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

public class AppletNumericTextFieldProcessKeyEvent extends Applet implements ActionListener
{
	private NumericTextField numberField;
	private Button clear;

	public void init()
	{
		// Set the size and background color of the applet
		setSize(300, 100);
		setBackground(Color.CYAN);

		// Set the layout manager for the applet
		setLayout(new FlowLayout());
		
		// Create an instance of the 'NumericTextField' class and add it to the applet
		numberField = new NumericTextField(10);
		add(numberField);

		// Create an instance of a 'Button', add it to the
		//  applet, and then "listen" for it to be pressed
		clear = new Button("Clear");
		add(clear);
		clear.addActionListener(this);
	}

	public void actionPerformed(ActionEvent ae)
	{
		Button source = (Button) ae.getSource();

		// If the button was pressed, clear the textfield
		//  and then return the focus to the textfield
		if (source.getLabel() == "Clear")
		{
			numberField.setText("");
			numberField.requestFocus();
		}
	}

	// Extend the 'TextField' class to create a new type of 'TextField' that
	//  accepts only numbers; provide the capability to use "key" events to
	//  filter characters entered into the 'NumericTextField' object
	public class NumericTextField extends TextField
	{
		public NumericTextField(int cols)
		{
			// Superclass constructor calls must be the FIRST statement in a constructor
			super(cols);

			// Enable "key" events on all objects of the class; this allows the
			//  'processKeyEvent' method to be invoked when a key is pressed
			enableEvents(AWTEvent.KEY_EVENT_MASK);
		}

		// Note that this method is actually called THREE TIMES for each time that
		//  a key is pressed (probably for KEY_PRESSED, KEY_RELEASED, KEY_TYPED);
		//  it is overridden to provide the code needed to filter out non-numeric
		//  characters without the need to instantiate a separate 'Listener' object
		public void processKeyEvent(KeyEvent e)
		{
			// Get and store the key that was pressed
			char keyChar = e.getKeyChar();

			// To avoid having the pressed key processed three times (see above),
			//  'KEY_TYPED' is used here because it represents key-down and key-up;
			//  since key-down occurs before all other key events, the key will
			//  either be processed (and then cleared) or changed into an illegal
			//  character (below) so that it will be rejected when processed again
			if (e.getID() == KeyEvent.KEY_TYPED)
			{
				// Accept digits (0 - 9), decimal points (.),
				//  negative signs (-), BACKSPACE, and DELETE
				if (((keyChar >= '0') && (keyChar <= '9')) || (keyChar == '.') ||
					 (keyChar == '-') ||(keyChar == 8) || (keyChar == 127))
				{
					// Since the 'processKeyEvent' method has been overridden, the same
					//  method should be invoked in the superclass by passing the event
					//  object (the character) as a parameter; this ensures that all
					//  necessary default processing occurs
					super.processKeyEvent(e);
				}
				else
				{
					// If the key is not one of the accepted characters, change it to an 
					//  illegal character (for the textfield) so that it will be rejected
					//  and the user will hear a system beep
					e.setKeyChar('\000');
				}
			}
		}
	}
}