Java - Using Generics Types

Question

What is the correct way to use the generic type in this situation? Consider the following code:

public class GTree<T> {
    private int level;
    private T value;
    private String key;
    private LinkedList<GTree<T>> subTrees;
    private GTree<T> parent;

    public class NodeEntry<T extends Comparable<? super T>> implements Comparable<T>{
        private T value;
        private String key;
        
        public NodeEntry(T v, String k) {
            this.value = v;
            this.key = k;
        }
        
        public T getValue() {
            return value;
        }
        
        public String getKey() {
            return key;
        }
        
        public int compareTo(T o) {
             return getValue().compareTo(o);
         }

        public int compareTo(GTree<T>.NodeEntry<T> o) {
            // TODO Auto-generated method stub
            return value.compareTo(o.getValue());
        }
    }

    private void getAllBranches(LinkedList<LinkedList<NodeEntry<T>>> path)
    {
        if(leaf())
        {
            LinkedList<NodeEntry<T>> lastPath = path.getLast();
            NodeEntry<T> p = new NodeEntry<T>(value, key);
            lastPath.addLast(p);
            return;
        }
        else
        {
            LinkedList<NodeEntry<T>> lastPath = path.getLast();
            NodeEntry<T> p = new NodeEntry<T>(value, key);
            lastPath.addLast(p);

            ....

            for(int i = 0; i < subTrees.size(); i++)
            {
                T x = subTrees.get(i).getValue();
                if(subTrees.get(i).leaf())
                {
                    lastPath.addLast(new NodeEntry<T>(subTrees.get(i).getValue(), subTrees.get(i).getKey()));
                    ....
                }
                else
                    subTrees.get(i).getAllBranches(path);
            }
        }
    }
}

The compiler is complaining about the generic type T whenever I use it, except when I define the variable x: Bound mismatch: The type T is not a valid substitute for the bounded parameter <T extends Comparable<? super T>> of the type GTree. NodeEntry

Answer 1

The issue is that NodeEntry specifically is looking for a class that is of type T that extends Comparable but the GTree class does not have this bound. To resolve this, you must either remove the bound requirement from NodeEntry to define the type parameter T from GTree with the upper bound T extends Comparable .

If NodeEntry expects the same value as GTree then in this specific case GTree's type parameter T should be bound to Comparable.

    public static class GTree<T extends Comparable<T>> {
        
        private int level;
        
        private T value;
        
        private String key;
        
        private LinkedList<GTree<T>> subTrees;
        
        private GTree<T> parent;

        public static class NodeEntry<T extends Comparable<T>> implements Comparable<T> {
            
            private T value;
            
            private String key;

            public NodeEntry(T v, String k) {
                this.value = v;
                this.key = k;
            }

            public T getValue() {
                return value;
            }

            public String getKey() {
                return key;
            }

            @Override
            public int compareTo(T o) {
                return value.compareTo(o);
            }
        }

        private void getAllBranches(LinkedList<LinkedList<NodeEntry<T>>> path) {
            // more
        }
    }