"Error: Main method not found in class MyClass, please define the main method as..."

Question

When you use the java command to run a Java application from the command line, e.g.,

java some.AppName arg1 arg2 ...

the command loads the class that you nominated and then looks for the entry point method called main. More specifically, it is looking for a method that is declared as follows:

package some;
public class AppName {
    ...
    public static void main(final String[] args) {
        // body of main method follows
        ...
    }
}

The specific requirements for the entry point method are:

1. The method must be in the nominated class.
2. The name of the method must be "main" with exactly that capitalization¹.
3. The method must be public.
4. The method must be static ².
5. The method's return type must be void.
6. The method must have exactly one argument and argument's type must be String[] ³.

(The argument may be declared using varargs syntax; e.g. String... args. See this question for more information. The String[] argument is used to pass the arguments from the command line, and is required even if your application takes no command-line arguments.)

If anyone of the above requirements is not satisfied, the java command will fail with some variant of the message:

Error: Main method not found in class MyClass, please define the main method as:
   public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application

Or, if you are running an extremely old version of Java:

java.lang.NoSuchMethodError: main
Exception in thread "main"

If you encounter this error, check that you have a main method and that it satisfies all of the six requirements listed above.

---

^{1 - One really obscure variation of this is when one or more of the characters in "main" is NOT a LATIN-1 character … but a Unicode character that looks like the corresponding LATIN-1 character when displayed.
2 - Here is an explanation of why the method is required to be static.
3 - String must correspond to java.lang.String and not to a custom class named String hiding it.}

The problem is that you do not have a public void main(String[] args) method in the class you attempt to invoke.

It

• must be static
• must have exactly one String array argument (which may be named anything)
• must be spelled m-a-i-n in lowercase.

Note, that you HAVE actually specified an existing class (otherwise the error would have been different), but that class lacks the main method.

Other answers are doing a good job of summarizing the requirements of main. I want to gather references to where those requirements are documented.

The most authoritative source is the VM spec (second edition cited). As main is not a language feature, it is not considered in the Java Language Specification.

• 2.17.1 Execution - Virtual Machine Start-up
• 5.2 Virtual Machine Start-up

Another good resource is the documentation for the application launcher itself:

• java - the Java application launcher

If you are running the correct class and the main is properly defined, also check if you have a class called String defined in the same package. This definition of String class will be considered and since it doesn't confirm to main(java.lang.String[] args), you will get the same exception.

• It's not a compile time error since compiler just assumes you are defining a custom main method.

Suggestion is to never hide library java classes in your package.