Applets and Frames. Copyright 2005, by the authors of these slides, and Ateneo de Manila University. All rights reserved L14: GUI Slide 2 Applets Usually

Applets and Frames

Copyright 2005, by the authors of these slides, and Ateneo de Manila University. All rights reserved

L14: GUISlide 2

Applets

Usually graphical in content One way of writing Java programs

(besides applications) Java programs that can be embedded

in HTML documents (web pages) Note: Not all web browsers automatically

support Java 2. You will have to install the Java Plug-in, which is done automatically if you installed Java on your machines.


L14: GUISlide 3

JApplet

This is the “swing” version of Applets JApplet is the newer version of the

Applet class that is used with Java’s swing GUI components

We will be using this in class


L14: GUISlide 4

Java Applet classes

Abstract Window Toolkit (AWT) Contain classes that provide the Java

graphical user interface (GUI) components Java Foundation Classes

Was an extension to java Contains swing components written

entirely in java


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Writing Java Appletsimport javax.swing.*;import java.awt.*;

/**This applet displays “Hello, world!” on a label.**/public class MyApplet extends JApplet{ private JLabel helloLabel; public void init() { Container c = getContentPane(); c.setLayout( new FlowLayout() );

this.helloLabel = new JLabel( "Hello, world!" ); c.add( helloLabel ); }}


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Running Java Applets Recall that an .html needs to be created to

execute applets Executing through an IDE

In BlueJ:, right-click on the class and select “Run Applet”, then Click on “OK” and wait for the applet to appear

In JCreator, create the .html file and execute that file Executing through the Command Prompt

appletviewer file.html Executing through a browser:

Go to the folder where the .html and the .class resides

Double-click on that and wait for the browser to load


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Writing Web Pages with Applets

You will have to include this in the body of the HTML file to tell it you are embedding an applet:

<APPLET CODE=AppletSubclass.class

WIDTH=anInt HEIGHT=anInt> </APPLET>

To embed the class file of MyApplet.java:

<APPLET CODE=MyApplet.class WIDTH=100

HEIGHT=100> </APPLET>


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Container and getContentPane()

Container A generic AWT container a component that can contain other AWT

components Most important methods: setLayout() and add()

getContentPane() A method found in JApplet This method returns the contentPane object (a

Container) for this applet. You’ll need these to be able to add

components to your applet.


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public void init() Every applet must implement one or more

of the init(), start(), and paint() methods. This is what a browser usually looks for when it runs applets.

You will have to include the init() method to give your applet its intended appearance and functionality Think of init() as a constructor for your applet

This is where you add the components to a container The applet is in effect an aggregate class

containing the labels, buttons, text fields, and other visual components


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Importing packages

To be able to use JApplet and swing components you will have to import the javax.swing package and to be able to use some AWT components, you will have to import the java.awt package.

At the top of your java class:import javax.swing.*;import java.awt.*;


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The Java Swing Components

Java’s framework for programming lightweight GUI Components Lightweight here means that it’s written in and it

runs entirely in Java

Components JButton, JTextField, JTextArea, JLabel,

JPanel Others (self-study)

You will have to add these components to an AWT container


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The Java Abstract Window Toolkit (AWT)

Java’s framework for programming Graphical User Interfaces (GUIs)

Containers Layout Managers

FlowLayout, GridLayout, BorderLayout CardLayout, GridBagLayout (self-study)


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Components

JButton: clickable visual object JLabel: text JTextField

contains editable text methods setText() and getText()

JTextArea same as JTextField but can support multiple

lines JPanel

may contain other visual components methods setLayout() and add()


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Layout Managers

FlowLayout objects are placed row by row, left to

right GridLayout

divides container into an m by n grid BorderLayout

divides container into 5 parts Center, North, South, East, West e.g., add( component, “North” );

See examples


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Panel

A Panel is a component that can contain other objects

We can do more complex layouts by using Panels within the Applet or even Panels within Panels


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Designing a GUI for an Applet

import java.awt.*; import javax.swing.*; extends JApplet

inherits built-in features of JApplet Declare variables for the different visual

components to be placed on the applet In init() method,

create visual components (use new) establish layout manager (use setLayout())

default for JApplets: BorderLayout add visual components (use MyContainer.add()) nested layouts possible (use Panels)


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Applet Exampleimport javax.swing.*;

import java.awt.*;

public class HelloWorldApplet extends JApplet

{ private JButton button;

private JTextField textField;

private JLabel label;

public void init()

{ Container c = getContentPane();

c.setLayout( new FlowLayout() );

label = new JLabel( "Hello, World!" );

c.add( label );

textField = new JTextField( "Enter your name here", 15 );

c.add( textField );

button = new JButton( "Click Me" );

c.add( button );

}

...

}


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HTML file for the applet

<applet code=HelloWorldApplet.class width=300 height=200></applet>


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JFrame

Use JFrame instead of JApplet if you want a GUI application (instead of a web-based applet)

Extend JFrame instead of JApplet Instead of the init() method, place

initialization code in the class’ constructor

Need a main method (perhaps in a separate class) that instantiates and displays the frame


L14: GUISlide 20

Frame exampleimport javax.swing.*;

import java.awt.*;

public class HelloWorldFrame extends JFrame

{ private JButton button;

private JTextField textField;

private JLabel label;

public HelloWorldFrame()

{ Container c = this.getContentPane();

c.setLayout( new FlowLayout() );

label = new JLabel( "Hello, World!" );

c.add( label );

textField = new JTextField( "Enter your name here", 15 );

c.add( textField );

button = new JButton( "Click Me" );

c.add( button );

}

...

}


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Driver class for the frame

import javax.swing.*;

public class FrameRunner{

public static void main( String args[] ){

JFrame f = new HelloWorldFrame();f.setSize( 200, 300 );

f.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );f.setVisible( true );

}}


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About frames

When using frames, you could call setLayout() and add() directly on the frame object No need to declare a Container object or

call getContentPane() Other methods applicable on a frame

object (these can be called in the frame’s constructor, or from the driver) setSize(): sets the dimensions of the

frame setTitle(): sets the title bar of the frame

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Applets and Frames. Copyright 2005, by the authors of these slides, and Ateneo de Manila University. All rights reserved L14: GUI Slide 2 Applets Usually