The Java error “Could not find or load main class” is thrown when the JVM fails to find or load the main class while executing a program. This is often due to simple mistakes like typing the wrong class name or having the class file in the wrong place. It usually occurs when executing a Java program from the command line.

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What Causes "Error: Could not find or load main class"

It typically arises from issues like an incorrect class name, a mismatch in the directory and package structure, or a misconfigured classpath. Here's a full list of things to check:

• The class being declared in the incorrect package.
• The file path of the class not matching the fully qualified name.
• Incorrectly specified classpath of the application.
• Missing dependencies from the classpath.
• Incorrect directory path on the classpath.
• A typo in the class name.

"Error: Could not find or load main class" Example

Here’s an example of the Java "Could not find or load main class" error thrown when an incorrect class name is specified during execution:

Say you have an example Java class MyClass.java:

public class MyClass {
    public static void main[String[] args] {
        System.out.println["Hello World"];
    }
}

You then compile the above class using the command line:

$ javac MyClass.java

The compiler generates an executable .class file for MyClass:

$ ls
MyClass.class   MyClass.java

Now if the java command is used to execute the .class file with an incorrect name, the "Could not find or load main class" error is thrown:

$ java Myclass
Error: Could not find or load main class Myclass

The generated .class file has the exact same name as the Java class, which in this case is MyClass.class. Specifying the correct name will execute the program successfully:

$ java MyClass
Hello World

How to Fix "Error: Could not find or load main class"

Most of the time, the error occurs because of specifying an incorrect class name, class file extension, file path or classpath.

Follow these tips to resolve it:

• **Use the correct class name** - The spelling and casing of the class name should be checked when executing the program.
• **Use the class name without the .class extension** - The java command expects the class name for executing the program, without the .class extension. Therefore, the following syntax should be used to execute Java classes: java
• **Use the correct file path** - The path to the .class file should be checked and corrected if the error occurs. Remember to use the fully qualified name of the class that is in a package if executing it from outside the directory structure of the package.
• **Correct the classpath definition** - The classpath should be checked and defined correctly if the error comes up. It can also be specified using the java -cp or

  $ javac MyClass.java

  0 command line arguments.

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First of all, you need to understand the correct way to launch a program using the

    packagename.packagename2.packagename3.ClassName

9 [or

    packagename/packagename2/packagename3/ClassName

1. command.

The normal syntax1 is this:

    java [  ]  [ ...]

where

    packagename/packagename2/packagename3/ClassName

1 is a command line option [starting with a "-" character],

    packagename/packagename2/packagename3/ClassName

2 is a fully qualified Java class name, and

    packagename/packagename2/packagename3/ClassName

3 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

    packagename.packagename2.packagename3.ClassName

9 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

    packagename.packagename2.packagename3.ClassName

9 command should look like:

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

The above is going to cause the

    packagename.packagename2.packagename3.ClassName

9 command to do the following:

1. Search for the compiled version of the

   packagename/packagename2/packagename3/ClassName  
   

   7 class.
2. Load the class.
3. Check that the class has a

   packagename/packagename2/packagename3/ClassName  
   

   8 method with signature, return type and modifiers given by

   packagename/packagename2/packagename3/ClassName  
   

   9. [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

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

   0.

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

    packagename.packagename2.packagename3.ClassName

9 command was not able to find the class. And indeed, the "..." in the message will be the fully qualified class name that

    packagename.packagename2.packagename3.ClassName

9 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

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

  3, then you must use the full classname including the package name in the

  packagename.packagename2.packagename3.ClassName  
  

  9 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

  packagename.packagename2.packagename3.ClassName  
  

  0

Reason

2 - the application's classpath is incorrectly specified

The second likely cause is that the class name is correct, but that the

    packagename.packagename2.packagename3.ClassName

9 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

  packagename.packagename2.packagename3.ClassName  
  

  9 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! It is important that a Java programmer understands at least the basics of how the Java classpath mechanisms works.]
2. Look at command line and / or the CLASSPATH environment variable that is in effect when you run the

   packagename.packagename2.packagename3.ClassName  
   

   9 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

   packagename.packagename2.packagename3.ClassName  
   

   9 command.
4. Check that the class [mentioned in the error message] can be located on the effective classpath.
5. Note that the classpath syntax is different for Windows versus Linux and Mac OS. [The classpath separator is

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

   9 on Windows and

   java ListUser

   0 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 class path, then when the JVM looks for a class called

java ListUser

1, it will look for a ".class" file with this pathname:

    packagename.packagename2.packagename3.ClassName

1

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.

Reason

2b - the subdirectory path doesn't match the FQN

If your classes FQN is

java ListUser

1, then the JVM is going to look for "Foon.class" in the directory "com/acme/example":

• If your directory structure doesn't match the package naming as per the pattern above, the JVM won't find your class.
• If you attempt rename a class by moving it, that will fail as well ... but the exception stacktrace will be different. It is liable to say something like this:

  packagename.packagename2.packagename3.ClassName  
  

  2 because the FQN in the class file doesn't match what the class loader is expecting to find.

To give a concrete example, supposing that:

• you want to run

  java ListUser

  1 class,
• the full file path is

  java ListUser

  4,
• your current working directory is

  java ListUser

  5,

then:

    packagename.packagename2.packagename3.ClassName

3

Notes:

• The

  java ListUser

  6 option can be shortened to

  java ListUser

  7 in most Java releases. Check the respective manual entries for

  packagename.packagename2.packagename3.ClassName  
  

  9,

  java ListUser

  9 and so on.
• Think carefully when choosing between absolute and relative pathnames in classpaths. Remember that a relative pathname may "break" if the current directory changes.

Reason

2c - dependencies missing from the classpath

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.] For the main class to load correctly, the JVM needs to find:

• the class itself.
• all classes and interfaces in the superclass hierarchy [e.g. see Java class is present in classpath but startup fails with Error: Could not find or load main class]
• all classes and interfaces that are referred to by means of variable or variable declarations, or method call or field access expressions.

[Note: the JLS and JVM specifications allow some scope for a JVM to load classes "lazily", and this can affect when a classloader exception is thrown.]

Reason

3 - the class has been declared in the wrong package

It occasionally happens that someone puts a source code file into the the wrong folder in their source code tree, or they leave out the

java com.acme.example.ListUser

0 declaration. If you do this in an IDE, the IDE's compiler will tell you about this immediately. Similarly if you use a decent Java build tool, the tool will run

java ListUser

9 in a way that will detect the problem. However, if you build your Java code by hand, you can do it in such a way that the compiler doesn't notice the problem, and the resulting ".class" file is not in the place that you expect it to be.

Still can't find the problem?

There lots of things to check, and it is easy to miss something. Try adding the

java com.acme.example.ListUser

2 option to the

    packagename.packagename2.packagename3.ClassName

9 command line [as the first thing after

    packagename.packagename2.packagename3.ClassName

9]. It will output various things about class loading, and this may offer you clues as to what the real problem is.

Also, consider possible problems caused by copying and pasting invisible or non-ASCII characters from websites, documents and so on. And consider "homoglyphs", where two letters or symbols look the same ... but aren't.

You may run into this problem if you have invalid or incorrect signatures in

java com.acme.example.ListUser

5. You can try opening up the .jar in your favorite ZIP editor, and removing files from

java com.acme.example.ListUser

6 until all you have is your

java com.acme.example.ListUser

7. However this is NOT RECOMMENDED in general. [The invalid signature may be the result of someone having injected malware into the original signed JAR file. If you erase the invalid signature, you are in infecting your application with the malware!] The recommended approach is to get hold of JAR files with valid signatures, or rebuild them from the [authentic] original source code.

Finally, you can apparently run into this problem if there is a syntax error in the

java com.acme.example.ListUser

7 file [see //stackoverflow.com/a/67145190/139985].

Alternative syntaxes for

    packagename.packagename2.packagename3.ClassName

9

There are three alternative syntaxes for the launching Java programs using the

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

0.

1. The syntax used for launching an "executable" JAR file is as follows:

   packagename.packagename2.packagename3.ClassName  
   

   4 e.g.

   packagename.packagename2.packagename3.ClassName  
   

   5 The name of the entry-point class [i.e.

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

2. and the classpath are specified in the MANIFEST of the JAR file. Anything you specify as a classpath on the command line is ignored with this syntax: only the

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

   2 entry in the Manifest is used [and, transitively, those in any JAR files referenced by this entry]. Note also that URLs in this

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

   2 are relative to the location of the JAR it is contained in.
3. The syntax for launching an application from a module [Java 9 and later] is as follows:

   packagename.packagename2.packagename3.ClassName  
   

   6 The name of the entrypoint class is either defined by the

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

   4 itself, or is given by the optional

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

   5.
4. From Java 11 onwards, you can use the

   packagename.packagename2.packagename3.ClassName  
   

   9 command to compile and run a single source code file using the following syntax:

   packagename.packagename2.packagename3.ClassName  
   

   7 where

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

   7 is [typically] a file with the suffix ".java".

For more details, please refer to the official documentation for the

    packagename.packagename2.packagename3.ClassName

9 command for the Java release that you are using.

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

    packagename.packagename2.packagename3.ClassName

9 command line.

However it is still possible for this exception to occur, if you do things behind the back of the IDE. 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, broken project references or broken launcher configurations.

It is also possible for an IDE to simply get confused. IDE's are hugely complicated pieces of software comprising many interacting parts. Many of these parts adopt various caching strategies in order to make the IDE as a whole responsive. These can sometimes go wrong, and one possible symptom is problems when launching applications. If you suspect this could be happening, it is worth trying other things like restarting your IDE, rebuilding the project and so on.