How to resolve this "could not find or load main class"

class HelpClassDemo
{
    void helpOn(int what)
    {
        switch(what)
        {
            case '1': System.out.println("The if:\n");
                      System.out.println("if(condition) statement;");
                      System.out.println("else statement;");
                      break;
            case '2': System.out.println("The switch:\n");
                      System.out.println("switch(expression)");
                      System.out.println("{");
                      System.out.println(" case constant");
                      System.out.println("   statement sequence");
                      System.out.println(" break;");
                      System.out.println("//...");
                      System.out.println("}");
                      break;
            case '3': System.out.println("The for:\n");
                      System.out.println("for(integer;condition;iteration)");
                      System.out.println("  statement;");
                      break;
            case '4': System.out.println("The while:\n");
                      System.out.println("while(condition)");
                      System.out.println("  statement;");
                      break;
            case '5': System.out.println("The do-while:\n");
                      System.out.println("do");
                      System.out.println("{");
                      System.out.println("  statement;");
                      System.out.println("}");
                      System.out.println("while(condition)");
                      break;
            case '6': System.out.println("The break:\n");
                      System.out.println("break; or break label;");
                      break;
            case '7': System.out.println("The continue:\n");
                      System.out.println("continue; or continue label;");
                      break;
        }
        System.out.println();
    }   
    void showMenu()
    {
        System.out.println("Help on:");
        System.out.println("  1. if");
        System.out.println("  2. switch");
        System.out.println("  3. for");
        System.out.println("  4. while");
        System.out.println("  5. do-while");
        System.out.println("  6. break");
        System.out.println("  7. continue\n");
        System.out.print("Choose one (q to quit):");
    }
    boolean isValid(int ch)
    {
        if((ch<'1')||(ch>'7')&& (ch!='q'))
            return false;
        else
            return true;
    }
    public static void main(String args[])throws java.io.IOException
    {
        char choice,ignore;
        HelpClassDemo hlpobj = new HelpClassDemo();
        
        for(;;)
        {
            do
            {
                hlpobj.showMenu();
                choice = (char) System.in.read();
                do
                {
                    ignore=(char) System.in.read();
                }
                while(ignore != '\n');
            }
            while(!hlpobj.isValid(choice));
            if(choice=='q')
              break;
            System.out.println("\n");
            hlpobj.helpOn(choice);
        }
    }
}