Your requirements flip to describe your situation
abstract class Base → use this
Holds instance state / fields✓
Has a constructor✓
Concrete method implementations✓ full
How many can a class have?one
Forces a contract✓
interface Contract → use this
Holds instance state / fields✗
Has a constructor✗
Concrete method implementationsdefault only
How many can a class have?many
Forces a contract✓
recommendation log
▸ Flip a requirement above and the recommended column lights up.