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Each method should be written in terms of a single level of abstraction.
All statements of a method should belong to the same level of abstraction. If there is a statement which belongs to a lower level of abstraction, it should go to a private method which comprises statements on this level. Doing so will result in smaller methods.
Often the body of a loop can be extracted resulting in a separate private method. Loops should ideally contain a single statement (usually a method call). Sometimes this is not achievable without other drawbacks but certainly large loop bodies can be considered a small.
A further indicator for a missing method is the combination of a blank line, a comment and a block of code. In most of the cases the code block should go to a new private method. This also makes the comment obsolete as the new method carries a name which typically resembles the comment.
Sometimes extracting the method would result in the new method having a large number of parameters. Alternatively the parameters could be converted to fields of the class. But this would often result in bad cohesion. Because of that in such a case extracting a new class is the next step in adhering to the principle.
Switching between levels of abstraction makes code harder to read. While reading the code you have to mentally construct the missing abstractions by trying to find groups of statements which belong together (mental grouping).
An exception where it may be helpful not to adhere to the principle are certain algorithms. When implementing for instance a sorting algorithm, keeping the algorithm together can be considered more readable than splitting it up into several small functions. This is because certain algorithms are well known and splitting them up may disguise them. A suitable compromise has to be found in such a case.
See section contrary principles for further aspects.
(on page 36). The principles is maybe older.
Discuss this wiki article and the principle on the corresponding talk page.