Poltergeist (computer programming)

{{Short description|Computer programming object}}

In computer programming, a poltergeist (or gypsy wagon) is a short-lived, typically stateless object used to perform initialization or to invoke methods in another, more permanent class. It is considered an anti-pattern. The original definition is by Michael Akroyd at the 1996 Object World West Conference:{{cn|date=October 2024}}

{{blockquote|As a gypsy wagon or a poltergeist appears and disappears mysteriously, so does this short lived object. As a consequence the code is more difficult to maintain and there is unnecessary resource waste. The typical cause for this anti-pattern is poor object design.}}

A poltergeist can often be identified by its name; they are often called "manager_", "controller_", "supervisor", "start_process", etc.

Sometimes, poltergeist classes are created because the programmer anticipated the need for a more complex architecture. For example, a poltergeist arises if the same method acts as both the client and invoker in a command pattern, and the programmer anticipates separating the two phases. However, this more complex architecture may actually never materialize.

Poltergeists should not be confused with long-lived, state-bearing objects of a pattern such as model–view–controller, or tier-separating patterns such as business delegate pattern.

To remove a poltergeist, delete the class and insert its functionality in the invoked class, possibly by inheritance or as a mixin.

There have been proposed methods in detecting poltergeists in code for refactoring.{{cite book |last1=Al-Rubaye |first1=Samer Raad Azzawi |last2=Selcuk |first2=Yunus Emre |title=2017 8th IEEE International Conference on Software Engineering and Service Science (ICSESS) |chapter=An investigation of code cycles and Poltergeist anti-pattern |date=24–26 November 2017 |pages=139–140 |doi=10.1109/ICSESS.2017.8342882 |isbn=978-1-5386-0497-7 |chapter-url=https://ieeexplore.ieee.org/document/8342882/authors#authors}}