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 imple         


        

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".