Constructor overloading in Java - best practice

Question

There are a few topics similar to this  but I couldn t find one with a sufficient answer   I would like to know what is the best practice for constructor overloading in Java  I already have my own thoughts on the subject  but I d like to hear more advice   I m referring to both constructor overloading in a simple class and constructor overloading while inheriting an already overloaded class  meaning the base class has overloaded constructors    Thanks

User · Answer

I think the best practice is to have single primary constructor to which the overloaded constructors refer to by calling this   with the relevant parameter defaults  The reason for this is that it makes it much clearer what is the constructed state of the object is - really you can think of the primary constructor as the only real constructor  the others just delegate to it  One example of this might be JTable - the primary constructor takes a TableModel  plus column and selection models  and the other constructors call this primary constructor   For subclasses where the superclass already has overloaded constructors  I would tend to assume that it is reasonable to treat any of the parent class s constructors as primary and think it is perfectly legitimate not to have a single primary constructor  For example when extending Exception  I often provide 3 constructors  one taking just a String message  one taking a Throwable cause and the other taking both  Each of these constructors calls super directly

User · Answer

It really depends on the kind of classes as not all classes are created equal   As general guideline I would suggest 2 options    For value  amp  immutable classes  Exception  Integer  DTOs and such  use single primary constructor as suggested in above answer For everything else  session beans  services  mutable objects  JPA  amp  JAXB entities and so on  use default constructor only with sensible defaults on all the properties so it can be used without additional configuration

User · Answer

If you have a very complex class with a lot of options of which only some combinations are valid  consider using a Builder   Works very well both codewise but also logically   The Builder is a nested class with methods only designed to set fields  and then the ComplexClass constructor only takes such a Builder as an argument      Edit   The ComplexClass constructor can ensure that the state in the Builder is valid   This is very hard to do if you just use setters on ComplexClass

User · Answer

Constructor overloading is like method overloading  Constructors can be overloaded to create objects in different ways   The compiler differentiates constructors based on how many arguments are present in the constructor and other parameters like the order in which the arguments are passed   For further details about java constructor  please visit https   tecloger com constructor-in-java

User · Answer

Well  here s an example for overloaded constructors   public class Employee      private String name     private int age      public Employee              System out println  We are inside Employee   constructor             public Employee String name             System out println  We are inside Employee String name  constructor          this name   name           public Employee String name  int age             System out println  We are inside Employee String name  int age  constructor          this name   name        this age   age           public Employee int age             System out println  We are inside Employee int age  constructor          this age   age            public String getName              return name           public void setName String name             this name   name           public int getAge              return age           public void setAge int age             this age   age           In the above example you can see overloaded constructors  Name of the constructors is same but each constructors has different parameters   Here are some resources which throw more light on constructor overloading in java   Constructors   Constructor explanation

User · Answer

While there are no  official guidelines  I follow the principle of KISS and DRY  Make the overloaded constructors as simple as possible  and the simplest way is that they only call this       That way you only need to check and handle the parameters once and only once   public class Simple        public Simple             this null              public Simple Resource r            this r  null              public Simple Resource r1  Resource r2               Guard statements  initialize resources or throw exceptions if            the resources are wrong         if  r1    null                r1   new Resource                      if  r2    null                r2   new Resource                          do whatever with resources            From a unit testing standpoint  it ll become easy to test the class since you can put in the resources into it  If the class has many resources  or collaborators as some OO-geeks call it   consider one of these two things   Make a parameter class  public class SimpleParams       Resource r1      Resource r2         Imagine there are setters and getters here but I m too lazy         to write it out  you can make it the parameter class          immutable  if you don t have setters and only set the         resources through the SimpleParams constructor     The constructor in Simple only either needs to split the SimpleParams parameter    public Simple SimpleParams params        this params getR1    params getR2            or make SimpleParams an attribute   public Simple Resource r1  Resource r2        this new SimpleParams r1  r2       public Simple SimpleParams params        this params   params      Make a factory class  Make a factory class that initializes the resources for you  which is favorable if initializing the resources is a bit difficult   public interface ResourceFactory       public Resource createR1        public Resource createR2        The constructor is then done in the same manner as with the parameter class   public Simple ResourceFactory factory        this factory createR1    factory createR2          Make a combination of both  Yeah    you can mix and match both ways depending on what is easier for you at the time  Parameter classes and simple factory classes are pretty much the same thing considering the Simple class that they re used the same way

[java] Constructor overloading in Java - best practice

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