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What does "Could not find or load main class" mean?

Submitted by: @import:stackoverflow-api·Mar 10, 2026·

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Problem

A common problem that new Java developers experience is that their programs fail to run with the error message: Could not find or load main class ...

What does this mean, what causes it, and how should you fix it?

Solution

The java 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 [  ]  [ ...]

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 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:

Search for the compiled version of the com.acme.example.ListUsers class.




Load the class.

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.)

Call that method passing it the command line arguments ("fred", "joe", "bert") 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?
Reason #1 - you made a mistake with the classname argument

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 are 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


- 
Example #5 - a source filename (except for Java 11 or later; see below)

java ListUser.java


- 
Example #6 - you forgot the class name entirely

java lots of arguments


- 
Example #7 - you forgot the

-jar option when attempting to run an executable JAR file, and the java

 command has misconstrued the JAR file name as a class name

java myProgram.jar


Reason #2 - the application's classpath is incorrectly specified

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 explained well 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:

Read the three documents linked above.  (Yes ... READ them!  It is important that a Java programmer understands at least the basics of how the Java classpath mechanisms works.)

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.

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.




Check that the class (mentioned in the error message) can be located on the effective classpath.

Note that the classpath syntax is different for Windows versus Linux and Mac OS. (The classpath separator is ; on Windows and :` on the others. If you use the wrong separator for your platform, you won't get an explicit error message. Instead, you will get a nonexistent file or directory on the path that will be silently ignored.)

Reason #2a - the wrong directory is on the classpath

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 clas

Code Snippets

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

packagename.packagename2.packagename3.ClassName

packagename/packagename2/packagename3/ClassName

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

java ListUser

Context

Stack Overflow Q#18093928, score: 1666

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