Java - How to get concrete parameter type when overriding interface method of generic type

Question

I am a generics newbie and I am not able to find out the best way to achieve this. Please feel free to point out if I am doing something obviously wrong.

interface Node<T> {

    void addOne(Node<T> node);
}

class StringNode implements Node<String> {

    List<StringNode> data = new ArrayList<>();

    /**
     * {@inheritDoc}
     */
    @Override
    public void addOne(Node<String> node) {
        StringNode stringNode = (StringNode) node;
        data.add(stringNode);
    }

}

I was hoping to find a way to avoid the downcast from Node to StringNode in the implementation of addOne by having the function itself be of signature public void addOne(StringNode node) . Any ideas as to how the definition of Node can be changed to achieve this for any implementation of the interface?

I tried searching through the questions already asked, but was not able to find a close match. Any pointers to questions where this has already been discussed would also be greatly appreciated.

Thanks in advance!

Answer 1

Why are you using String and StringNode separatly? Couldn't you just say

interface Node<T> {

    void addOne(T node);
}

class StringNode implements Node<StringNode> {

    List<StringNode> data = new ArrayList<StringNode>();

    /**
     * {@inheritDoc}
     */
    @Override
    public void addOne(StringNode node) {
        StringNode stringNode =  node;
        data.add(stringNode);
    }

}

Answer 2

This is how I would have done it:

interface Node<T extends Node<T>> {

    void addOne(T node);
}

class StringNode implements Node<StringNode> {

    List<StringNode> data = new ArrayList<StringNode>();

    /**
     * {@inheritDoc}
     */
    @Override
    public void addOne(StringNode node) {
        data.add(node);
    }

}

Similar to U-No-Poo's suggestion, but a little more strict (forces T to actually be a node)

Answer 3

You could introduce another generic parameter:

public interface Node<VALUE_TYPE, SELF> {
  public void addOne(SELF u);
}

public class StringNode implements Node<String, StringNode> {

  @Override
  public void addOne(StringNode u) {
    // TODO Auto-generated method stub  
  }
}

Use VALUE_TYPE whenever you need the actual value type (in this case String) and SELF whenever you want to pass a node of that type.