Java generics: Bound mismatch

Question

I have a generic class with this definition:

public class AcoProblemSolver<C, E extends Environment, A extends AntColony<E, Ant<C, E>>> {

Where AntColony goes this way:

public abstract class AntColony<E extends Environment, A extends Ant<?, E>> {

And Ant goes like this:

public abstract class Ant<C, E extends Environment> {

I was hoping to extend AntColony in this fashion:

public class FlowShopProblemSolver extends
    AcoProblemSolver<Integer, FlowShopEnvironment, FlowShopAntColony> {

But Eclipse is showing an error on the FlowShopAntColony parameter class:

Bound mismatch: The type FlowShopAntColony is not a valid substitute for the bounded parameter <A extends AntColony<E,Ant<C,E>>> of the type AcoProblemSolver<C,E,A>

Which confuses me, since FlowShopAntColony is defined this way:

public class FlowShopAntColony extends
    AntColony<FlowShopEnvironment, AntForFlowShop> {

And AntForFlowShop goes like this:

public class AntForFlowShop extends Ant<Integer, FlowShopEnvironment> {

Why isn't FlowShopAntColony accepted as a valid parameter?

Answer 1

A extends AntColony<E, Ant<C, E>>

The third parameter of AcoProblemSolver has the restriction extends AntColony<E, Ant<C, E>>. The second parameter of AntColony must be exactly Ant<C, E> and you're trying to pass a subclass of Ant. Try:

A extends AntColony<E, ? extends Ant<C, E>>

You may want other similar ? extends clauses elsewhere.