What is the difference between Type and Class?

Question

What makes a type different from class and vice versa?

(In the general language-agnostic sense)

Answer 1

Different classes may describe the same type.

Type consists of these parts:

1. Operations = syntax
2. Description of operations = semantics

Class consists of these parts:

1. Operations = syntax
2. Implementation (= various implementations describe same semantics)

Some notes:

• Interface (as in Java) is not type, because it does not describe semantics (describes only syntax)

• Subclass is not subtype, because subclass may change semantics defined in superclass, subtype cannot change supertype semantics (see Liskov Substitution Principle, e.g. this LSP example).