Should I initialize variable within constructor or outside constructor

Question

When I use Java based on my C   knowledge  I love to initialize variable using the following way   public class ME       private int i       public ME              this i   100            After some time  I change the habit to  public class ME       private int i   100       public ME               I came across others source code  some are using 1st convention  others are using 2nd convention   May I know which convention do you all recommend  and why

User · Answer

The only problem I see with the first method is if you are planning to add more constructors  Then you will be repeating code and maintainability would suffer

User · Answer

I recommend initializing variables in constructors  That s why they exist  to ensure your objects are constructed  initialized  properly   Either way will work  and it s a matter of style  but I prefer constructors for member initialization

User · Answer

I have the practice  habit  of almost always initializing in the contructor for two reasons  one in my opinion it adds to readablitiy  cleaner   and two there is more logic control in the constructor than in one line  Even if initially the instance variable doesn t require logic  having it in the constructor gives more flexibility to add logic in the future if needed   As to the concern mentioned above about multiple constructors  that s easily solved by having one no-arg constructor that initializes all the instance variables that are initilized the same for all constructors and then each constructor calls this   at the first line  That solves your reduncancy issues

User · Answer

I think both are correct programming wise   But i think your first option is more correct in an object oriented way  because in the constructor is when the object is created  and it is when the variable should initialized   I think it is the  by the book  convention  but it is open for discussion   Wikipedia

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I find the second style  declaration   initialization in one go  superior  Reasons    It makes it clear at a glance how the variable is initialized  Typically  when reading a program and coming across a variable  you ll first go to its declaration  often automatic in IDEs   With style 2  you see the default value right away  With style 1  you need to look at the constructor as well  If you have more than one constructor  you don t have to repeat the initializations  and you cannot forget them     Of course  if the initialization value is different in different constructors  or even calculated in the constructor   you must do it in the constructor

User · Answer

I would say  it depends on the default  For example  public Bar     ArrayList lt Foo gt  foos      I would make a new ArrayList outside of the constructor  if I always assume foos can not be null  If Bar is a valid object  not caring if foos is null or not  I would put it in the constructor    You might disagree and say that it s the constructors job to put the object in a valid state  However  if clearly all the constructors should do exactly the same thing initialize foos   why duplicate that code

User · Answer

I tend to use the second one to avoid a complicated constructor  or a useless one   also I don t really consider this as an initialization  even if it is an initialization   but more like giving a default value   For example in your second snippet  you can remove the constructor and have a clearer code

User · Answer

Both the options can be correct depending on your situation   A very simple example would be  If you have multiple constructors all of which initialize the variable the same way int x 2 for each one of them   It makes sense to initialize the variable at declaration to avoid redundancy   It also makes sense to consider final variables in such a situation  If you know what value a final variable will have at declaration  it makes sense to initialize it outside the constructors  However  if you want the users of your class to initialize the final variable through a constructor  delay the initialization until the constructor

User · Answer

If you initialize in the top or in constructor it doesn t make much difference  But in some case initializing in constructor makes sense   class String       char   arr   char  20       Here initializing char   over here will not make sense      String                 this arr new char 0             String char   arr                this arr arr            So depending on the situation sometime you will have to initialize in the top and sometimes in a constructor    FYI other option s for initialization without using a constructor    class Foo       int i      static int k         instance initializer block                 run s every time a new object is created         i 20               static initializer block     static            run s only one time when the class is loaded         k 18

User · Answer

It can depend on what your are initialising  for example you cannot just use field initialisation if a checked exception is involved  For example  the following   public class Foo       FileInputStream fis   new FileInputStream   tmp       throws FileNotFoundException     Will cause a compile-time error unless you also include a constructor declaring that checked exception  or extend a superclass which does  e g    public Foo   throws FileNotFoundException

User · Answer

One thing  regardless of how you initialize the field  use of the final qualifier  if possible  will ensure the visibility of the field s value in a multi-threaded environment

[java] Should I initialize variable within constructor or outside constructor

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