code: protected String check() {
      String		error;
  
      if (!m_JavaHome.exists())
        return "Java home directory does not exist: " + m_JavaHome;
      if (!m_JavaHome.isDirectory())
        return "Java home does not point to a directory: " + m_JavaHome;
      if (System.getProperty("os.name").toLowerCase().contains("windows"))
        m_Jdeps = new File(m_JavaHome.getAbsolutePath() + File.separator + "bin" + File.separator + "jdeps.exe");
      else
        m_Jdeps = new File(m_JavaHome.getAbsolutePath() + File.separator + "bin" + File.separator + "jdeps");
      if (!m_Jdeps.exists())
        return "jdeps binary does not exist: " + m_Jdeps;
  
      if (!m_ClassesFile.exists())
        return "File with class names does not exist: " + m_ClassesFile;
      if (m_ClassesFile.isDirectory())
        return "File with class names points to directory: " + m_ClassesFile;
  
      // read classes
      error = readFile(m_ClassesFile, m_Classes);
      if (error != null)
        return error;
  
      // read resources
      if ((m_AdditionalFile != null) && m_AdditionalFile.exists() && (!m_AdditionalFile.isDirectory())) {
        error = readFile(m_AdditionalFile, m_Resources);
        if (error != null)
  	return error;
      }
  
      return null;
    }
docs: Performs some checks.
  
  @return		null if successful, otherwise error message