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Acyclic Visitor

Difficulty-Intermediate Java Behavioral

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Intent

Allow new functions to be added to existing class hierarchies without affecting those hierarchies, and without creating the troublesome dependency cycles that are inherent to the GOF VISITOR Pattern.

Applicability

This pattern can be used: * When you need to add a new function to an existing hierarchy without the need to alter or affect that hierarchy. * When there are functions that operate upon a hierarchy, but which do not belong in the hierarchy itself. e.g. the ConfigureForDOS / ConfigureForUnix / ConfigureForX issue. * When you need to perform very different operations on an object depending upon its type. * When the visited class hierarchy will be frequently extended with new derivatives of the Element class. * When the recompilation, relinking, retesting or redistribution of the derivatives of Element is very expensive.

Consequences

The good: * No dependency cycles between class hierarchies. * No need to recompile all the visitors if a new one is added. * Does not cause compilation failure in existing visitors if class hierarchy has a new member.

The bad: * Violates the principle of least surprise or Liskov's Substitution principle by showing that it can accept all visitors but actually only being interested in particular visitors. * Parallel hierarchy of visitors has to be created for all members in visitable class hierarchy.

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