why .java filename constraint ?

[prashant p joshi]

why .java filename must be same as public class name defined in that file?

[Sayed Ibrahim Hashimi]

I think its because that is how the compiler finds the class files. By package and class name.

[Frank Carver]

It's just a convenience for the compiler.
It's one of the tricks which enable recompiling a whole Java project to be acceptably quick, without needing something like a "makefile" to tell it what to compile.