java - Can I say "if class A is subclass of class B, type A must be subtype of type B"? -

java - Can I say "if class A is subclass of class B, type A must be subtype of type B"? -

i have understands relationship between subtype , subclass, i'm not sure if correct.

concrete classes (not generic/higher-order class)

say in java or scala, if declare concrete classes or types:

class b class extends b

if class a subclass of class b, type a must subtype of type b?

and also, if type a subtype of type b, class a subclass of class b? (assume a , b both classes)

but true if it's concrete class or types

higher-order classes/types

but higher-order types, not true, right? say:

class a[+t]

then type a[string] subtype of type a[object], a[string] not subclass of a[object], right?

almost yes in question - can describe a , b beingness subtypes when generic classes, explain below.

there 2 ways "a subclasses b" , "a subtypes b" may not equivalent:

1, traits/interfaces can part of inheritance hierarchy can't described classes, , hence can't part of subclassing relationship. can utilize "extends" express more general relationship, in a extends b.

2, generic classes aren't simple types, type constructors (i.e. higher-kinded types). with:

class b[t] class a[t] extends b[t]

it right "a[t] subtypes b[t]" a[t] , b[t] aren't classes , can't have subclassing relationship. right "a subclasses b" , "a subtypes b", in case describing relationships between higher-kinded types.

note scala, unlike java, allows type parameters declared co- or contravariant. if have class c[-t, +u] , 2 types a , b , a subtypes b, c[b, a] subtypes c[a, b], there no subclassing relationship there.

java scala subclassing type-systems subtyping