What does “Could not find or load main class” mean?

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 [ <option> ... ] <class-name> [<argument> ...]

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

^{1 - There is a second syntax for "executable" JAR files which I will describe at the bottom.}

For example:

    java -Xmx100m com.acme.example.ListUser fred

What this is going to do is the following:

1. Search for the compiled version of the com.acme.example.ListUser class.
2. Load the class.
3. Check that the class has a main method with signature static void main(String[]).
4. Call that method passing it the command line arguments as a String[].

Reasons why Java cannot find the class

When you get the message "Could not find or load main class ...", that means that the first step has failed. The java command was not able to find the class. And indeed, the "..." in the message will be the fully qualified class name that java is looking for.

So why might it be unable to find the class? Basically, there are two main causes:

The first likely cause is that you may have provided the wrong class name. (Or ... the right class name, but in the wrong form.) Considering the example above, here a variety of wrong ways to specify the class name:

• Example #1 - a simple class name:

  java ListUser

  When the class is declared in a package such as com.acme.example, then you must use the full classname including the package name in the java command; e.g.

  java com.acme.example.ListUser

• Example #2 - a filename or pathname rather than a class name:

  java ListUser.class
  java com/acme/example/ListUser.class

• Example #3 - a class name with the casing incorrect:

  java com.acme.example.listuser

• Example #4 - a typo

  java com.acme.example.mistuser

The second likely cause is that the class name is correct, but that the java command cannot find the class. To understand this, you need to understand the concept of the "classpath". This is explainedwell by the Oracle documentation:

• The java command documentation
• Setting the Classpath.
• The Java Tutorial - PATH and CLASSPATH

So ... if you have specified the class name correctly, the next thing to check is that you have specified the classpath correctly:

1. Read the three documents linked above. (Yes ... READ them.)
2. Look at command line and / or the CLASSPATH environment variable that is in effect when you run the java command. Check that the directory names and JAR file names are correct.
3. If there are relative pathnames in the classpath, check that they resolve correctly ... from the current directory that is in effect when you run the java command.
4. Check that the class (mentioned in the error message) can be located on the effective classpath.

Additional Notes:

1. When you put a directory on the classpath, it notionally corresponds to the root of the qualified name space. Classes are located in the directory structure beneath that root, by mapping the fully qualified name to a pathname. So for example, if "/usr/local/acme/classes" is on the class path, then when the JVM looks for a class called com.acme.example.Foon, it will look for a ".class" file with this pathname:

       /usr/local/acme/classes/com/acme/example/Foon.class

   If you had put "/usr/local/acme/classes/com/acme/example" on the classpath, then the JVM wouldn't be able to find the class.
2. The classpath needs to include all of the other (non-system) classes that your application depends on. (The system classes are located automatically, and you rarely need to concern yourself with this.)

The java -jar  syntax

The alternative syntax used for "executable" JAR files is as follows:

    java [ <option> ... ] -jar <jar-file-name> [<argument> ...]

e.g.

    java -Xmx100m -jar /usr/local/acme-example/listuser.jar fred

In this case the name of the entry-point class (i.e. com.acme.example.ListUser) and the classpath are specified in the MANIFEST of the JAR file.

IDEs

A typical Java IDE has support for running Java applications in the IDE JVM itself or in a child JVM. These are generally immune from this particular exception, because the IDE uses its own mechanisms to construct the runtime classpath, identify the main class and create the javacommand line.

However it is still possible for this exception to occur, if you do things behind the back of the IDE to break things. For example, if you have previously set up an Application Launcher for your Java app in Eclipse, and you then moved the JAR file containing the "main" class to a different place in the file system without telling Eclipse, Eclipse would unwittingly launch the JVM with an incorrect classpath.

In short, if you get this problem in an IDE, check for things like stale IDE state and broken project references or launcher configurations.