User Tools

Site Tools


principles:murphy_s_law

Differences

This shows you the differences between two versions of the page.

Link to this comparison view

Both sides previous revision Previous revision
principles:murphy_s_law [2020-10-12 20:30]
159.69.186.191 old revision restored (2020-10-12 12:09)
principles:murphy_s_law [2020-10-12 20:32] (current)
159.69.186.191 old revision restored (2013-01-16 13:53)
Line 3: Line 3:
 ===== Variants and Alternative Names ===== ===== Variants and Alternative Names =====
  
-  * Design for Errors(({{page>resources:201 Principles#reference}}))+  * Design for Errors((Alan M. Davis//201 Principles of Software Development//))
  
 ===== Context ===== ===== Context =====
 /* fill in contexts here: */ /* fill in contexts here: */
   * [[contexts:Object-Oriented Design]]    * [[contexts:Object-Oriented Design]] 
-  * [[contexts:API Design]] +
-  * [[contexts:User Interface Design]]  +
-  * [[contexts:Implementation]]+
  
 ===== Principle Statement ===== ===== Principle Statement =====
Line 25: Line 23:
 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 the future. So a design is better the fewer possibilities there are to introduce faults while doing maintenance work.+
  
  
Line 36: Line 32:
 This means the fewer possibilities there are that a mistake is made, the fewer there will be. As mistakes are generally undesirable, a design is better when there are fewer possibilities for something to go wrong. This means the fewer possibilities there are that a mistake is made, the fewer there will be. As mistakes are generally undesirable, a design is better when there are fewer possibilities for something to go wrong.
  
-Note that ML does //not// claim that everything constantly fails unless there is no possibility to do so. It simply says that statistically in the long run a system will fail if it can.+
 ===== Strategies ===== ===== Strategies =====
  
Line 42: Line 38:
  
   * 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|IAP]]) +
-  * Use assertions to detect problems early.+
   * ...   * ...
- 
- 
-===== Caveats ===== 
- 
-See section [[#contrary principles]]. 
  
  
Line 68: Line 57:
 /*  * [[wiki:Examined]]*/ /*  * [[wiki:Examined]]*/
  
-  * [[wiki:Accepted]] The principle is widely known and it's validity is assumed. Nevertheless sometimes it is rather used as a kind of joke instead of as design advice. See for example Jargon File: //[[http://www.catb.org/jargon/html/M/Murphys-Law.html|Murphy's Law]]//+  * [[wiki:Accepted]] The principle is widely known and it's validity is assumed. Nevertheless sometimes it is rather used as a kind of joke instead of an design advice. See for example Jargon File: //[[http://www.catb.org/jargon/html/M/Murphys-Law.html|Murphy's Law]]//
  
 /*  * [[wiki:Questioned]]*/ /*  * [[wiki:Questioned]]*/
  
-Furthermore every defect in any system is a manifestation of ML. If there is a fault then obviously something went wrong. The correlation between the number of possibilities for introducing defects and the actual defect count can be regarded trivially intuitive.+
 ===== Relations to Other Principles ===== ===== Relations to Other Principles =====
  
Line 82: Line 71:
   * [[Easy to Use and Hard to Misuse]] (EUHM): Because of ML an interface should be crafted so it is easy to use and hard to misuse. EUHM is the application of ML to interfaces.   * [[Easy to Use and Hard to Misuse]] (EUHM): Because of ML an interface should be crafted so it is easy to use and hard to misuse. EUHM is the application of ML to interfaces.
   * [[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 to 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 ====
  
-  * **[[Fail Fast]] (FF)**: Sometimes it is impossible to actually prevent an error. In such a case it is advisable to fail fast so the error is recognized early.+  * **[[Fail Fast]] (FF)**: Sometimes it is impossible to actually prevent an error. In such a case it is advisable to fail fast to the mistake is recognized early.
  
  
Line 98: Line 86:
  
  
-===== Examples =====+===== Example =====
  
 ==== Example 1: Parameters ==== ==== Example 1: Parameters ====
Line 106: Line 94:
 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. 
Line 113: Line 101:
 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]].
Line 120: Line 108:
 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: 
Line 158: Line 146:
 Note that the typecast is rather a symptom than the actual problem here. The problem is, that the ''List'' interface is not generic and the symptom is the typecast. The reason for this flaw is, that the ''List'' interface predates the introduction of generics in Java. Note that the typecast is rather a symptom than the actual problem here. The problem is, that the ''List'' interface is not generic and the symptom is the typecast. The reason for this flaw is, that the ''List'' interface predates the introduction of generics in Java.
  
-==== Example 3: Date, Mutability/Aliasing ==== 
- 
-In Java the classes [[http://docs.oracle.com/javase/7/docs/api/java/util/Date.html|''Date'']] as well as the newer [[http://docs.oracle.com/javase/7/docs/api/java/util/Calendar.html|''Calendar'']] are mutable which means the reference semantics of Java objects may cause unintended alternations of date values. Eventually someone will copy the reference to a date object instead of copying the object itself, which is usually a mistake when programming with dates. 
- 
-<code java> 
-Date date1 = new Date(2013, 01, 12); 
-Date date2 = date1; 
-System.out.println(date1); // Sun Feb 12 00:00:00 CET 3913 
-System.out.println(date2); // Sun Feb 12 00:00:00 CET 3913 
-date1.setMonth(2); 
-System.out.println(date1); // Sun Mar 12 00:00:00 CET 3913 
-System.out.println(date2); // Sun Mar 12 00:00:00 CET 3913 
-</code> 
- 
-Furthermore as can be seen in the code above, the month value counterintuitively is zero-based, which results in 1 meaning February. This obviously is another source for mistakes. Also the order of the parameters can be mixed up easily. And lastly this does not refer to a date in 2013 but to one in 3913! The year value is meant to be "two-digit", so 1900 is added to it. So there are plenty of possibilities for making mistakes. And sooner or later someone will make them. 
- 
-Because of these and several other flaws in the design of the Java date API, most of the methods in ''Date'' are deprecated and also the newer ''Calendar'' API will be replaced by a [[http://openjdk.java.net/jeps/150|new API]] in Java 8. 
  
 ===== Description Status ===== ===== Description Status =====
Line 188: Line 159:
   * [[wiki>MurphysLaw]]   * [[wiki>MurphysLaw]]
  
-===== Discussion ===== 
- 
-Discuss this wiki article and the principle on the corresponding [[talk:principles:Murphy's Law|talk page]]. 
principles/murphy_s_law.txt · Last modified: 2020-10-12 20:32 by 159.69.186.191