles on the screen

This method must be named paint

paint needs to be told where on the screen it can draw This will be the only parameter it needs

paint doesn’t return any result



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 objectthat 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 doesn’t remember what youhave painted)



The applet so far


import java.awt.*;


public void paint(Graphics g) {

…we still need to put some code in here… }

}



Colors

The java.awt package defines a class named Color There are 13 predefined colors—here 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



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)



Setting a color

To use a color, we tell our Graphics g what color we

want:

g.setColor(Color.RED);

g will remember this color and use it for everything untilwe tell it some different color



The paint method so far


g.setColor(Color.BLUE);

…draw a rectangle…

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

}

}



Java’s 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)



Drawing rectangles

There are two ways to draw rectangles: g.drawRect( left , top , width , height );

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



The complete applet


import java.awt.*;



g.setColor(Color.BLUE);

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

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

}

}



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:

<html>

<body>

<h1>DrawingApplet Applet</h1><applet code="DrawingApplet.class"

width="250" height="200">

</applet>

</body></html>



Java

GUI building with Swing



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AWT (Abstract Window Toolkit)

Present in all Java implementations Described in (almost) every Java textbook

Adequate for many applications

Uses the controls defined by your OS (whatever it is) therefore it's “least common denominator”

Difficult to build an attractive GUI

import java.awt.*;import java.awt.event.*;



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Swing

Present in all modern Java implementations (since 1.2) More controls, and they are more flexible Gives a choice of “look and feel” packages Much easier to build an attractive GUI import javax.swing.*;

import javax.swing.event.*;

and

import java.awt.*;import java.awt.event.*; You may not need all of these packages



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Swing vs. AWT

Swing is built “on top of” AWT, so you need to import AWT

and use a few things from it

Swing is bigger and slower

Swing is more flexible and better looking

Swing and AWT are incompatible--you can use either, but you

can’t mix them Actually, you can, but it’s tricky and not worth doing

Basic controls are practically the same in both AWT: Button b = new Button ("OK");

Swing: JButton b = new JButton("OK");

Swing gives far more options for everything (buttons with

pictures on them, etc.)



AWT Class Hierarchy

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To build a GUI...

1. Make somewhere to display things (a Container) Usually you would use a JFrame or a JApplet

2. Create some Components (buttons, text areas, panels, etc.) It’s usually best to declare Components as instance variables, and

Define them in your applet’s init() method or in some applicationmethod

3. Add your Components to your display area Choose a layout manager Add your Components to your JFrame or JApplet according to the

rules for your layout manager

4. Attach Listeners to your Components Interacting with a Component causes an Event to occur A Listener gets a message when an interesting event occurs, and

executes some code to deal with it



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Containers and Components

A GUI is built by putting components into containers

The job of a Container is to hold and display Components

Some frequently used types (subclasses) of Component are

JButton, JCheckbox, JLabel, JTextField, and JTextArea

A Container is also a Component This allows Containers to be nested

Important Container classes are JFrame, JApplet, and JPanel JFrame and JApplet both contain other containers; use

getContentPane() to get to the container you want

You typically create and use JPanels directly



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Starting with a Container

First, import some packages: import javax.swing.*;

import javax.swing.event.*;import java.awt.*;import java.awt.event.*;

Second, extend a Container type: For an application, extend JFrame

public class MyClass extends JFrame { ... }

For an applet, extend JApplet public class MyApplet extends JApplet { ... }

Neither of these returns a Container that you use directly; instead, Both JFrame and JApplet have a getContentPane() method

that returns a Container you can use: getContentPane( ).doSomething( );



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Example: A "Life" applet

Container (Applet)

Containers (Panels)

Component (Canvas)

Components (Buttons)

Components (Labels)

Components (TextFields)



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Applets

An application has a public static void main(String args[ ]) method, but

an applet usually does not

An applet's main method is in the Browser

To write an applet, you extend JApplet and override

some of its methods

The most important methods are init( ), start( ), and

paint(Graphics g)



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To create an applet

public class MyApplet extends JApplet { … } The only way to make an applet is to extend Applet or

JApplet

You can add components to the applet

The best place to add components is in init( )

You can paint directly on the applet, but…

…it’s better to paint on a contained component

Do all painting from paint(Graphics g)



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Some types of components

Label Button

Button

Checkbox

Choice

List

Scrollbar

TextField TextArea

CheckboxGroupCheckbox



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Creating components

JLabel lab = new JLabel ("Hi, Dave!");

JButton but = new JButton ("Click me!");

JCheckBox toggle = new JCheckBox ("toggle");

JTextField txt =new JextField ("Initial text.", 20);

JScrollbar scrolly = new JScrollbar

(JScrollbar.HORIZONTAL, initialValue,

bubbleSize, minValue, maxValue);



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Adding components to the Applet

class MyApplet extends JApplet { public void init () {

getContentPane().add(lab); // or this.getContentPane().add(lab);

getContentPane().add(but); getContentPane().add(toggle); getContentPane().add(txt);

getContentPane().add(scrolly); ...



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Creating a JFrame

When you create an JApplet, The size of the applet is determined by the HTML page The browser makes the applet visible

When you write a GUI for an application, you need to do

these things yourself public class MyApplication extends JFrame {

void createMyGUI() { ... add components ... pack(); // compute the size and lay it out setVisible(true); // make the JFrame visible

}}



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Arranging components

Every Container has a layout manager You do not directly control where components are placed; the

layout manager does this for you

The default layout for a Panel or JPanel is FlowLayout A JApplet is a Panel; therefore, the default layout for a

JApplet is FlowLayout

The default layout for a JFrame is BorderLayout

You can set the layout manager explicitly; for example, setLayout(new FlowLayout());

setLayout(new BorderLayout());



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FlowLayout

Use add(component ); to add to a component when using aFlowLayout Components are added left-to-right If no room, a new row is started

Exact layout depends on size of Applet Components are made as small as possible FlowLayout is convenient but often ugly

Note: The following code and screenshots use AWTComponents rather than Swing Components I haven’t had a chance to make new screenshots The layout managers are identical, not just similar There is no real difference with Swing instead of AWT



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Complete example: FlowLayout

import java.awt.*;import java.applet.*;

public class FlowLayoutExample extends Applet {

public void init () {

setLayout (new FlowLayout ()); // default

add (new Button ("One"));

add (new Button ("Two"));

add (new Button ("Three"));

add (new Button ("Four"));

add (new Button ("Five"));

add (new Button ("Six"));}

}



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BorderLayout

At most five components can beadded

If you want more components, add a

Panel, then add components to it.

setLayout (new BorderLayout());

add (new Button("NORTH"), BorderLayout.NORTH);



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BorderLayout with five Buttons

public void init() {

setLayout (new BorderLayout ());

add (new Button ("NORTH"), BorderLayout.NORTH);

add (new Button ("SOUTH"), BorderLayout.SOUTH);add (new Button ("EAST"), BorderLayout.EAST);

add (new Button ("WEST"), BorderLayout.WEST);

add (new Button ("CENTER"), BorderLayout.CENTER);

}



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Complete example: BorderLayout

import java.awt.*;

import java.applet.*;

public class BorderLayoutExample extends Applet {

public void init () {setLayout(new BorderLayout());

add(new Button("One"), BorderLayout.NORTH);

add(new Button("Two"), BorderLayout.WEST);

add(new Button("Three"), BorderLayout.CENTER);

add(new Button("Four"), BorderLayout.EAST);

add(new Button("Five"), BorderLayout.SOUTH);

add(new Button("Six"), BorderLayout.SOUTH);

}

}



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Using a Panel

Panel p = new Panel();

add(p, BorderLayout.SOUTH);

p.add(new Button ("Button 1"));

p.add(new Button ("Button 2"));



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GridLayout

The GridLayout manager

divides the container up into

a given number of rows and

columns:

new GridLayout(rows , columns)

All sections of the grid are equally sized and as large as

possible



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Complete example: GridLayout

import java.awt.*;

import java.applet.*;

public class GridLayoutExample extends Applet {


setLayout(new GridLayout(2, 3));add(new Button("One"));

add(new Button("Two"));

add(new Button("Three"));

add(new Button("Four"));

add(new Button("Five"));}

}



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Making components active

Most components already appear to do something--

buttons click, text appears

To associate an action with a component, attach a

listener to it Components send events, listeners listen for events

Different components may send different events, and

require different listeners



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Listeners

Listeners are interfaces, not classes class MyButtonListener implements

ActionListener {

An interface is a group of methods that must be supplied When you say implements, you are promising to supply

those methods



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Writing a Listener

For a JButton, you need an ActionListener

b1.addActionListener(new MyButtonListener ( ));

An ActionListener must have anactionPerformed(ActionEvent) method

public void actionPerformed(ActionEvent e) {…

}



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MyButtonListener


...b1.addActionListener (new MyButtonListener ());

}

class MyButtonListener implements ActionListener {public void actionPerformed (ActionEvent e) {

showStatus ("Ouch!");

}

}



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Listeners for JTextFields

An ActionListener listens for someone hitting the Enter key

An ActionListener requires this method: public void actionPerformed (ActionEvent e)

You can use getText( ) to get the text

A TextListener listens for any and all keys

A TextListener requires this method: public void textValueChanged(TextEvent e)



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Summary I: Building a GUI

Create a container, such as JFrame or JApplet

Choose a layout manager

Create more complex layouts by adding JPanels; each

JPanel can have its own layout manager Create other components and add them to whichever

JPanels you like



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Summary II: Building a GUI

For each active component, look up what kind of Listeners it can have

Create (implement) the Listeners often there is one Listener for each active component

Active components can share the same Listener

For each Listener you implement, supply the methods

that it requires

For applets, write the necessary HTML



The End