访问外部类引用时Java内部类出错

Question

I am implementing Trie data structure. When I use the outer class reference, I am getting the below error in Eclipse IDE.

The method compare(capture#1-of ? super E, capture#1-of ? super E) in the type Comparator is not applicable for the arguments (E, E)

public class Trie<E> implements Collection<E> {

    private Comparator<? super E> comparator;

    private class Entry<E> {        
        private Entry<E> parent;
        private Set<Entry<E>> children;

        private E data;

        Entry(Entry<E> parent, E data){
            this.parent = parent;
            this.data = data;

            parent.addChild(this);
        }

        boolean addChild(Entry<E> child){
            if (children == null)
                children = new TreeSet<Entry<E>>(
                        new Comparator<Entry<E>>() {                            
                            public int compare(Entry<E> o1, Entry<E> o2) {
                                return Trie.this.comparator.compare(o1.data, o2.data);
                            };
                        }
                        );

            return children.add(child);
        }

        boolean removeChild(Entry<E> child){
            boolean result = false;
            if (children != null && children.contains(child)) {
                result = children.remove(child);

                if (children.size() == 0)
                    children = null;
            }

            return result;          
        }
    }
}

How to resolve?

Answer 1

You omitted one crucial warning in your post: The type parameter E is hiding the type E on the line private class Entry<E> .

There is no relation between the type parameter E in inner class Entry and the one in outer class Trie . You can change E to F in Entry and you will get exactly the same error.

However if you don't redefine <E> in the class declaration Entry it will work because the inner class has access to the type parameter of the outer class (initially I forgot that)

What you need to do is this: replace every occurrence of Entry<E> with just Entry .

public class Trie<E> implements Collection<E> {

    private Comparator<? super E> comparator;

    private class Entry {
        private Entry parent;
        private Set<Entry> children;

        private E data;

        Entry(Entry parent, E data) {
            this.parent = parent;
            this.data = data;

            parent.addChild(this);
        }

        boolean addChild(Entry child) {
            if (children == null)
                children = new TreeSet<Entry>(new Comparator<Entry>() {
                    public int compare(Entry o1, Entry o2) {
                        return Trie.this.comparator.compare(o1.data, o2.data);
                    };
                });

            return children.add(child);
        }

        boolean removeChild(Entry child) {
            boolean result = false;
            if (children != null && children.contains(child)) {
                result = children.remove(child);
                if (children.size() == 0)
                    children = null;
            }
            return result;
        }
    }
}

That will make your problem go away.