Question

To be brief, I have:

public interface Dictionary<E extends Comparable<E>> extends Iterable<E> {

And

public class TreeDictionary implements Dictionary<TreeDictionary> {

When compiled, caused an error

TreeDictionary.java:4: type parameter TreeDictionary is not within its
bound public class TreeDictionary implements
Dictionary {

I guess the reason is probably because I’m not fully clear with the declaration of the Dictionary interface.

Would really appreciate if somebody could explain me this 🙂

Answer 1

public interface Dictionary<E extends Comparable<E>>

That says that whichever type you want to use for E, it must implement the Comparable interface.

Given yourTreeDictionary definition:

public class TreeDictionary implements Dictionary<TreeDictionary>

TreeDictionary does not implement Comparable, and so cannot be substituted for E in the generic type of Dictionary.

Try this instead:

public class TreeDictionary implements Dictionary<TreeDictionary>, Comparable<TreeDictionary>

TreeDictionary should now conform to the constraints of E.