What Does "Could Not Find or Load Main Class" Mean?

The java <class-name> command syntax

First of all, you need to understand the correct way to launch a program using the java (or javaw) command.

The normal syntax¹ is this:

    java [ <options> ] <class-name> [<arg> ...]

where <option> is a command line option (starting with a "-" character), <class-name> is a fully qualified Java class name, and <arg> is an arbitrary command line argument that gets passed to your application.

^{1 - There are some other syntaxes which are described near the end of this answer.}

The fully qualified name (FQN) for the class is conventionally written as you would in Java source code; e.g.

    packagename.packagename2.packagename3.ClassName

However some versions of the java command allow you to use slashes instead of periods; e.g.

    packagename/packagename2/packagename3/ClassName

which (confusingly) looks like a file pathname, but isn't one. Note that the term fully qualified name is standard Java terminology ... not something I just made up to confuse you :-)

Here is an example of what a java command should look like:

    java -Xmx100m com.acme.example.ListUsers fred joe bert

The above is going to cause the java command to do the following:

1. Search for the compiled version of the com.acme.example.ListUsers class.
2. Load the class.
3. Check that the class has a main method with signature, return type and modifiers given by public static void main(String[]). (Note, the method argument's name is NOT part of the signature.)
4. Call that method passing it the command line arguments ("fred", "joe", "bert") as a String[].