Constructor in an Interface

Question

I know it s not possible to define a constructor in an interface  But I m wondering why  because I think it could be very useful   So you could be sure that some fields in a class are defined for every implementation of this interface   For example consider the following message class   public class MyMessage       public MyMessage String receiver          this receiver   receiver           private String receiver      public void send             some implementation for sending the mssage to the receiver          If a define an interface for this class so that I can have more classes which implement the message interface  I can only define the send method and not the constructor  So how can I ensure that every implementation of this class really has an receiver set  If I use a method like setReceiver String receiver  I can t be sure that this method is really called  In the constructor I could ensure it

User · Answer

Generally constructors are for initializing non-static members of particular class with respect to object.

There is no object creation for interface as there is only declared methods but not defined methods. Why we can’t create object to declared methods is-object creation is nothing but allocating some memory (in heap memory) for non-static members.

JVM will create memory for members which are fully developed and ready to use.Based on those members , JVM calculates how much of memory required for them and creates memory.

Incase of declared methods, JVM is unable to calculate the how much memory will required to these declared methods as the implementation will be in future which is not done by this time. so object creation is not possible for interface.

conclusion:

without object creation, there is no chance to initialize non-static members through a constructor.That is why constructor is not allowed inside a interface.(as there is no use of constructor inside a interface)

User · Answer

Taking some of the things you have described       So you could be sure that some fields in a class are defined for   every implementation of this interface         If a define a Interface for this class so that I can have more   classes which implement the message interface  I can only define the   send method and not the constructor       these requirements are exactly what abstract classes are for

User · Answer

Dependencies that are not referenced in an interfaces methods should be regarded as implementation details  not something that the interface enforces  Of course there can be exceptions  but as a rule  you should define your interface as what the behavior is expected to be  Internal state of a given implementation shouldn t be a design concern of the interface

User · Answer

If you want to make sure that every implementation of the interface contains specific field  you simply need to add to your interface the getter for that field   interface IMyMessage         NonNull String getReceiver         it won t break encapsulation it will let know to everyone who use your interface that the Receiver object has to be passed to the class in some way  either by constructor or by setter

User · Answer

A problem that you get when you allow constructors in interfaces comes from the possibility to implement several interfaces at the same time  When a class implements several interfaces that define different constructors  the class would have to implement several constructors  each one satisfying only one interface  but not the others  It will be impossible to construct an object that calls each of these constructors   Or in code   interface Named   Named String name     interface HasList   HasList List list      class A implements Named  HasList          implements Named constructor       This constructor should not be used from outside        because List parameter is missing         public A String name                         implements HasList constructor       This constructor should not be used from outside        because String parameter is missing         public A List list                       This is the constructor that we would actually       need to satisfy both interfaces at the same time          public A String name  List list        this name          the next line is illegal  you can only call one other super constructor     this list

User · Answer

There is only static fields in interface that dosen t need to initialized during object creation in subclass and the method of interface has to provide actual implementation in subclass  So there is no need of constructor in interface   Second reason-during the object creation of subclass  the parent constructor is called  But if there will be more than one interface implemented then a conflict will occur during call of interface constructor  as to which interface s constructor will call first

User · Answer

An interface defines a contract for an API  that is a set of methods that both implementer and user of the API agree upon  An interface does not have an instanced implementation  hence no constructor   The use case you describe is akin to an abstract class in which the constructor calls a method of an abstract method which is implemented in an child class   The inherent problem here is that while the base constructor is being executed  the child object is not constructed yet  and therfore in an unpredictable state   To summarize  is it asking for trouble when you call overloaded methods from parent constructors  to quote mindprod      In general you must avoid calling any   non-final methods in a constructor    The problem is that instance   initialisers   variable initialisation   in the derived class is performed   after the constructor of the base   class

User · Answer

See this question for the why  taken from the comments    If you really need to do something like this  you may want an abstract base class rather than an interface

User · Answer

Here  s an example using this Technic  In this specifik example the code is making a call to Firebase using a mock MyCompletionListener that is an interface masked as an abstract class  an interface with a constructor  private interface Listener       void onComplete databaseError  databaseReference      public abstract class MyCompletionListener implements Listener      String id      String name      public MyCompletionListener String id  String name            this id   id          this name   name           private void removeUserPresenceOnCurrentItem         mFirebase removeValue child  some key    new MyCompletionListener UUID randomUUID   toString     removeUserPresenceOnCurrentItem              Override         public void onComplete DatabaseError databaseError  DatabaseReference databaseReference                                 Override public void removeValue DatabaseReference ref  final MyCompletionListener var1        CompletionListener cListener   new CompletionListener                      Override                 public void onComplete DatabaseError databaseError  DatabaseReference databaseReference                        if  var1    null                           System out println  Im back and my id is    var1 is     and my name is    var1 name                           var1 onComplete databaseError  databaseReference                                                              ref removeValue cListener

User · Answer

This is because interfaces do not allow to define the method body in it but we should have to define the constructor in the same class as interfaces have by default abstract modifier for all the methods to define  That s why we can not define constructor in the interfaces

User · Answer

A work around you can try is defining a getInstance   method in your interface so the implementer is aware of what parameters need to be handled  It isn t as solid as an abstract class  but it allows more flexibility as being an interface   However this workaround does require you to use the getInstance   to instantiate all objects of this interface   E g   public interface Module       Module getInstance Receiver receiver

[java] Constructor in an Interface?

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