Problems with Constructor Methods

From: JS (dsa._at_asdf.com)
Date: 11/27/04 

Date: Sat, 27 Nov 2004 01:50:49 +0100

This code works:

import javax.swing.*;
public class HelperClass
{
  public void computeSquareRoot()
  { String s = JOptionPane.showInputDialog("Type a number:");
    double d = new Double(s).doubleValue();
    double root = Math.sqrt(d);
    JOptionPane.showMessageDialog(null,
                     "The square root of " + d + " is " + root);
  }
}

class test
{public static void main(String[] args)
  {HelperClass writer = new HelperClass();
  writer.computeSquareRoot();

}}

But I am trying to make another which calculates the length of two names and
show the names and their length in a dialog windows. But I can't get it to
work:

class NameLength
{ public static void main(String[] args)
  { HelperClass c = new HelperClass();
    c.readNameAndDisplayItsLength();
    c.readNameAndDisplayItsLength();
    JOptionPane.showMessageDialog(null, "Finished!");
}}

class HelperClass
{
  public void readNameAndDisplayItsLength()
  { String a = JOptionPane.showInputDialog("Type a name:");
    int z = a.length();
    JOptionPane.showMessageDialog(null, a + " Length is " + z);
  }
}

I guess class test and class NameLength has the same function. But why then
has NameLength the line JOptionPane.showMessageDialog(null,
"Finished!");

And why does it have two c.readNameAndDisplayItsLength();

When i write: java NameLength I get the following error:

NoSuchMethodError: HelperClass.readNameAndDisplayItsLength()V
  at NameLength.main(Johannes_Serup_48.java:149)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
  at java.lang.reflect.Method.invoke(Unknown Source)

JS