Java Generics - Type parameter `K` is not within its bound; should implement X

Question

I am reading the java docs on generics, specifically this page on bounded types and am lost. Here is a simple version of my code, for context I generically want a compare-able key, one with an Integer and another abstract class with a Date, here is just the base abstract class and the integer implementation:

Abstract class:

public abstract class A<K extends Comparable<K>, V> extends TreeMap<K, V>

Integer Abstract class:

public abstract class B<K extends Integer, V> extends A<K,V>

The error I get is in the type reference of the second class at the part extends A<K,V>. Specifically, my IDE is underlying the K and says Type parameter 'K' is not within its bound; should implement 'java.lang.Comparable<K>'

I am confused on this because in the definition of Integer, it implements a Comparable<Integer>, is that not enough to satisfy this? Moreover, when I try to do the following, public abstract class B<K extends Integer & Comparable<K>, V> extends A<K,V>, that does not work either.

Am I misunderstanding something with generics?

Answer 1

Since Integer is a final class, the correct definition of B would be:

public abstract class B<V> extends A<Integer,V> {

}

You don't need the K type parameter.

Answer 2

The compiler error occurs because although Date implements Comparable<Date>, a subclass of Date might not implement Comparable<SubclassOfDate>, so the compiler complains.

You can fix this by changing A's declaration to use a wildcard:

public abstract class A<K extends Comparable<? super K>, V> extends TreeMap<K, V>

However, you probably intended to declare B like this:

public abstract class B<V> extends A<Integer,V> {

}

Integer is final, no other class can inherit it. So B does not have to be generic on K.

As for the class with Date as the key, you probably want to do it like this as well:

public abstract class C<V> extends A<Date,V> {

}

But since Date is not final, you could make a generic parameter K extends Date:

public abstract class C<K extends Date, V> extends A<K,V> {

}