Encapsulation, IS-A, HAS-A

Encapsulation helps hide implementation behind an interface (or API).
Encapsulated code has two features:
Instance variables are kept protected (usually with the private modifier).
Getter and setter methods provide access to instance variables.
IS-A refers to inheritance.
IS-A is expressed with the keyword extends.
IS-A, "inherits from," and "is a subtype of" are all equivalent expressions.
HAS-A means an instance of one class "has a" reference to an instance of another class.