Constructors & this/super

this() delegates to another constructor in the same class. super() calls the parent constructor. One of them must be the first statement, and if you write neither, super() is inserted automatically.

Constructor chaining means one constructor calls another via this() to reuse initialization logic. The chain must end at a constructor that calls super().

Yes. Abstract classes can have constructors. They are called via super() from concrete subclass constructors, never directly via new.