Note: all code except "remove" part comes from textbook chapter 3 section 4.

• File BagApplet.java is an applet that provides a GUI-based interactive driver for the IntArrayBag class.
• It currently has facilities for testing the "add()" "size()" and "countOccurrences()" methods.
• We will study the components of this applet, then enhance it with a facility for testing "remove()"
• This is an example of event-driven interaction. The user, not the program, decides the sequence in which execution will occur.

Compile as usual. Applet is activated through "appletviewer" command or from browser. In either case, an HTML <applet></applet> tag pair is required, so you need a little HTML file containing at minimum the following content: <applet code="BagApplet.class" width=480 height=340></applet>

The width and height indicate size of applet window in pixels. Save this short bit of code in BagApplet.html, then point your browser or appletview to it. Run the BagApplet here.

• Import statements to gain access to Applet and AWT (Abstract Windowing Toolkit) classes. AWT defines GUI components and their behaviors.
• Declare the class to "extends Applet"
• Declare GUI components as instance variables
• Define an "init()" method to place GUI components on the UI and associate action listener objects with the buttons. Each specifies the code to be executed when corresponding button is clicked.
• Define action listener classes. Each will be a different "inner class". Components of an inner class have access to components of its enveloping class. This allows action listeners to access the various GUI components.

Three types are demonstrated here: Button, TextField, and TextArea.

• Button is rectangle that triggers action listener when clicked on. In this applet, buttons are used to trigger various IntArrayBag methods.
• TextField is rectangle used to display and enter one line of text. Horizontal scrolling occurs if text is longer than rectangle. In this applet, textfields are used to enter parameter values needed by IntArrayBag methods.
• TextArea is rectangle used to display and enter multiple lines of text. Scrollbars are provided. In this applet, it is used only to display information.

Each component is declared and created as an instance variable. Important: it is not yet displayed at this time.

When your applet is run, the init() method in your class is executed. In that sense, it is like main(), except it is not static. Main is not used for applets, but by defining both an init and a main, it is possible for a class to be run either as an applet or an application.

This method serves three major functions:

1. place the GUI components onto the UI window.

1. AWT defines various layout methods,
2. This example uses the default layout: left-to-right, top-to-bottom, no pixel positions specified.
3. Allows window to be specified in system-independent (abstract) way.
4. GUI component (created upon declaration) specified as parameter to "add()" method.
5. "add()" inherited from Applet class
6. Look of interface changes if order of "add"s are changed.
7. This example defines utility methods to add horizontal lines and force next component to be placed on next line.

2. associate action listeners with active GUI components (buttons).

1. Each button has "addActionListener()" method.
2. Parameter must be object of class that "implements ActionListener".
3. Such a class must define method "public void actionPerformed(ActionEvent event)"
4. This method specifies code executed when button is clicked.
5. Generally, one such class is defined for each button, as an "inner class"

3. When execution reaches the end of init, the UI window is displayed and user can interact with the applet.

One class is defined for each button, and one object of this class is instantiated and associated with the button in init().

This class is defined inside the BagApplet class. This is called "inner class" and is necessary to allow your code to access the GUI components.

Each class has a standard template:

Class action_listener_class_name_here implements ActionListener
{
	public void actionPerformed(ActionEvent event)
	{
		// your code here
	}
}

1. In declaration of BagApplet class components. elementText is a TextField into which user will type in the value to be added to the bag.

Button addButton = new Button("add( )");
TextField elementText = new TextField(10);

2. Adding the button and textfield to UI, in init:

add(addButton);
add(elementText);

3. Associating button with action listener, in init:

addButton.addActionListener(new AddListener( ));

4. Define AddListener inner class:

class AddListener implements ActionListener
{
	public void actionPerformed(ActionEvent event)
	{
		try
		{
			String userInput = elementText.getText( );
			int element = Integer.parseInt(userInput);
			b.add(element);
			feedback.append(element + " has been added to the bag.\n");
		}
		catch (NumberFormatException e)
		{
			feedback.append("Type an integer before clicking button.\n");
			elementText.requestFocus( );
			elementText.selectAll( );
		}
	} 
}

1. In declaration of class components, add two more instance variables:

Button removeButton = new Button("remove( )");
TextField removeElemText = new TextField(10);

2. Add those GUI components to UI layout, in init method.

add(removeButton);
add(removeElemText);

3. Associate a listener object (class specified below) with the button, in init method.

removeButton.addActionListener(new RemoveListener( ));

4. Develop listener inner class to be used with this button. In this case, you can copy the AddListener class and make a couple modifications.

class RemoveListener implements ActionListener
{
	public void actionPerformed(ActionEvent event)
	{
		try
		{
			String userInput = removeElemText.getText( );
			int element = Integer.parseInt(userInput);
			b.remove(element);
			feedback.append(element + " has been removed from the bag.\n");
		}
		catch (NumberFormatException e)
		{
			feedback.append("Type an integer before clicking button.\n");
			removeElemText.requestFocus( );
			removeElemText.selectAll( );
		}
	} 
}

Run the modified BagAppletRemove here.

[ Lectures | CSC 132 | Peter Sanderson | Computer Science | SMSU ]

Peter Sanderson ( PeteSanderson@smsu.edu )