Why it is not possible to define a static method in a Java interface

Question

Here's the example:

public interface IXMLizable<T>
{
  static T newInstanceFromXML(Element e);
  Element toXMLElement();
}

This is not working. Can anyone help me to find out why it is not working?

Answer 1

There is never a point to declaring a static method in an interface. They cannot be executed by the normal call MyInterface.staticMethod(). If you call them by specifying the implementing class MyImplementor.staticMethod() then you must know the actual class, so it is irrelevant whether the interface contains it or not.

More importantly, static methods are never overridden, and if you try to do:

MyInterface var = new MyImplementingClass();
var.staticMethod();

the rules for static say that the method defined in the declared type of var must be executed. Since this is an interface, this is impossible.

The reason you can't execute "result=MyInterface.staticMethod()" is that it would have to execute the version of the method defined in MyInterface. But there can't be a version defined in MyInterface, because it's an interface. It doesn't have code by definition.

While you can say that this amounts to "because Java does it that way", in reality the decision is a logical consequence of other design decisions, also made for very good reason.

Answer 2

Interfaces are concerned with polymorphism which is inherently tied to object instances, not classes. Therefore static doesn't make sense in the context of an interface.

Static methods cannot be overridden in subclasses, and hence they cannot be abstract. And all methods in an interface are, de facto, abstract.