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principles:murphy_s_law [2020-10-12 12:41] – old revision restored (2013-03-19 16:58) 159.69.186.191principles:murphy_s_law [2020-10-12 12:41] – old revision restored (2013-02-10 18:07) 159.69.186.191
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 It is not always possible to design a system in such a way. But as systems are built and used by humans, one should strive for such "fool-proof" designs. It is not always possible to design a system in such a way. But as systems are built and used by humans, one should strive for such "fool-proof" designs.
  
-There are different kinds of possible errors that can and according to ML eventually will occur in some way: Replicated data can get out of sync, invariants can be broken, preconditions can be violated, interfaces can be misunderstood, parameters can be given in the wrong order, typos can occur, values can be mixed up, etc. +Note that Murphy's law also applies to every chunk of code. According to the paw the programmer will make mistakes while implementing the system. So it is better to implement a simple design, as this will have fewer possibilities to make implementation mistakes. Furthermore code is maintained. Bugfixes will be necessary present functionality will be changed and enhanced, so every piece of code will potentially be touched in future. So a design is better the fewer possibilities there are to introduce faults while doing maintenance work.
- +
-Note that Murphy's law also applies to every chunk of code. According to the law the programmer will make mistakes while implementing the system. So it is better to implement a simple design, as this will have fewer possibilities to make implementation mistakes. Furthermore code is maintained. Bugfixes will be necessary present functionality will be changed and enhanced, so every piece of code will potentially be touched in future. So a design is better the fewer possibilities there are to introduce faults while doing maintenance work.+
  
  
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   * Make use of static typing, so the compiler will report faults   * Make use of static typing, so the compiler will report faults
-  * Make the design simple, so there will be fewer implementation defects (see [[Keep It Simple Stupid|KISS]])+  * Make the design simple, so there will be fewer implementation defects (see [[Keep It Simple Stupid]])
   * Use automatic testing to find defects   * Use automatic testing to find defects
-  * Avoid duplication and manual tasks, so necessary changes are not forgotten (see [[Don't Repeat Yourself|DRY]])+  * Avoid duplication and manual tasks, so necessary changes are not forgotten (see [[Don't Repeat Yourself]])
   * Use polymorphism instead of repeated switch statements   * Use polymorphism instead of repeated switch statements
-  * Use the same mechanisms wherever reasonably possible (see [[Uniformity Principle|UP]]) +  * Use the same mechanisms wherever reasonably possible (see [[Uniformity Principle]]) 
-  * Use consistent naming and models throughout the design (see [[Model Principle|MP]]) +  * Use consistent naming and models throughout the design (see [[Model Principle]])
-  * Avoid Preconditions and Invariants (see [[Invariant Avoidance Principle]])+
   * ...   * ...
- 
- 
-===== Caveats ===== 
- 
-See section [[#contrary principles]]. 
  
  
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   * [[Uniformity Principle]] (UP): A typical source of mistakes are differences. If similar things work similarly, they are more understandable. But if there are subtle differences in how things work, it is likely that someone will make the mistake to mix this up.   * [[Uniformity Principle]] (UP): A typical source of mistakes are differences. If similar things work similarly, they are more understandable. But if there are subtle differences in how things work, it is likely that someone will make the mistake to mix this up.
   * [[Invariant Avoidance Principle]] (IAP): Invariants are statements that have t be true in order to keep a module in a consistent state. ML states that eventually an invariant will be broken resulting in a hard to detect defect. IAP states that invariants should therefore be avoided. So IAP is the application of ML to invariants.   * [[Invariant Avoidance Principle]] (IAP): Invariants are statements that have t be true in order to keep a module in a consistent state. ML states that eventually an invariant will be broken resulting in a hard to detect defect. IAP states that invariants should therefore be avoided. So IAP is the application of ML to invariants.
- 
 ==== Contrary Principles ==== ==== Contrary Principles ====
  
-  * **[[Keep It Simple Stupid]] (KISS)**: On the one hand a simpler design is less prone to implementation errors. In this aspect KISS is similar to ML. On the other hand it is sometimes more complicated to make a design "fool-proof" so usage and maintenance mistakes are prevented. In this aspect KISS is rather a contrary principle. Both applies at the same time so a tradeoff has to be made whether correct implementation or correct usage and maintenance are more important in the given case. This means, it is necessary to consider KISS in addition to ML in order to find a suitable compromise. See [[#example 1: parameters]].+  * **[[Keep It Simple Stupid]] (KISS)**: On the one hand a simpler design is less prone to implementation errors. In this aspect KISS is similar to ML. On the other hand it is sometimes more complicated to make a design "fool-proof" so usage and maintenance mistakes are prevented. In this aspect KISS is rather a contrary principle. Both applies at the same time so a tradeoff has to be made whether correct implementation or correct usage and maintenance are more important in the given case. This means, it is necessary to consider KISS in addition to ML in order to find a suitable compromise.
  
 ==== Complementary Principles ==== ==== Complementary Principles ====
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-===== Examples =====+===== Example =====
  
 ==== Example 1: Parameters ==== ==== Example 1: Parameters ====
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 The following method signatures are a bad choice: The following method signatures are a bad choice:
 <code java> <code java>
-replaceFirst(String pattern, String replacement) +replaceFirst(String pattern, string replacement) 
-replaceAll(String replacement, String pattern)+replaceAll(String replacement, string pattern)
 </code> </code>
 Eventually someone will mix up the order of the parameters leading to a fault in the software which is not detectable by the compiler.  Eventually someone will mix up the order of the parameters leading to a fault in the software which is not detectable by the compiler. 
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 So it is better to make parameter lists consistent: So it is better to make parameter lists consistent:
 <code java> <code java>
-replaceFirst(String pattern, String replacement) +replaceFirst(String pattern, string replacement) 
-replaceAll(String pattern, String replacement)+replaceAll(String pattern, string replacement)
 </code> </code>
 This is less error prone. When for example a call to ''replaceFirst()'' is replaced by a call to ''replaceAll()'', one cannot forget to exchange the parameters anymore. This is how is is done in the [[http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#replaceFirst(java.lang.String, java.lang.String)|Java API]]. This is less error prone. When for example a call to ''replaceFirst()'' is replaced by a call to ''replaceAll()'', one cannot forget to exchange the parameters anymore. This is how is is done in the [[http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#replaceFirst(java.lang.String, java.lang.String)|Java API]].
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 But here still one could mix up the two string parameters. Although this is less likely, as having the substring to look for first is "natural", such a mistake is still possible. An alternative would be the following: But here still one could mix up the two string parameters. Although this is less likely, as having the substring to look for first is "natural", such a mistake is still possible. An alternative would be the following:
 <code java> <code java>
-replaceFirst(Pattern pattern, String replacement) +replaceFirst(Pattern pattern, string replacement) 
-replaceAll(Pattern pattern, String replacement)+replaceAll(Pattern pattern, string replacement)
 </code> </code>
 Here both methods expect a ''Pattern'' object instead of a regular expression expressed in a string. Mixing up the parameters is impossible in this case as the compiler would report that error. On the other hand using these methods becomes a bit more complicated:  Here both methods expect a ''Pattern'' object instead of a regular expression expressed in a string. Mixing up the parameters is impossible in this case as the compiler would report that error. On the other hand using these methods becomes a bit more complicated: 
principles/murphy_s_law.txt · Last modified: 2021-10-20 21:18 by christian