Format Of Java File Names

In Java® identifiers, you are allowed letters and numbers (also _ $£¢€ etc, but you should avoid them in normal identifiers). So you cannot have spaces. You cannot write public class Hello World because the javac tool will see World as a separate identifier and not understand what it means and will fail to compile the code. You must write public class HelloWorld instead. And because the class is labelled public you must call the source file Hello World. java. Since you can't have two classes with the same [fully‑qualified] name, you cannot write two public classes in the same source file.

“What about names of source files?” somebody will ask. Well, some file systems will permit spaces in file names; ext4 will and I suspect so will NTFS. Can you write file names with spaces in? You would have to have a different name of the class inside the file, because you can't have spaces, and you therefore cannot make the class public, but maybe you can write a package‑private class with a different name.

On ext4, you have to write out the name of the file and the shell will interpret the space as meaning there are two different file, so you have to escape the space.

campbell@campbellsComputer:~/java$ gedit My First Class.java
// My First Class.java
class Foo
{
public static void main(String... args)
{
System.out.println("Hello, World!");
}
}
campbell@campbellsComputer:~/java$ javac My First Class.java
campbell@campbellsComputer:~/java$ java Foo