Why does Java allow us to compile a class with a name different than the file name?

Question

I have a file Test.java and the following code inside it.

public class Abcd
{
        //some code here

}

Now the class does n

Answer 1

Note that Java is case-sensitive, but the filesystem need not be. If the file's base name is "abcd", but the class is "Abcd", would that conform to the rule on a case-insensitive filesystem? Certainly not when ported to a case-sensitive one.

Or suppose you happened to have a class called ABCD, and a class Abcd (let's not get into that being a bad idea: it could happen) and the program is ported to a case insensitive filesystem. Now you not only have to rename files, but also classes, oops!

Or what if there is no file? Suppose you have a Java compiler which can take input on standard input. So then the class has to be named "StandardInput"?

If you rationally explore the implications of requiring file names to follow class names, you will find that it's a bad idea in more than one way.

Answer 2

Also one other point that many answers missed to point out is that without the public declaration, the JVM would never know which classes' main method needs to be invoked. All classes declared in one .java file can all have main methods, but the main method is run on only the class marked as public. HTH