Parameterized inheritance and implementation

Question

I'm learning generics, and I'm having a problem getting the following to work
I have a base class with some data/behavior
public abstract Base<E> {
//...Then there is a concrete class that extends Base and implements Map
public Concrete<E extends Object & Map<K,V>> extends Base<E extends Object & Map<K,V>> implements Map<K, V>{
//...I cannot seem to specify that the type for Base must be a Map<K, V>.
I've thought about containing and delegating, but really Concrete IS-A Base and Concrete IS-A Map.
Am I missing something about inheritance and generics?
Any suggestions?

Answer 1

James,
Based on your previous posts I think you understand generics as well as I do, and in some aspects, probably better.
I was trying to capture my thought processes (and that is quite hard since mostly this happens sub-consciously - its quite hard to drag it up into consciousness then document it, but the effort was worthwhile for me and hopefully for others as well).
In this particular case, E is Map<K,V>, but there could be all sorts of relationships between the type variables, the idea is to identify these and back substitute them, slowly getting rid of type variables until you can't get rid of any more. The ones you are left with are the ones that need to be type parameters.
take the other post from yesterday
http://forum.java.sun.com/thread.jsp?thread=554360&forum=316&message=2719355
as another worked example...
from
"And I'm trying to create a special generic container (List) which would only take elements that
implement the Keyable interface above. For example:
public class ListOfKeyables <E extends Keyable> extends AbstractList<E>{..."
and
"Now, besides standard List methods that use type varable 'E' in their definitions, I would also
like to have a special method like this:
   public E getByKey(Object key)   {   ... "
I would start with (ignoring their "E extends Keyable" suggestion, its too premature for that).
public class ListOfKeyables < ???? > extends AbstractList<E> {
    public E getByKey(Object key) { ... }
} but "only take elements that
implement the Keyable interface" suggests replacing E with Keyable<K> thus
public class ListOfKeyables < ???? > extends AbstractList<Keyable<K>> {
    public Keyable<K> getByKey(Object key) { ... }
Note In this case the substitution getting rid of E introduced another type variable (K), thats OK, its still a step toward a solution.
Then the original poster said "But I would like to constrain the 'key' argument in this method to be of type that is specified as a type parameter to the Keyable interface"
So after implementing this requirement we have
public class ListOfKeyables < ???? > extends AbstractList<Keyable<K>> {
    public Keyable<K> getByKey(K key) { ... }
}At this point there is no more substitutions or requirements to fulfill.
So we look through and see what type variables are still unresolved. These (in this case there is only one K), are then the type parameters of the class, and thus we have what I suggested in that post
public class ListOfKeyables <K> extends AbstractList<Keyable<K>> {
    public Keyable<K> getByKey(K key) { ... }
}As a final step, we need to look at the type parameters and decide if they need any constraints (K extends something), These might come from some use of the type parameter ( such as if Keyable constrained its K parameter, we would need to do the same), or from the application requirements. But in this case, there are not any.
At a certain level of abstraction, this process is not a whole lot different to the process you use when working out what parameters a method or constructor needs (and that process also tends to happen below the consciusness threshold).
In really simplistic terms, the answer to both "what parameters do I need" questions is "all the ones you can't get from somewhere else or derive from the ones you already have".
Bruce