Synonym: instance


Booch defines an object as

"Something you can do things to. An object has state, behavior, and identity; the structure and behavior of similar objects are defined in their common class. [...]"

[BOO91, p. 516]

What does that mean?

» state: An object has a condition, determined by the current values of its variables.

» behavior: An object's state can be changed by applying a certain method.

» identity: Each object can be distinguished from other objects.


Example: Object "MyCar"

An instance of a Car: The object MyCar
An instance of a Car: The object MyCar

The object looks similar to the class it had been instantiated from. This is, because the structure didn't change. By applying certain values to the variables, the object MyCar has now a certain state as well as an identity. It is still a car, for it has the same behaviour as all other cars, which had been defined in the methods of their common class Car.


Now we have created our real-world object and are driving constantly with 50 mph. After a while, we enter the highway, because we want to move faster. So we have to change the car's speed (change the object's state).


This can be done by applying one of an object's methods.

Neural Net Components in an Object Oriented Class Structure