Object Summary (Top)

Figure 1: Object Concepts

Objects (Top)

• An object is an abstraction of some real world thing or concept that is relevant to the problem that must be solved.
• An object encapsulates a set of attributes that represent its properties and a set of operations that simulate its behavior.
• An object's attributes are implemented using variables and its operations are implemented using methods (functions and procedures).
• The current state of an object is the current values for each of the object's attribute variables.
• Each object has an identity that is independent of its state.
• An object hides the implementation details of its attributes and operations in a private kernal.
• Each operation that an object can perform is implemented by a method which defines the process the object uses to complete the operation.
• The list of messages that a client can send to an object defines the public behavior of the object.
• Objects are polymorphic which means that different types of objects can respond to the same message, each in its own way.
• An object may have a temporary existence or it may persist across space and time.

Classes (Top)

• A class abstracts the common structure and behavior of a group of objects.
• The class is used like a template to create instances of the type of object the class represents.
• Through inheritance a subclass can inherit the attributes and operations of a superclass. The subclass is a subtype of the superclass.
• An abstract class is a superclass that defines a common set of attributes and behavior for each of its subclasses. An abstract class may provide methods for some of the operations.
• A concrete class can be used to instantiate objects. An abstract class can not be used to instantiate objects.

Relationships (Top)

• In a composition relationship, an object is part of its containing object. The container is responsible for the part. The persistence of the part is often the same as the persistence of the container.
• In an aggregation relationship, an object is contained in another object. If the container disappears, the contained objects may continue to exist. The contained object may also be related to several containers.
• In an association relationship, an object is known by another object. Associated objects may know about each other, exchange messages with each other, but neither is responsible for the existance of the other.
• In a utilization relationship, a client object uses the services of a server object. The client knows about the server but the server does not know about the client. In essense, this is one way association.

In a utilization relationship, the client delegates work to a server. But, delegation may occur in the other types of relationships. An aggregate often delegates work to its parts, and associates often delegate work to each other.