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Table of Contents
Single Level of Abstraction (SLA)
Variants and Alternative Names
Context
Principle Statement
Each module should be written in terms of a single level of abstraction.
Description
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.
Rationale
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).
Strategies
Caveats
See section contrary principles.
Origin
Evidence
Relations to Other Principles
Generalizations
Specializations
Contrary Principles
- MIMC: Adhering to SLA results in more methods and classes.
- PSU: The purpose of SLA is to avoid mental grouping. On the other hand just adhering to SLA and neglecting PSU may result in the opposite: The reader of the code has to do mental inlining. Sometimes it can be more readable to allow a small amount of statements on the “wrong” level of abstraction (like having a guarding if statement in a higher level method).
Complementary Principles
Principle Collections
Examples
Example1:
Description Status
Further Reading
Discussion
Discuss this wiki article and the principle on the corresponding talk page.