Initialize class fields in constructor or at declaration

Question

I ve been programming in C  and Java recently and I am curious where the best place is to initialize my class fields   Should I do it at declaration    public class Dice       private int topFace   1      private Random myRand   new Random         public void Roll                                    or in a constructor    public class Dice       private int topFace      private Random myRand       public Dice                 topFace   1          myRand   new Random               public void Roll                                    I m really curious what some of you veterans think is the best practice  I want to be consistent and stick to one approach

User · Answer

When you don t need some logic or error handling    Initialize class fields at declaration   When you need some logic or error handling    Initialize class fields in constructor      This works well when the initialization value is available and the   initialization can be put on one line  However  this form of   initialization has limitations because of its simplicity  If   initialization requires some logic  for example  error handling or a   for loop to fill a complex array   simple assignment is inadequate    Instance variables can be initialized in constructors  where error   handling or other logic can be used    From https   docs oracle com javase tutorial java javaOO initial html

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The semantics of C  differs slightly from Java here  In C  assignment in declaration is performed before calling the superclass constructor  In Java it is done immediately after which allows  this  to be used  particularly useful for anonymous inner classes   and means that the semantics of the two forms really do match   If you can  make the fields final

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Consider the situation where you have more than one constructor  Will the initialization be different for the different constructors  If they will be the same  then why repeat for each constructor  This is in line with kokos statement  but may not be related to parameters  Let s say  for example  you want to keep a flag which shows how the object was created  Then that flag would be initialized differently for different constructors regardless of the constructor parameters  On the other hand  if you repeat the same initialization for each constructor you leave the possibility that you  unintentionally  change the initialization parameter in some of the constructors but not in others  So  the basic concept here is that common code should have a common location and not be potentially repeated in different locations  So I would say always put it in the declaration until you have a specific situation where that no longer works for you

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There are many and various situations   I just need an empty list  The situation is clear  I just need to prepare my list and prevent an exception from being thrown when someone adds an item to the list   public class CsvFile       private List lt CsvRow gt  lines   new List lt CsvRow gt          public CsvFile                   I know the values  I exactly know what values I want to have by default or I need to use some other logic   public class AdminTeam       private List lt string gt  usernames       public AdminTeam                  usernames   new List lt string gt      usernameA    usernameB              or  public class AdminTeam       private List lt string gt  usernames       public AdminTeam                  usernames   GetDefaultUsers 2             Empty list with possible values  Sometimes I expect an empty list by default with a possibility of adding values through another constructor   public class AdminTeam       private List lt string gt  usernames   new List lt string gt          public AdminTeam                    public AdminTeam List lt string gt  admins                 admins ForEach x   gt  usernames Add x

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Not a direct answer to your question about the best practice but an important and related refresher point is that in the case of a generic class definition  either leave it on compiler to initialize with default values or we have to use a special method to initialize fields to their default values  if that is absolute necessary for code readability    class MyGeneric lt T gt        T data        T data           lt -- ERROR       T data   0      lt -- ERROR       T data   null      lt -- ERROR              public MyGeneric                    All of the above errors would be errors here in constructor as well           And the special method to initialize a generic field to its default value is the following   class MyGeneric lt T gt        T data   default T        public MyGeneric                               The same method can be used here in constructor

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The design of C  suggests that inline initialization is preferred  or it wouldn t be in the language  Any time you can avoid a cross-reference between different places in the code  you re generally better off   There is also the matter of consistency with static field initialization  which needs to be inline for best performance  The Framework Design Guidelines for Constructor Design say this        CONSIDER initializing static fields inline rather than explicitly using static constructors  because the runtime is able to optimize the performance of types that don   t have an explicitly defined static constructor     Consider  in this context means to do so unless there s a good reason not to  In the case of static initializer fields  a good reason would be if initialization is too complex to be coded inline

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I normally try the constructor to do nothing but getting the dependencies and initializing the related instance members with them  This will make you life easier if you want to unit test your classes   If the value you are going to assign to an instance variable does not get influenced by any of the parameters you are going to pass to you constructor then assign it at declaration time

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I think there is one caveat  I once committed such an error  Inside of a derived class  I tried to  initialize at declaration  the fields inherited from an abstract base class  The result was that there existed two sets of fields  one is  base  and another is the newly declared ones  and it cost me quite some time to debug   The lesson  to initialize inherited fields  you d do it inside of the constructor

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Being consistent is important  but this is the question to ask yourself   Do I have a constructor for anything else    Typically  I am creating models for data transfers that the class itself does nothing except work as housing for variables   In these scenarios  I usually don t have any methods or constructors  It would feel silly to me to create a constructor for the exclusive purpose of initializing my lists  especially since I can initialize them in-line with the declaration   So as many others have said  it depends on your usage  Keep it simple  and don t make anything extra that you don t have to

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There is a slight performance benefit to setting the value in the declaration   If you set it in the constructor it is actually being set twice  first to the default value  then reset in the ctor

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What if I told you  it depends   I in general initialize everything and do it in a consistent way  Yes it s overly explicit but it s also a little easier to maintain    If we are worried about performance  well then I initialize only what has to be done and place it in the areas it gives the most bang for the buck   In a real time system  I question if I even need the variable or constant at all   And in C   I often do next to no initialization in either place and move it into an Init   function  Why  Well  in C   if you re initializing something that can throw an exception during object construction you open yourself to memory leaks

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My rules    Don t initialize with the default values in declaration  null  false  0  0 0      Prefer initialization in declaration if you don t have a constructor parameter that changes the value of the field  If the value of the field changes because of a constructor parameter put the initialization in the constructors  Be consistent in your practice  the most important rule

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In Java  an initializer with the declaration means the field is always initialized the same way  regardless of which constructor is used  if you have more than one  or the parameters of your constructors  if they have arguments   although a constructor might subsequently change the value  if it is not final   So using an initializer with a declaration suggests to a reader that the initialized value is the value that the field has in all cases  regardless of which constructor is used and regardless of the parameters passed to any constructor  Therefore use an initializer with the declaration only if  and always if  the value for all constructed objects is the same

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Assuming the type in your example  definitely prefer to initialize fields in the constructor  The exceptional cases are    Fields in static classes methods Fields typed as static final et al   I always think of the field listing at the top of a class as the table of contents  what is contained herein  not how it is used   and the constructor as the introduction  Methods of course are chapters

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In C  it doesn t matter  The two code samples you give are utterly equivalent  In the first example the C  compiler  or is it the CLR   will construct an empty constructor and initialise the variables as if they were in the constructor  there s a slight nuance to this that Jon Skeet explains in the comments below   If there is already a constructor then any initialisation  above  will be moved into the top of it   In terms of best practice the former is less error prone than the latter as someone could easily add another constructor and forget to chain it

[java] Initialize class fields in constructor or at declaration?

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