A special Java class that doesn't run in a dos window

a special Java class that doesn't run in a DOS window

• a special Java class that doesn't run in a DOS window

• runs in a web browser on client, in a window, on a HTML page

• Java has security features to keep the applet from acting like a virus on the client machine

need import statements to bring in classes, etc. that Applet will use

• need import statements to bring in classes, etc. that Applet will use

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

• put extends Applet after class name

• has Applet functionality, plus what you add

• has a public void init() method

• has no main() method // code below

components are objects (or like objects)

• components are objects (or like objects)

• GUI means graphic user interface

• graphic is the opposite of text

• you see something other than just plain text

• interface is what the user works with when using a computer program

some "heavyweight" (platform dependent) components in Java

• some "heavyweight" (platform dependent) components in Java

• Label: writes text on the Applet
• Button: push button to trigger action
• TextField: one line input box
• TextArea: multi-line input box
• others: scrollbar, etc. (similar to many components in Windows programs)

earlier Java versions had "heavyweight" platform dependent AWT (abstract windowing toolkit) components

• earlier Java versions had "heavyweight" platform dependent AWT (abstract windowing toolkit) components

• different on different operating systems

• "lightweight" Swing components are platform independent in many ways

• e.g., look the same on PC or Apple

Java Swing class (lightweight) "bean" that includes components including:

• Java Swing class (lightweight) "bean" that includes components including:

• Label,
• TextField
• Buttons

• Java beans: classes with extra features

• makes them easier to use with other classes, languages, and on different platforms

the Java Swing class has additional GUI components (that are NOT platform dependent)

• the Java Swing class has additional GUI components (that are NOT platform dependent)

• JCheckBox: yes or no choices
• JRadioButton: can only select 1 in a group
• JComboBox: drop down list that user can type one or more characters into

need to declare the object

• need to declare the object

• Label aLabel = new Label("Hello");
• Button aButton = new Button("Click");

• need to add objects to the Applet, usually in its init() method

• public void init()
• {
• add(aLabel);
• }

can use methods to set or change properties on components

• can use methods to set or change properties on components

• font on labels (type, style, size)
• text on a button e.g., OK
• input box size(s)
• default content inside an input box
• make content of input box editable or not

can use methods to manipulate components

• can use methods to manipulate components

• set focus (cursor position) onto a TextField
• get contents (user input) from a TextField
• e.g., getting String from JOptionPane

event = something that happens

• event = something that happens

• mouse click, mouse over, mouse out, button push, key stroke, time of day, etc.

• event-driven program flow is (partially) controlled by external events

• rather than just by internal logic

object that waits for an event and then triggers a method that responds to it

• object that waits for an event and then triggers a method that responds to it

• associated with another object e.g., button or text component

need another import statement at top

• need another import statement at top

• import java.awt.event.*;

• the class also needs to "implement" an ActionListener

• public class Greet extends Applet implements ActionListener

attach a listener method to an object

• attach a listener method to an object

• public void init()
• {
• add(aButton); // declared above init
• aButton.addActionListener(this);
• }

add an actionPerformed method

• add an actionPerformed method

• public void actionPerformed(ActionEvent
• thisEvent)
• {
• aLabel.setText("Hi");
• }
• /*will get fatal error if have an ActionListener without actionPerformed method in class*/

can add statements to actionPerformed method

• can add statements to actionPerformed method

• e.g., labels with text from user inputs

• may need to force Applet to redraw itself by adding following methods:

• invalidate(); // marks Applet as out-of-date
• validate(); // redraws out-of-date Applet

can also add methods to actionPerformed method to remove GUI components

• can also add methods to actionPerformed method to remove GUI components

• remove(aLabel);

when add components without a layout manager, Java chooses where to put each item

• when add components without a layout manager, Java chooses where to put each item

• left to right until row is filled, and then starts a new row below previous
• as space permits, centered in row

• can subsequently move components to locations identified by coordinates

1st number is x (horizontal) position

• 1st number is x (horizontal) position

• 2nd number is y (vertical) position

• upper left corner is 0, 0
• 100, 0 is top of window, 100 pixels to right of upper left corner
• 0, 100 is left side of window, 100 pixels down from upper left corner
• 100, 100 is 100 pixels over, 100 down

use setLocation method with a component

• use setLocation method with a component

• myLabel.setLocation(50, 150); // where?

disable makes it impossible to click a button, checkbox, etc. e.g.

• disable makes it impossible to click a button, checkbox, etc. e.g.

• disable Print button until inputs are entered and calculations are completed

• enable makes the item functional again

• enable Print button when data is OK

clickButton.setEnabled(false);

• clickButton.setEnabled(false);

• button previously declared by user will not respond to clicks

• clickButton.setEnabled(true);

• default condition, don't have to set to true if didn't previously set it to false