[JAVA] Eccezione strana "NoClassDefFoundError"

[Aki]

Ave populus
ho questo sorgente:

import java.sql.*;
import java.util.*;

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

public class contabilita extends JFrame {
		
	// OGGETTI SWING
	JLabel lb_data;
	
	public contabilita() {
		super("Jorganizer");
		this.setSize(640, 480);
		this.setLocationRelativeTo(null);
		this.setResizable(false);
		this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		this.getContentPane().setLayout(null);
		
		lb_data = new JLabel();
		lb_data.setBounds(10, 10, 150, 20);
		this.getContentPane().add(lb_data);
		
		this.setVisible(true);
	}
	
	public static void main(String[] args) {	
		JFrame.setDefaultLookAndFeelDecorated(true);
		contabilita c = new contabilita();
	}
}

che compila regolarmente, ma quando lo eseguo dà Exception in thread "main" java.lang.NoClassDefFoundError: contabilita
come mai???