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It states that two instances of similar code do not require refactoring, but when similar code is used three times, it should be extracted into a new procedure. The rule was popularised by Martin Fowler in Refactoring [1] and attributed to Don Roberts. Duplication is considered a bad practice in programming because it makes the code harder to ...
Refactoring is usually motivated by noticing a code smell. [2] For example, the method at hand may be very long, or it may be a near duplicate of another nearby method. Once recognized, such problems can be addressed by refactoring the source code, or transforming it into a new form that behaves the same as before but that no longer "smells".
[1] [2] In object-oriented programming, the purpose of objects is to encapsulate both relevant data (fields) and operations ( methods ) that can be performed on this data. [ 3 ] The failure to group fields together into a true object can discourage the association of relevant actions.
[1] Object-oriented decomposition breaks a large system down into progressively smaller classes or objects that are responsible for part of the problem domain. According to Booch, algorithmic decomposition is a necessary part of object-oriented analysis and design, but object-oriented systems start with and emphasize decomposition into objects. [2]
For example, in the expression (f(x)-1)/(f(x)+1), the function f cannot be called only once with its value used two times since the two calls may return different results. Moreover, in the few languages which define the order of evaluation of the division operator's operands, the value of x must be fetched again before the second call, since ...
Code reuse may be achieved different ways depending on a complexity of a programming language chosen and range from a lower-level approaches like code copy-pasting (e.g. via snippets), [3] simple functions (procedures or subroutines) or a bunch of objects or functions organized into modules (e.g. libraries) [4] [2]: 7 or custom namespaces, and ...
For example, the expression: print length([2+1, 3*2, 1/0, 5-4]) fails under strict evaluation because of the division by zero in the third element of the list. Under lazy evaluation, the length function returns the value 4 (i.e., the number of items in the list), since evaluating it does not attempt to evaluate the terms making up the list.
The main planning process within extreme programming is called the Planning Game. The game is a meeting that occurs once per iteration, typically once a week.