2aa9e78ddd
* Made minor changes in some patterns such as removed throws clause where not needed, changed incorrect order of arguments in assertEquals * Minor refactorings and code style changes. 1) Removed several use of raw types 2) Removed unnecessary throws clauses 3) Used lambda expressions wherever applicable 4) Used apt assertion methods for readability 5) Use of try with resources wherever applicable 6) Corrected incorrect order of assertXXX arguments * Removed unused import from Promise * Addressed review comments * Addressed checkstyle issue
layout, title, folder, permalink, categories, tags
| layout | title | folder | permalink | categories | tags | ||
|---|---|---|---|---|---|---|---|
| pattern | Acyclic Visitor | acyclic-visitor | /patterns/acyclic-visitor/ | Behavioral |
|
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.