Java Language Specification, Version Java SE 21 Edition

8.3. Field Declarations

Formal constructor parameters are equivalent to instance variables

A class with a recursive method represents part of a recursive data structure

A class referring to a recursive data structure is (indirectly) part of that recursion as well

Variable declarations don't need a type

If a class has a method that has a local variable, parameter, or return value with the class as its type, the class is a recursive type