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

Question

New Java programmers often encounter these messages when they attempt to run a Java program.

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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

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Error: Main method is not static in class MyClass, please define the main method as:
   public static void main(String[] args)

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Error: Main method must return a value of type void in class MyClass, please
define the main method as:
   public static void main(String[] args)

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

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What does this mean, what can cause it, and what should one do to fix it?

Answer 1

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.

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^{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.}