Why can't implementing classes define an overriding method as static?

Question

I'm confused why the following is not allowed:

public interface MyInterface {
  MyInterface getInstance(String name);
}

public class MyImplementation implements MyInterface {
  public MyImplementation(String name) {
  }
  @Override
  public static MyInterface getInstance(String name) { // static is not allowed here
    return new MyImplementation(name)
  }
}

I understand why a method in the interface cannot be static, but why can't the overriding method be?

I want all classes to implement the getInstance(String name) method, but I'm currently limited to only being able to call the method if the object has already been instantiated which kind of defeats the purpose...

---

*update:* Thanks for the answers, I understand it better now. Basically I shouldn't be trying to make a utility class (or a factory class for that matter) implement an interface (or at least, not in this way)...

Answer 1

Invoking static methods in Java requires you to specify the exact type. It is not possible to invoke static methods polymorphically, eliminating the need for @Override.

Please note that this approach is not universal across all languages: for example, you can override class methods in Objective-C, and Apple's cocoa frameworks make good use of this mechanism to customize their "factory" classes. However, in Java, C++, and C# class methods do not support polymorphic behavior.

Theoretically, Java designers could have let you provide interface method implementations through static methods in case an implementation does not need to access the state from the instance. But the same behavior is simple to achieve with a trivial wrapper:

public class MyImplementation implements MyInterface {
    public MyImplementation(String name) {
    }
    @Override
    public MyInterface getInstance() { // static is not allowed here
        return getInstanceImpl();
    }
    public static getInstanceImpl() {
        return new MyImplementation(name)
    }
}

Java compiler could have done the same thing on your behalf, but seeing a static method implement an instance method is both unusual and confusing, so my guess is that Java designers decided against providing this "piece of magic".

Answer 2

Static methods cannot be subject to polymorphic behavior. That would not make much sense. Image this use case, assuming what you want would be possible:

public void foo(MyInterface i) {
    i.getInstance("abc");
}

now I want to call this method with an implementation of MyInterface (class A), but since I cannot pass the class itself, I need to pass an object:

A a = new A();
foo(a);

now inside foo the static override of getInstance is called on the instance of class A. So now I am stuck with creating an object just to call a static method.

My point is that you would still be constrained to create an object in most use cases of polymorphism since in your original interface the method was an instance method.

Answer 3

because implementing an interface makes the implementor the type of the interface. That means instances need to have the methods defined by the type, not the class of the instances.

To put it another way,

public void mymethod

and

public static void mymethod

are NOT the same method declaration. They are completely distinct. If mymethod is defined on an interface, having the second definition simply does not satisfy implementing the interface.

Answer 4

The answer comes down to what it means to implement an interface. When a class implements an interface, that is a promise that every instance of the class will respond to every method in the interface. When you implement the method as static, you make it possible to call the method without an instance of the class - but that doesn't fulfill the inheritance implementation's promise that the method will be callable on every instance of the class.