Solution

HTML Interface to Applet

<APPLET CODEBASE="/applets/magelang/AWT-Training/classes/" CODE="FormElement.class" WIDTH=300 HEIGHT=150 ALIGN=CENTER></APPLET>

Java Code

import java.awt.Panel;
import java.awt.Event;

/**
 * Abstract class to describe the functionality needed for
 * every field of a form.  A form contains spaces for user
 * input, therefore we want to be able to query whether there
 * is any contents.  We also we want the field to be able
 * to print itself.
 */
public abstract class FormElement extends Panel {
    /**
     * Returns a String containing text description of this
     * field and an appropriate label.
     * @return String text in this field
     */
    public abstract String getContents();

    /**
     * Query whether the field has valid contents
     * @return boolean true if empty else false
     */
    public boolean isEmpty() {
    	return false;
    }
    
    /** Verify the contents of the element.  Empty elements
     *  are always ok (until they hit submit anyway). 
     */
    public void verify() {}
    
    public boolean handleEvent(Event event) {
        switch (event.id) {
            // if they click mouse outside of element or activate it
            // we verify the element.
            case Event.LOST_FOCUS:
            case Event.ACTION_EVENT:
                if ( !isEmpty() ) verify();
                return true;
            default:
                return super.handleEvent(event);
        }
    }

}