Private vs Protected - Visibility Good-Practice Concern

Question

I ve been searching and I know the theoretic difference    public - Any class function may access the method property  protected - Only this class and any subclasses may access the method property  private - Only this class may access the method property  It won t even be inherited    That s all fine and well  the question is  what s the practical difference between them  When would you use private and when would you use protected  Is there a standard or acceptable good practice over this one   Up until now  to retain the concept of inheritance and polymorphism  I use public for anything that should be accessed from the outside  like constructors and main class functionality   and protected for internal methods  logic  helper methods etc   Am I on the right track    Note that this question is for me  but also for future reference as I haven t seen a question like this one SO

User · Accepted Answer

No  you re not on the right track  A good rule of thumb is  make everything as private as possible  This makes your class more encapsulated  and allows for changing the internals of the class without affecting the code using your class    If you design your class to be inheritable  then carefully choose what may be overridden and accessible from subclasses  and make that protected  and final  talking of Java  if you want to make it accessible but not overridable   But be aware that  as soon as you accept to have subclasses of your class  and there is a protected field or method  this field or method is part of the public API of the class  and may not be changed later without breaking subclasses   A class that is not intended to be inherited should be made final  in Java   You might relax some access rules  private to protected  final to non-final  for the sake of unit-testing  but then document it  and make it clear that although the method is protected  it s not supposed to be overridden

User · Answer

Let me preface this by saying I m talking primarily about method access here  and to a slightly lesser extent  marking classes final  not member access  The old wisdom   quot mark it private unless you have a good reason not to quot   made sense in days when it was written  before open source dominated the developer library space and VCS dependency mgmt  became hyper collaborative thanks to Github  Maven  etc   Back then there was also money to be made by constraining the way s  in which a library could be utilized   I spent probably the first 8 or 9 years of my career strictly adhering to this  quot best practice quot   Today  I believe it to be bad advice   Sometimes there s a reasonable argument to mark a method private  or a class final but it s exceedingly rare  and even then it s probably not improving anything  Have you ever   Been disappointed  surprised or hurt by a library etc  that had a bug that could have been fixed with inheritance and few lines of code  but due to private   final methods and classes were forced to wait for an official patch that might never come   I have  Wanted to use a library for a slightly different use case than was imagined by the authors but were unable to do so because of private   final methods and classes  I have  Been disappointed  surprised or hurt by a library etc  that was overly permissive in it s extensibility  I have not   These are the three biggest rationalizations I ve heard for marking methods private by default  Rationalization  1  It s unsafe and there s no reason to override a specific method I can t count the number of times I ve been wrong about whether or not there will ever be a need to override a specific method I ve written   Having worked on several popular open source libs  I learned the hard way the true cost of marking things private   It often eliminates the only practical solution to unforseen problems or use cases   Conversely  I ve never in 16  years of professional development regretted marking a method protected instead of private for reasons related to API safety   When a developer chooses to extend a class and override a method  they are consciously saying  quot I know what I m doing  quot  and for the sake of productivity that should be enough  period   If it s dangerous  note it in the class method Javadocs  don t just blindly slam the door shut  Marking methods protected by default is a mitigation for one of the major issues in modern SW development  failure of imagination  Rationalization  2  It keeps the public API   Javadocs clean This one is more reasonable  and depending on the target audience it might even be the right thing to do  but it s worth considering what the cost of keeping the API  quot clean quot  actually is  extensibility   For the reasons mentioned above  it probably makes more sense to mark things protected by default just in case  Rationalization  3  My software is commercial and I need to restrict it s use  This is reasonable too  but as a consumer I d go with the less restrictive competitor  assuming no significant quality differences exist  every time  Never say never I m not saying never mark methods private  I m saying the better rule of thumb is to  quot make methods protected unless there s a good reason not to quot   This advice is best suited for those working on libraries or larger scale projects that have been broken into modules   For smaller or more monolithic projects it doesn t tend to matter as much since you control all the code anyway and it s easy to change the access level of your code if when you need it   Even then though  I d still give the same advice  -

User · Answer

Well it is all about encapsulation if the paybill classes handles billing of payment then in product class why would it needs the whole process of billing process i e payment method how to pay where to pay    so only letting what are used for other classes and objects nothing more than that public for those where other classes would use too  protected for those limit only for extending classes  As you are madara uchiha the private  is like  limboo  you can see it  you class only single class

User · Answer

Stop abusing private fields     The comments here seem to be overwhelmingly supportive towards using private fields  Well  then I have something different to say   Are private fields good in principle  Yes  But saying that a golden rule is make everything private when you re not sure is definitely wrong  You won t see the problem until you run into one  In my opinion  you should mark fields as protected if you re not sure   There are two cases you want to extend a class     You want to add extra functionality to a base class You want to modify existing class that s outside the current package  in some libraries perhaps    There s nothing wrong with private fields in the first case  The fact that people are abusing private fields makes it so frustrating when you find out you can t modify shit   Consider a simple library that models cars   class Car       private screw      public assembleCar            screw install               private putScrewsTogether                           The library author thought  there s no reason the users of my library need to access the implementation detail of assembleCar   right  Let s mark screw as private   Well  the author is wrong  If you want to modify only the assembleCar   method without copying the whole class into your package  you re out of luck  You have to rewrite your own screw field  Let s say this car uses a dozen of screws  and each of them involves some untrivial initialization code in different private methods  and these screws are all marked private  At this point  it starts to suck   Yes  you can argue with me that well the library author could have written better code so there s nothing wrong with private fields  I m not arguing that private field is a problem with OOP  It is a problem when people are using them   The moral of the story is  if you re writing a library  you never know if your users want to access a particular field  If you re unsure  mark it protected so everyone would be happier later  At least don t abuse private field   I very much support Nick s answer

User · Answer

When would you use private and when would you use protected   Private Inheritance can be thought of Implemented in terms of relationship rather than a IS-A relationship  Simply put  the external interface of the inheriting class has no  visible  relationship to the inherited class  It uses the private inheritance only to implement a similar functionality which the Base class provides   Unlike  Private Inheritance  Protected inheritance is a restricted form of Inheritance wherein the deriving class IS-A kind of the Base class and it wants to restrict the access of the derived members only to the derived class

User · Answer

I read an article a while ago that talked about locking down every class as much as possible  Make everything final and private unless you have an immediate need to expose some data or functionality to the outside world  It s always easy to expand the scope to be more permissible later on  but not the other way around  First consider making as many things as possible final which will make choosing between private and protected much easier    Make all classes final unless you need to subclass them right away  Make all methods final unless you need to subclass and override them right away  Make all method parameters final unless you need to change them within the body of the method  which is kinda awkward most of the times anyways    Now if you re left with a final class  then make everything private unless something is absolutely needed by the world - make that public   If you re left with a class that does have subclass es   then carefully examine every property and method  First consider if you even want to expose that property method to subclasses  If you do  then consider whether a subclass can wreak havoc on your object if it messed up the property value or method implementation in the process of overriding  If it s possible  and you want to protect your class  property method even from subclasses  sounds ironic  I know   then make it private  Otherwise make it protected   Disclaimer  I don t program much in Java

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