Assigning Responsibility for Operation
Many operations might have obvious target objects, but some of these operations could be used at numerous places in an algorithm, by one of many objects. These operations are complex high level operations which can be overlooked in laying out the object classes as they are not an inherent part of any of the class.
Now, obvious question is, how do you know what class owns an operation? It is easy when only one of the objects is involved in operation: You are simply informing the object to perform an operation. But when more than one of the objects is involved in an operation it becomes quite hard. Therefore, we must know which object plays main role in operation. For this, ask yourself following questions:
- Is an object acted on when any other object performs action? In general, we shall associate the operation with the target of operation, instead of one initiating it.
- Whether an object is modified by operation or when another object is only performing query for getting some information from it. The object that has changed in whole process known as target of the operation.
- Which class is center of all classes and associations involved in operation? If classes and associations form a star around any single class, it is target of the operation.
- If object is some real world object represented internally, then what real object could you push, activate , move, or otherwise manipulate to initiate operation?