Originally posted by veeransenthil kumar: hi! why UML people needs both class diagram and object diagram? and both collaboration diagram and sequence diagram?
A class diagram represents a static view to the design, while an object diagram visualizes the runtime interaction between objects. "Object != Class"... The difference between a collaboration diagram and a sequence diagram is similar. They both communicate the same information but from a different view. A sequence diagram is good for visualizing the time-ordering of messages while the collaboration diagram is good at visualizing relationships between objects.
Originally posted by veeransenthil kumar: why UML people needs both class diagram and object diagram?
Because they show different information. From a class diagram, you can't see how many objects of class will exist in the system at a specific point. The only thing you see the *potential* for existing objects and links between them. For example, class A has an association to B with a "*" multiplicity. How many instances of B will exist after you started the system? You simply can't tell. An object diagram, on the other hand, does show a "snapshot" of the existing objects at a specific moment. Say you have an object a with links to objects b1 to b3. Can the number of b's change later? You can't tell from the object diagram. Typically, there is an infinite number of possible object diagrams to a class diagram. Does that compute?
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