Managing constants in large applications

Question

This is an issue that has come up many times in my projects. While I try to avoid the practice, I almost always have some sort of Constants.java class that contains a bunch of public static final constants. This includes things like application name, filenames for configuration files, version numbers, network ports, etc. Sometimes I'll split them up into multiple classes (e.g., AppConstants.java and NetworkConstants.java).
Is this an accepted practice in larger applications, or are global constants typically handled differently? I've always considered my solution to be inelegant, but I haven't come up with a better way of doing it. Suggestions?

Answer 1

> Yes, I do typically use an interface to hold my
constants. I know I said "class" above.
This is considered a bad practice, or "antipattern", to wit:
Effective Java - Joshua Bloch, Chapter 4, Item #17: Use interfaces only to define types.
The constant interface pattern is a poor use of interfaces. That a class uses some constants internally is an implementation detail. Implementing a constant interface causes this implementation detail to leak into the class's exported API. It is of no consequence to the users of a class that the class implements a constant interface. In fact, it may even confuse them. Worse, it represents a commitment: if in a future release the class is modified so that it no longer needs to use the constants, it still must implement the interface to ensure binary compatibility. If a nonfinal class implements a constant interface, all of its subclasses will have their namespaces polluted by the constants in the interface.
In summary, interfaces should only be used to define types. They should not be used to export constants.
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