NoClassDefFoundError

C:\Prog>javac LeapYear.java
C:\Prog>java LeapYear
Exception in thread "main" java.lang.NoClassDefFoundError: LeapYear

Inhold af Leapyear.java:
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import javax.swing.JOptionPane;

public class LeapYear {
  public static void main(String arguments[]) {
    String userInput = JOptionPane.showInputDialog(null, "Type a number");
    int year = Integer.parseInt(userInput);

    if(year%100 == 0) {
      if(year%400 == 0)
        JOptionPane.showMessageDialog(null, "Year " + year + " is a leap year");
      else
        JOptionPane.showMessageDialog(null, "Year " + year + " is not a leap year");
    }

    else if(year%4 == 0) {
      JOptionPane.showMessageDialog(null, "Year " + year + " is a leap year");
    }

    else
      JOptionPane.showMessageDialog(null, "Year " + year + " is not a leap year");

    System.exit(0);
  }
}
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Hvad betyder java.lang.NoClassDefFoundError ?