Question

I have inherited code which contains static nested classes as:

public class Foo {

// Foo fields and functions
// ...
    private static class SGroup {
        private static Map<Integer, SGroup> idMap = new HashMap<Integer, SGroup>();

        public SGroup(int id, String type) {
// ...
        }
    }
}

From reading SO (e.g. Java inner class and static nested class) I believe that this is equivalent to two separate classes in two separate files:

 public class Foo {

    // Foo fields and functions
    // ...
}

and

public class SGroup {
    static Map<Integer, SGroup> idMap = new HashMap<Integer, SGroup>();

    public SGroup(int id, String type) {
// ...
    }
}

If this is correct is there any advantage to maintaining the static nested class structure or should I refactor?

Answer 1

Jorn statement is correct and it’s usually manifests itself as the following rule of thumb:

Nested classes should be made private, Meaning that the hold auxiliary logic for the hosting class and nothing more. If you cant make them private- thet probably should not be nested.

The exception is when you define a nested class to allow easy access to the state of the hosting class, in that case you should consider simply merging both classes to increase cohesion.