constructor methods in interfaces

Question

Are constructor methods in interfaces bad?

Answer 1

Although interfaces can't have constructors in most languages, the Factory pattern provides a contract for constructing objects, similar to an interface. Take a look at that.

Answer 2

Why do people think that anybody wants to instantiate the interface?

What we want to do is to force implementers to implement the constructor, just like other interface methods.

An interface is like a contract. Let's say I have an interface Queue, and I want to make sure that implementers create a constructor with one argument, which creates a singleton queue (A new queue with just that element). Why should that not be part of the contract? With at least Java interfaces, that cannot be specified.

Answer 3

Whether or not they are bad, I am not aware of any language that has the ability to specify a constructor on an interface.

That being said, however, I personally do not believe that the constructor of an object is part of that object's interface and as such adding a constructor to an interface would inhibit the natural flexibility that interfaces afford.

Answer 4

You have to instantiate immutable polymorphic objects sometime via their constructor that requires parameters and you may need that constructor in the interface for the exact same reasons you may need the other public methods in the interface, for example…

Say you have to instantiate a polymorphic object, its class implementing your interface and being supplied by the client code. As a dumb but simple scenario let's say this object is a value object and as such should be immutable, which mean the object's state should be valid from the moment it's instantiated…

$immutablePolymorphe = $userConfig['immutable_polymorphe_class'];
$immutablePolymorphe = new $immutablePolymorphe($state);
// Then do something with that polymorphe...

So what if you don't define the constructor with its parameter in the interface? Hence the reason why I believe a constructor in an interface can be as much legitimate as any other public method in an interface…

Answer 5

They are bad in that they serve no purpose. At its core, an interface is simply a data passing contract. There is no implemenation attached with an interface and hence there is nothing to initialize and no need for a constructor.

If you need some sort of initialization your much better off using an abstract class.

Answer 6

First off, I disagree that interface is just a data passing contract. If that were true you would be allowed to define properties in an interface.

I wouldn't exactly think it's weird to do something like:

interface IDBConnection
{
    function __construct( $connectionString );
    function executeNonQuery( $commandText, $paramters=null);
    function executeScalar( $commandText, $paramters=null);
    function executeSingle( $commandText, $paramters=null);
    function executeArray( $commandText, $paramters=null);
}

This would enable you to create instances of third party classes for data access based on simple reflection instead of just being a data contract.

I'm pretty sure that this isn't the best example, I'd go for an abstract base class here in the real world, but I'm also pretty sure that there are perfectly valid reasons for defining a constructor methods' contract in an interface that I haven't thought of.

I haven't seen it done, but i wouldn't think it to be weird or bad.