Interface inheritance and Generics

Question

Hello!
For an application I need several 'stores' (classes which contain some elements) which share certain characteristics. I would like to create a general StoreInterface, and some specialized interfaces which inherit from this, e.g. a 'KwicStoreInterface'.
Also I would like to specify that the Store takes elements of a general type 'Storable', but the KwicStore only takes objects of the type 'KwicInterface' which inherits from 'Storable'.
I thought that specifying these restrictions using Generics would be a good idea, but I can't get the hang of it.
This is a simplyfied version of what I tried:
// interfaces for the stores
public interface StoreInterface<T extends Storable>{
    public T getElem(Long id);
    public void setElem(T elem);
    public void setElems(Map<Long, T> elems);
public interface KwicStoreInterface<T extends KwicInterface> extends StoreInterface{
    public void SomeKwicStoreSpecificMethod();
// Interfaces for the elements
public interface Storable {
    public Long getID();
    public void setID(Long id);
public interface KwicInterface extends Storable{
    public void SomeKwicSpecificMethod();
}When I try to instantiate KwicStoreInterface, this code doesn't compile, because the methods are not recognized as valid implementations of the interface's methods.
public KwicStore<T extends KwicInterface> implements KwicStoreInterface {
     private Map<Long, T> elements = new HashMap<Long, T>();
   // Only his method seems to be recognized correctly:
    public T getElem(Long id){
          return elements.get(id);
   // not recognized as interface method
    public void setElem(T elem){
           elements.put(elem.getID(), elem);
    // not recognized as interface method
    public void setElems(Map<Long, T> elems){
            elements = elems;
   // this one seems okay, too, of course:
   public void SomeKwicStoreSpecificMethod() {}
}If I allow the Netbeans IDE to automatically instantiate the methods, all the type information is reverted to Storable or is lost completely (in case of the Map), resulting in methods like:
@Override
    public void setElem(Storable elem) {
        throw new UnsupportedOperationException("Not supported yet.");
    @Override
    public void setElems(Map elems) {
        throw new UnsupportedOperationException("Not supported yet.");
    @Override
    public Storable getElem(Long id) {
        throw new UnsupportedOperationException("Not supported yet.");
    }What am I doing wrong and how can I achieve the functionality I want?
Thanks in advance for any help!

Answer 1

Thanks, that was really helpful!
However, I ran into new problem now.
The KwicStore should be built using a Reader object (which parses an XML file) and the reader contains a Map with concrete types.
However, it seems impossible to initialize a 'variable parameter' with a concrete type.
public class KwicStore<T extends KwicInterface> implements KwicStoreInterface<T>{
    Map<Long, T> elements;
    // doesn't work:
    KwicStore() {
        KwicStoreReader reader = new KwicStoreReader();
        reader.read();
        this.elements = reader.readerElems;
   // other methods
// the very simplified reader
public class KwicStoreReader {
     public Map<Long, KwicInterface> readerElems = new HashMap<Long, KwicInterface>();
    // do something to fill readerElems with Kwic objects (Kwic instantiates KwicInterface)
    public void read() {
         KwicInterface kwic = new Kwic();
         readerElems.put(kwic.getID(), kwic);
}Is there any way around this?
I already tried to use Generics and variables within the reader, too, but then I couldn't put Kwic objects into the readerElems map.
public class KwicStoreReader<T extends KwicInterface> {
     public Map<Long, T> readerElems = new HashMap<Long, T>();
    // do something to fill elems with Kwic objects (Kwic instantiates KwicInterface)
    public void read() {
         T kwic = new Kwic(); // this doesn't work
         readerElems.put(kwic.getID(), kwic);
}Actually, I would also be fine with restricting KwicStoreInterface to take KwicInterface objects only (without the extends), but I don't know how to declare that correctly.