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Monday, August 1, 2011

How to avoid the violation of the DRY-Principle

Don’t Repeat Yourself (DRY-Principle) ?

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Why do you need source code conventions in your company?
An other common violation of the DRY principle is this one here :
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How to avoid the violation of the DRY-principle?

In this blog i'm gonna show you how to avoid violation of the DRY principle. The most important thing: i will tell you WHY you should not. I'm gonna give you some arguments, so that you are able to explain and motivate anybody at anytime.

What i hate, is the violation of the DRY principle by copy-pate or auto-comments. It seams to be a world wide "disease", because we all know it, but still continually violate this principle almost all the time. Let's figure out what i mean by doing a very trivial example:

Bad example 1 - DRY violation by repeating comments / making javaDoc obsolete:
/**
* The id
* @return id
*/ 
public long getId(){
    return this.id;
This example is so trivial, the most of the programmers would say one of those sentences: 
  • it's ok, any programmer with a little experience is able to understand that.
  • if a programmer does not understand that, then he is probabily on the false job
  • it would be better without comments.
  • it was auto-generated from eclipse. 
  • My checkstyle tool would fail if a did not comment it.
it sounds legtmin right? Well, if i was forced to do that, i would probabily say to my boss:
hey boss, i know you need me to do the work XYZ, but i have to rewrite (even better repeat)  some pieces of my code to accomplish the rules we have in your company
What do you think, he would say? OK, i think you got it...let's jump this part and see how it should be and why!
Good example 1: 
/**
* With this id you are able to retrieve the object from the 
* database by calling {@link DAO#get(id)} 
* @return id from the object to be retrieved
*/ 
public long getId(){
    return this.id;
1 rule: we comment not what, but why we do something.
2. rule: we write an example how to use it, because in a year nobody knows that anymore
3. rule: writing a good comment, motivates and encourages junior programmers to do it correctly also.
4. rule: the javaDoc does not become obsolete this way and really helps
5. rule: a good comment can reduce maintenance efforts, when a bug is issued
6. rule: agility has nothing to do with zero documents! thats not the meaning. agility, means sustainability means, re-usability, understandable clean code and so on....

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An other common violation of the DRY principle is this one here :


Bad example 2 - DRY violation unconsciously:

/**comment ommited */ 
public void doThis(Object o){
    if(o !=null){
         // do something here... 
        }
/**comment ommited */ 
public void doThat(Object o){
    if(o !=null){
         // do something here... 
        }
this is an also very trivial example, but a really good one. such constructs are very often. instead of doing this way, do better this way here:

Good example 2: 
/**comment ommited */ 
public void doThis(Object o){
    doNotAllowNull ( o ) ;    
     // do something here... 
/**comment ommited */ 
public void doThat(Object o){
    doNotAllowNull ( o ) ;    
     // do something here...
/**comment ommited */ 
private <T>  void  doNotAllowNull(T anyObjectOfYourChoice){
    final String msg = "YourExceptionHelpMessage"; 
    if( anyObjectOfYourChoice  == null){
          throw new NullPointerException(msg);
        }
Or this way here:
/**comment ommited */ 
private <T>  boolean  isNotNull(T anyObjectOfYourChoice){
    return ( anyObjectOfYourChoice != null ) ?  true  :  false
Even better: 
Even better it would be to define an interface and a class that does that for you so that you don't have to write or implement it always from the scratch. you just have to delegate it to this class.

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