Why would you ever want a Java file with no public classes declared in it?

Question

There is a statement in the book I'm reading for the SCJP qualification, it says :

Files with no public classes have no naming restrictions

That has made me ask, why would you ever want to do this?

If there are no public classes, then how could other classes ever import and use the file? The only purpose I can see is if the file runs standalone in itself, which could also be odd, such as have an entire application in one file

Answer 1

This is valid for package-private classes as well. And you can use package-private classes within the same package. (And in that case you don't have to import it, because it's in the same package.)

For example, the JapaneseImperialCalendar class is package-private, because it is only used from Calendar.createCalendar(..) - it is not part of the public API. You can't directly instantiate the japanese calendar, but you can still use it by its interface. Same goes for all unmodifiable collections that are obtained by methods like Collections.unmodifiableList(..) - they are package-private.

So the .java file of JapaneseImperialCalendar could've been arbitrary. However, it is advisable not to diverge from the established practice of naming even package-private files after the class name.

Answer 2

You can create a file named package-info.java, which contains only a package statement. The javadoc 1.5+ tool treats a javadoc comment on this package statement exactly like a package.html file. In addition, you can add package-level annotations such as @Generated to this statement, which you can't do in package.html.

Because package-info is not a valid Java identifier, there is no risk of this file ever clashing with an existing Java class (i.e. backwards compatibility).