Can you specify a generic type that subclasses another generic type _and_ a concrete interface?

Question

I have a family of domain models, each of which has a subclass that extends it and implements a specific interface, like this (Cloneable is not the interface in question, it's for example purposes only):

class A{}
class B extends A implements Cloneable{}

class C{}
class D extends C implements Cloneable{}

I want to create a generic type signature that will enforce this pairing, I've tried this which FAILS:

<T1,T2 extends T1 & Cloneable> void f ( T1 t1, T2 t2 ){}

but I get message in my IntelliJ IDE "Type parameter cannot be followed by other bounds"; it still FAILS if I switch the order around to:

<T1,T2 extends Cloneable & T1> void f ( T1 t1, T2 t2 ){}

I get the message "Interface expected here."

Confusingly, both of these signatures WORK:

<T extends A & Cloneable> void f( A a, T t ){}
<T1,T2 extends T1> void f ( T1 t1, T2 t2 ){}

Is this just a weird limitation of Java's generic type system that I cannot have a generic class (ie T2) extend both another generic class (ie T1) and a concrete interface (eg Cloneable)?

tl;dr: So, why won't <T1,T2 extends Cloneable & T1> void f ( T1 t1, T2 t2 ){} compile: is it a limitation of the Java generic syntax or am I using the wrong syntax?

Answer 1

The answer is in JLS 4.4:

TypeParameter:
    TypeVariable TypeBoundopt

TypeBound:
    extends TypeVariable
    extends ClassOrInterfaceType AdditionalBoundListopt

AdditionalBoundList:
    AdditionalBound AdditionalBoundList
    AdditionalBound

AdditionalBound:
    & InterfaceType

The & Cloneable is an AdditionalBound, which can only be used in an AdditionalBoundList. An AdditionalBoundList can only be used after ClassOrInterfaceType. And T1 is a TypeVariable, not a ClassOrInterfaceType.

So, yes, it's a limitation of the Java generic syntax.