17.1) Objects in the World

The world consists of objects, with distinct identities. These objects can be physical (you, a pen, a car, etc) or purely conceptual (a doctor’s appointment, your bank account, etc).

Objects in the world have well-defined states. The state of an object is captured by the values of a set of attributes. For example, you are characterised by attributes such as your name, your date of birth, your nationality; your bank account is characterised by a unique account number and by the current balance.

Attribute values may stay fixed over the lifetime of the object (e.g., your account number) or may change (e.g., your bank balance). If an attribute’s value changes, the overall state of the object has changed.

Objects also have behaviour, characterised by the operations that they can perform or have performed on them. For example, your bank account supports operations such as ‘deposit money’, ‘withdraw money’ and ‘check balance’.