$ cat HelloWorld.java
public class HelloWorld {
	public static void main(String[] args) {
		System.out.println("Hello World!");
	}
}

#compile
$ javac HelloWorld.java


#File is created
$ ll HelloWorld.class
-rw-r--r--  1 root  staff   426B  2  1 15:18 HelloWorld.class

#No extension is added at runtime! !! !!
$ java HelloWorld
Hello World!

#When I add an extension, I get this error
$ java HelloWorld.class
error:Main class Hello World.Could not find and load class
Cause: java.lang.ClassNotFoundException: HelloWorld.class

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