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

java runtime-error program-entry-point nosuchmethoderror

2022-08-31 15:56:32

New Java programmers often encounter messages like the following when they attempt to run a Java program. (Different Java tools, IDEs and so on give a variety of diagnostics for this problem.)

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

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

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

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)

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

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

答案 1

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

java some.AppName arg1 arg2 ...

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

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:

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

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

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

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 method and that it satisfies all of the six requirements listed above.main

^{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 be the standard java.lang.String class and not to a custom class named String that is hiding the standard class.}

答案 2

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

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.