Use javac only sitting in the root of the source tree? What does it mean?

Question

I have a professor who always insists on

"sitting in the root of the source tree when invoking javac (and in the root of the directory when invoking java )".

Although, for the basic programs I have done, I have never found the need of doing that.

So, what is the need of doing it?

Answer 1

It means that if you have a package, foo , and a class or classes in that package:

package foo;
class Nifty {
    // ...
}

...and your source tree looks like (and it mostly has to):

+ source
  + foo
    + Nifty.java

javac will expect to be in the source directory when you're compiling, not in the foo directory. It's just how javac is.

java will expect that the classpath will include source (not foo ), and like javac work relative to that (looking in foo for the classes in the package foo ).