The JEP states this:
Restrictions on records
Records cannot extend any other class, and cannot declare instance fields other than the private final fields which correspond to components of the state description. Any other fields which are declared must be static. These restrictions ensure that the state description alone defines the representation.
The Java 17 JLS 8.10 notes1 this:
recorddeclaration does not have an
extendsclause, so it is not possible to explicitly declare a direct superclass type, even
implement interfaces, so you can use them polymorphically. Furthermore, since records will inherit any
default methods in the interfaces that they implement, they do support a limited form of inheritance.
1 – This is a non-normative statement, but it is obviously true since the specified syntax for RecordDeclaration clearly does not allow
extends to be used.