Question

I have an application that has several classes used for storing application-wide settings (locations of resources, user settings, and such). Right now these classes are just full of static fields and methods, but I never instantiate them.

Someone suggested I make them Singletons, What’s the case for/against?

Answer 1

Effective Java says:

Singletons typically represent some system component that is intrinsically 
unique, such as a video display or file system.

So if your component warrants single instance accross the entire application and it has some state, it makes sense to make it a singleton

In your case, the settings of the application is a good candidate for singleton.

On the other hand, a class can only have static methods if you want to group certain functions together, such as utility classes, examples in jdk are java.util.Arrays java.util.Collections. These have several related methods that act on arrays or collections