Error while trying to accept generic types that extend a class in an interface

Question

In this code:

public interface LinkedList<T> {
   <N extends Node<T>> Optional<N> findNodeByData(T data);
   // ...
}

public class SinglyLinkedListNode<T> extends Node<T> {
   // ...
}

public class Node<T> {
   // ...
}

public class SinglyLinkedList<T> implements LinkedList<T> {
   public Optional<SinglyLinkedListNode<T>> findNodeByData(T data){
      // ...
   }
}

My goal is to have the method in interface accept any class that extends Node<T>, as N.

I get the error 'findNodeByData(T)' in 'com.starosti.datastructures.linkedlist.singly.SinglyLinkedList' clashes with 'findNodeByData(T)' in 'com.starosti.datastructures.linkedlist.LinkedList'; both methods have same erasure, yet neither overrides the other in the line that findNodeByData is defined in SinglyLinkedList<T>.

How do I fix this issue? Is there a better way to do this?

score 0 · Accepted Answer · answered Sep 29 '22 at 20:44

The erasure of the return types is Optional, yet Optional<SinglyLinkedListNode<T>> is not a subtype of Optional<Node<T>>, explaining your compiler error.

By PECS (Producer: Extends, Consumer: Super), your Node produces the value, so it's safe to use a wildcard upper bound ("extends") on the return type of the method findNodeByData.

interface LinkedList<T> {
    Optional<? extends Node<T>> findNodeByData(T data);
}

This allows you to override the method in the concrete subclass with a subtype, according to covariant return types. Also, when overriding, always include the @Override annotation, so the compiler can check for you if you're truly overriding the method or merely overloading it accidentally.

class SinglyLinkedList<T> implements LinkedList<T> {
    @Override
    public Optional<SinglyLinkedListNode<T>> findNodeByData(T data){
        // ...
    }
}

Error while trying to accept generic types that extend a class in an interface

1 Answers1