Java Constructor Inheritance

Question

I was wondering why in java constructors are not inherited  You know when you have a class like this   public class Super      public Super ServiceA serviceA  ServiceB serviceB  ServiceC serviceC       this serviceA   serviceA        etc           Later when you inherit from Super  java will complain that there is no default constructor defined  The solution is obviously something like   public class Son extends Super     public Son ServiceA serviceA  ServiceB serviceB  ServiceC serviceC       super serviceA serviceB serviceC            This code is repetitive  not DRY and useless  IMHO     so that brings the question again   Why java doesn t support constructor inheritance  Is there any benefit in not allowing this inheritance

User · Answer

I don t know any language where subclasses inherit constructors  but then  I am not much of a programming polyglott     Here s a discussion about the same question concerning C   The general consensus seems to be that it would complicate the language  introduce the potential for nasty side effects to changes in a base class  and generally shouldn t be necessary in a good design

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A derived class is not the the same class as its base class and you may or may not care whether any members of the base class are initialized at the time of the construction of the derived class  That is a determination made by the programmer not by the compiler

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Suppose constructors were inherited    then because every class eventually derives from Object  every class would end up with a parameterless constructor  That s a bad idea  What exactly would you expect   FileInputStream stream   new FileInputStream      to do   Now potentially there should be a way of easily creating the  pass-through  constructors which are fairly common  but I don t think it should be the default  The parameters needed to construct a subclass are often different from those required by the superclass

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When you inherit from Super this is what in reality happens   public class Son extends Super        If you dont declare a constructor of any type  adefault one will appear    public Son           If you dont call any other constructor in the first line a call to super   will be placed instead      super             So  that is the reason  because you have to call your unique constructor  since Super  doesn t have a default one   Now  trying to guess why Java doesn t support constructor inheritance  probably because a constructor only makes sense if it s talking about concrete instances  and you shouldn t be able to create an instance of something when you don t know how it s defined  by polymorphism

User · Answer

You essentially do inherit the constuctors in the sense that you can simply call super if and when appropriate  it s just that it would be error prone for reasons others have mentioned if it happened by default  The compiler can t presume when it is appropriate and when it isn t    The job of the compiler is to provide as much flexibility as possible while reducing complexity and risk of unintended side-effects

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Because constructors are an implementation detail - they re not something that a user of an interface superclass can actually invoke at all   By the time they get an instance  it s already been constructed  and vice-versa  at the time you construct an object there s by definition no variable it s currently assigned to   Think about what it would mean to force all subclasses to have an inherited constructor   I argue it s clearer to pass the variables in directly than for the class to  magically  have a constructor with a certain number of arguments just because it s parent does

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Because constructing your subclass object may be done in a different way from how your superclass is constructed  You may not want clients of the subclass to be able to call certain constructors available in the superclass    A silly example   class Super       protected final Number value      public Super Number value           this value   value           class Sub       public Sub    super Integer valueOf 0          void doSomeStuff               We know this value is an Integer  so it s safe to cast          doSomethingWithAnInteger  Integer this value               Client code  Sub s   new Sub Long valueOf 666L       Devilish invocation of Super constructor  s doSomeStuff       throws ClassCastException   Or even simpler   class Super       private final String msg      Super String msg           if  msg    null  throw new NullPointerException            this msg   msg          class Sub       private final String detail      Sub String msg  String detail           super msg           if  detail    null  throw new NullPointerException            this detail   detail            void print               detail is never null  so this method won t fail         System out println detail concat       concat msg               Client code  Sub s   new Sub  message       Calling Super constructor - detail is never initialized  s print       throws NullPointerException   From this example  you see that you d need some way of declaring that  I want to inherit these constructors  or  I want to inherit all constructors except for these   and then you d also have to specify a default constructor inheritance preference just in case someone adds a new constructor in the superclass    or you could just require that you repeat the constructors from the superclass if you want to  inherit  them  which arguably is the more obvious way of doing it

User · Answer

Constructors are not polymorphic  When dealing with already constructed classes  you could be dealing with the declared type of the object  or any of its subclasses  That s what inheritance is useful for  Constructor are always called on the specific type eg new String    Hypothetical subclasses have no role in this

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Because a  super class must have complete control over how it is constructed  If the programmer decides that it doesn t make sense to provide a default  no args  constructor as part of the class s contract  then the compiler should not provide one

User · Answer

David s answer is correct  I d like to add that you might be getting a sign from God that your design is messed up  and that  Son  ought not to be a subclass of  Super   but that  instead  Super has some implementation detail best expressed by having the functionality that Son provides  as a strategy of sorts   EDIT  Jon Skeet s answer is awesomest

[java] Java Constructor Inheritance

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