An Applet with Buttons

The applet's real estate is the area within the border

Source

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

public class AnAppletWithButtons extends Applet implements ActionListener {
	public void init() {
		button1 = new Button("Button 1");
		add(button1);
		button1.addActionListener(this);

		button2 = new Button("Button 2");
		add(button2);
		button2.addActionListener(this);
	}

	public void actionPerformed(ActionEvent e) {
		if (e.getSource() == button1) 
			System.out.println("Button 1 was pressed");
		else
			System.out.println("Button 2 was pressed");
	}

	Button button1, button2;
}

Description and Objective

An applet with some buttons

Shows how to create and position interface components (in particular, buttons) in an applet
Discusses how to handle events (react to graphical user interface (GUI) components)

Notes

Buttons and GUI Components

An applet can contain various graphical user interface (GUI) components that allow a user to interact with the applet.
- Some components are for user input
- Others display information
- Some components can do both
A button is primarily used for user input
- It can provide a limited feedback
  - The button can being disabled
  - A disabled button has a different appearance (grayed out)
  - This does provide the user with some inmformation about the state of the applet

Creating a Button

Java has a Button class
Like any other object, a Button object is created using the new operator
```
Button b = new Button("Click me!");
				
```
The parameter to the constructor is the text that appears on the button

Placing (Laying Out) the Button

Simply creating the Button object does not cause a button to appear on the applet's surface
- The Button object must be explicitly added the applet
- This is achieved using the Applet's add method

Reacting to the Button (Event Handling)

The button must be wired to logic that does something when the button is pressed
- Similar to a light switch-- putting it in the wall is not enogh
- Must be wired to the bulb/outlet
The user clicking the button is called an event We speak of the click as generating the event In general, the user interacting with the applets interface generates events
Reacting to the button is referred to as handling the event
The applet, is called the listener because it wishes to be notified when an event occurs (i.e., it wishes to listen for the event)
The listener provides a method that is invoked when the event is generated The method is called an event handler The event handler is executed when the event occurs Each event has a specific event-handling method that is call when the event is generated The method has a specific name/signature It is this method that the listener must provide We thus speak of 'the listener executing the event-handler when the event is generated'
In addition to providing the event-handling method, the listener (i.e., the applet) must explicitly declare that it wishes to be a listener This is achieved using the implements keyword in the class header



This Applet's Behavior
	
		The Class Header
			
				In addition to extending Applet, the AnAppletWithButtons
					 class declares that it intends to be an ActionListener, i.e., to listen 
					 to button clicks.
					
						It does this through the implements keyword
						
Making this declaration obligates the AnAppletWithButtons class 
							to provide the appropriate even-handling method
							
								The method for an ActionListener is addActionPerformed (see below)
							
					
			
		
The init Method
			
				Two buttons (named button1 and button2) are created in init
				
The buttons are placed on the applet (using the add method)
					
						In the absence of anything else, adding interface elemnts to the applets places them one
							after the other from left-to-right. 
						
When the right edge of the applet's real estate is reached, the placement continues below,
							again starting at the left edge. 
						
The placement of the elements thus flows from right-to-left, top-to-bottom.
					
				
The applet notifies each button that it wishes to be a listener to the button, i.e., that it wishes to be 
					notified when the button is clicked
					
						The applet sends a reference to itself (this) to the button's addActionListener method
						
The applet is said to register itself as a listener with the button.
					
			
		
The actionPerformed Method
			
				This is the event-handling method for button clicks
				
Having registered the applet as a listener with each button, this method
					is invoked whenever either of the two buttons is clicked.
				
An ActionEvent object is passed a a parameter to this method
					
						The ActionEvent parameter contains information pertaining to
							the specific event that occurred.
						
In particular, the method, getSource can be invoked to determine 
							which button generated the event (i.e., which button was clicked).
							
								getSource returns a reference to the interface component that
									generated the event
								
Knowing which component (button) generated the event allows the
									listener to determine what should be done
							
					
				
Using the getSource method to distinguish betwen the two buttons, 
					the actionPerformed method simply prints a message stating which
					button was clicked. 
			
	
					
Things to Do
	API work
		
			The Button class
				
					What package is it in?
				
			
The ActionListener class (actually interface)
				
					What package is it in?
				
			
The ActionEvent class (actually interface)
				
					What package is it in?
					
Name a couple of other event classes whose names seem to make sense to you
				
			

Playing With the Applet
Place the following three applets on the same page:
		
			Modify AppletWithButtons so that buttons' foreground colors are set using RGB values passed in as parameters to the applet.
								 Each applet should have its own color capability.
					
							Button inherits behavior from Component which had a method that allows you to specify the foreground color.
							
In the API, after the methods defined by the class you can find a list of the methods inherited from the
										 superclasses (i.e., parent classes) of the class. 
					 
			
Modify AppletWithButtons so that when the first button is pressed, draw a line on the surface of the applet; the second button should cause a oval/circle to 
							 be drawn
 		  (Optional) Same as #2; make the line a diagonal from upper left to lower right corners; the oval/circle should fit exactly within the applet