Java – NullPointerException when trying to run Jar file

I've just started learning Java and only know a small amount of code, but it's still a simple program This is a prank program, but most of them just test whether I can make a jar file

This is the code:

import java.awt.*;  
import java.awt.event.*;  
import java.lang.*;  
import java.util.Random;  
public class randommouse {  
    public static void main(String[] args) {  
        for (int i=1; i<1000; i++) {  
            Random rand = new Random();  
            int w = rand.nextInt(1024) + 1;  
            int h = rand.nextInt(768) + 1;  
            int t = rand.nextInt(2000) + 1;  
            try {  
                Robot r = new Robot();  
                r.mouseMove(w,h);  
                Thread.sleep(t);  
            } catch (AWTException e) {}  
            catch (InterruptedException e) {}  
            catch (NullPointerException e) {}  
        }  
    }  
}

I saved it to a file named randommouse Java file, and then use it to compile

javac randommouse.java

This is effective when I run it

java randommouse

It's also very good

So I tried to create a jar file I use commands

jar cvf randommouse.jar randommouse.class

After it works, I double - click the jar file and an error Java exception appears So I run it with CMD

java -jar randommouse.jar

And get this error

F:\Java>java -jar randommouse.jar
Exception in thread "main" java.lang.NullPointerException
        at sun.launcher.LauncherHelper.getMainClassFromJar(LauncherHelper.java:3
99)
        at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:463)

F:\Java>

I need to put forward an argument. If so, what should I do and what should I do?

Thank you first, Sam

Solution

From the JDK doc:

Main-Class: classname