1. State-Space and Behavior
State-Space and Behavior of a Class
State-Space of a Subclass
Behavior of a Subclass
Class Invariant as a Restriction on a State-Space
Preconditions and Postconditions

2. Abstraction by Class
A class should represent a uniform abstraction of the properties of the individual objects that belong to that class
abstraction: not necessarily every property in the real world
uniform: apply to all objects in the class
Two properties of a class
state-space
allowed behavior

3. State-Space of a Class
The ensemble of all the permitted states of any object of the class
state: the ordered set of object attribute values at a given time
Dimensions of a state-space
the coordinates needed to specify the state of a given object
basic attributes defined on the class
attributes derived from others are not considered as dimensions

4. Behavior of a Class
The set of transitions that an object of the class is permitted to make between states in the state-space
transition: change of object state
when one or more of the attributes change values
usually hop along the dimensions
Not all paths in the state-space are allowed in a class

5. Example: Chess Knight vs. Queen Both have the same state-space
chessboard
The two classes are different in behavior
different moving rules
Special Knight (Fig in P. 261)
has different state-space from the original knight class

6. State-Space of a Subclass
The state-space of a subclass must be entirely contained within the state-space of its superclass
the state-space of a subclass is confined by the state-space of its superclass
a subclass usually has narrower ranges of values for the dimensions defined in the superclass
The state-space of a subclass may have more dimensions
the state-space extends from superclass

7. Behavior of a Subclass
The behaviors of subclass and superclass have the similar relationship as the state-spaces
Behavior Confinement
closed behavior
Behavior Extension
extended behavior due to the extended state-space

8. Class Invariant
A class invariant is a condition that every object of that class must satisfy at all times
when the object is in equilibrium
not in the middle of changing states
special rules for attribute values
Invariant Checking Points
when the object is initialized at instantiation
before any operation is executed
after any operation is executed
A subclass must obey the invariant of its superclass

9. Preconditions and Postconditions
To govern individual operations
Every operation has a precondition and a postcondition
Precondition
a condition that must be true when the operation begins to execute, otherwise, refuse to execute
Postcondition
a condition that must be true when the operation ends its execution, otherwise, recover and fix

10. Design by Contract
Use preconditions and postconditions as a contract between an operation and the message senders
Message senders guarantee that the precondition is true before execution
The operations guarantee that the postcondition is true after execution
Basic Object-Oriented Design Approach
formed by class invariant, preconditions, and postconditions