Why make private inner class member public in Java?

Question

What is the reason of declaring a member of a private inner class public in Java if it still can\'t be accessed outside of containing class? Or can it?

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Answer 1

If the inner class is private it cannot be accessed by name outside of the outer class. Inner and outer classes have access to each other's private methods and private instance variables. As long as you are within the inner or outer class, the modifiers public and private have the same effect. In your code example:

public class DataStructure {
    // ...

    private class InnerEvenIterator {
        // ...

        public boolean hasNext() { // Why public?
            // ...
        }
    }
}

As far as the class DataStructure is concerned, this is completely equivalent to:

public class DataStructure {
    // ...

    private class InnerEvenIterator {
        // ...

        private boolean hasNext() {
            // ...
        }
    }
}

This is because only DataStructure can access it, so it doesn't matter if you set it to public or private. Either way, DataStructure is still the only class that can access it. Use whichever modifier you like, it makes no functional difference. The only time you can't choose at random is when you are implementing or extending, in which case you can't reduce the access, but you can increase it. So if an abstract method has protected access you can change it to public. Granted neither one actually makes any difference.

If you plan on using an inner class in other classes, and therefore making it public, you probably shouldn't make it an inner class in the first place.

Additionally, I don't see any requirement for inner classes extending or implementing other classes. It might be common for them to do so, but it's certainly not required.