How can I run an Eclipse compiled Java class from the command line?

Question

I am writing a basic TCP chat program and one of the requirements is that it can be run from the command line with the following argument formats:

java Server 8888
java Client localhost 8888

Which will start a server listening on 8888 and which waits to accept incoming connections from clients. Then start a client and connect to the server at localhost:8888. These classes can both be compiled and run from within Eclipse and I have added the above variables to the run configurations for the classes, respectively.

If I navigate to the directory of the files in CMD I can see the compiled .class files, but when I try to run the server with:

java Server 8888

I get the error

Error: Could not find or load main class Server

Eclipse>Window>Preferences>Java>Compiler shows JDK 1.7.

Running java -version from the command line shows

java version "1.7.0_02"
Java(TM) SE Runtime Environment (build 1.7.0_02-b13)
Java HotSpot(TM) 64-Bit Server VM (build 22.0-b10, mixed mode)

I want to be able to run both the classes from separate prompts in parallel. Any ideas?

• Jeff: You might like to check out the answer to this relevant question
• Mattias Isegran Bergander: Do you have your classes in a package name?
• Mattias Isegran Bergander: Then you need to specify basicChat.Server not just Server
• Xeon: Do your classes have public modifier? And if they do - can you show us some minimal code that gives the same error?

Answer 1

java -cp . basicChat.Server 8888

You need to specify the fully qualified class name (including the package name)

Reason:

The class full name (known as fully qualified name) is not Server, it is basicChat.Server. The file is located under a directory named basicChat. So java is looking for a a directory structure matching the package name. The Server.class file is located under the basicChat directory in the file system.

Else consider how we would have a problem to select the intended class if you had several classes called Server in different packages (name spaces).

• Mattias Isegran Bergander: Because your class's full name is not Server, it is basicChat.Server. The file is located under a directory named basicChat
• Dan Prince: But my current directory was basicChat when I ran java Server 8888?
• Mattias Isegran Bergander: That doesn't work, need to be a the root of your packages. Else in your case it would be looking for a class named Server, but it found one named basicChat.Server. Again classes are identified by the full name and you specified a class with an empty package.

Answer 2

Are you certain that in both these classes you have main method?

public static void main(String[] args) { ... }

And do your classes have public modifier?

if you have this class in a package e.g.: test.Server you'll run it from bin folder like that:

java test.Server 8888

Compiling this class:

public class Server {
 public static void main(String[] args) {
 System.out.println(Arrays.deepToString(args));
 }
}

and running:

java Server 8888

gives output:

[8888]

• Dan Prince: Yes, I'm positive they do. As I said, they both compile and run in Eclipse. I tried running it from bin using the complete package name as well, same main class not found error.

Answer 3

Your CLASSPATH is incorrect. Try the following (on Windows) from bin directory:

java -cp .;%CLASSPATH% basicChat.Server 8888

Also, you can run your Server in Eclipse. Create a run configuration for it, specify a port in arguments. Then you can launch it in Eclipse, it will be running util you stop it or close Eclipse.

• Dan Prince: Error: Could not find or load main class Files\Java\jre7\bin\java.exe
• tcb: what if you just try java -cp Server 8888
• Dan Prince: Error: Could not find or load main class 8888