Simple program wont run java.lang.NoClassDefFoundError

Question

Exception in thread "main" java.lang.NoClassDefFoundError: Server (wrong name: Simple_Server/Server)
i am new to java and i made this simple script to accept connections then pass them on to HandleClient.
but when i run it i get this output
Exception in thread "main" java.lang.NoClassDefFoundError: Server (wrong name: Simple_Server/Server)
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(Unknown Source)
        at java.security.SecureClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.access$000(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClassInternal(Unknown Source)here is the source code
package SMTP_Server;
import java.io.*;
import java.net.*;
public class Server {
    ServerSocket serverSocket = null;
    boolean listening = true;
    public Server(){
     System.out.println("New Server Created");
    public void main(String[] args) {
        try {
            this.serverSocket = new ServerSocket(4444);
        } catch (IOException e) {
            System.err.println("Could not listen on port: 4444.");
            System.exit(1);
     while(listening){
         try{
          new HandleClient(this.serverSocket.accept()).start();
         } catch (IOException e) {
          System.err.println("Caught IOException: " + e.getMessage());
}I thought it might be that main() is not static but it wont work if make it static because it cannot access the listening or serverSocket fields
Thanks for all the help
Scott.

Answer 1

The directory hierarchy of your .class files must mirror the package hierarchy of your classes.
So if you've got a class "Foo" in a package "p1" (i.e. a class with the fully qualified name of "p1.Foo") and a clas "Bar" in a package "p2" (i.e. a class with the fully qualified name of "p2.Bar"), then you need a file hierarchy such as this:
p1/Foo.class
p2/Bar.classAnd the directory in your classpath needs to be the directory that contains p1 and p2.
So what you need to do is go to a directory that contains a directory called "Simple_Server" (your source file mentions SMTP_Server, but I assume you changed it at some point).
That directory Simple_Server must contain a file called Server.class.
Then execute "java Simple_Server.Server".
Also please follow common naming conventions, as your code will become harder to understand if you don't. An important rule in those conventions is that package names should be in all-lowercase and not contain any non-letter characters if possible (i.e. use "smtpserver" instead of "SMTP_Server").