Why make private inner class member public in Java

问题:

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?

public class DataStructure {
    // ...

    private class InnerEvenIterator {
        // ...

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

回答1:

If the InnerEvenIterator class does not extend any class or implement any interface, I think it is nonsense because no other class can access any instance of it.

However, if it extends or implements any other non private class or interface, it makes sense. An example:

interface EvenIterator {
    public boolean hasNext();
}


public class DataStructure {
    // ...

    private class InnerEvenIterator implements EvenIterator{
        // ...

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

    InnerEvenIterator iterator;

    public EvenIterator getIterator(){
         return iterator;
    }     

}

回答2:

This method can be made public in order to indicate that it's semantically public, despite the fact that compiler doesn't enforce visibility rules in this particular case.

Imagine that during some refactoring you need to make this inner class top-level. If this method is private, how would you decide whether it should be made public, or some more restrictive modifier should be used? Declaring method as public tells reader the intentions of original author - this method shouldn't be considered an implementation detail.

回答3:

It is useful when you implement any interface.

class DataStructure implements Iterable<DataStructure> {

    @Override
    public Iterator<DataStructure> iterator() {
        return new InnerEvenIterator();
    }
    // ...        

    private class InnerEvenIterator implements Iterator<DataStructure> {
        // ...    
        public boolean hasNext() { // Why public?
            // ...
            return false;
        }

        @Override
        public DataStructure next() {
            throw new UnsupportedOperationException("Not supported yet.");
        }

        @Override
        public void remove() {
            throw new UnsupportedOperationException("Not supported yet.");
        }
    }

    public static void main(String[] ex) {
        DataStructure ds = new DataStructure();
        Iterator<DataStructure> ids = ds.iterator();
        ids.hasNext(); // accessable            
    }
}

回答4:

I think you are missing the implementing the Iterator interface part in your sample code. In that case, you can't make the hasNext() method have any other visibility identifier other than public since that would end up reducing its visibility (interface methods have public visibility) and it won't compile.

回答5:

There are many combinations of access modifiers which are not useful. A public method in a private inner class is only useful if it implements a public method in a public class/interface.

public class DataStructure {
    // ...

    private class InnerEvenIterator implements Iterator {
        // ...

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

    public Iterator iterator() {
        return new InnerEvenIterator();
    }
}

BTW: abstract classes often have public constructors when actually they are protected

回答6:

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.