Java中的多态性和继承与抽象类的静态方法

Question

I was going through the sample questions of Java 8 Programmer I exam on Oracle website and came across the following question:

abstract class Writer {
  public static  void write() {
    System.out.println("Writing...");
  }
 }
class Author extends Writer {
public static void write() {
       System.out.println("Writing book");
    }
 }
class Programmer extends Writer {
   public static void write() {
     System.out.println("Writing code");
   }
public static void main(String[] args) {
  Writer w = new Author();
   w.write();//What would be the ouput here?
    }
 }

The correct answer is that the method of the abstract class is called.

Now, my understanding was that in polymorphism, if a variable of type parent class contains a reference to an object of subclass, then the method of the subclass will be called.

Therefore, am I understanding right that in case of a static function, the method of the of the class whose variable contains the refrence would be called?

Answer 1

There is no polymorphism for static methods.

The compiler decides at compile time which method is going to be invoked.

It sees that w is a Writer, and it does neither know or care that the actual instance will be of that specific sub class at runtime. The compiler could know it sometimes, but Java takes the easy path here.

That is one of the reasons why you are really careful about using static methods in real world production code: doing so means sacrificing one of the essential elements of OOP.

Answer 2

Nope, you can't override static methods in a subclass, so there's no polymorphic runtime lookup of the method. Static methods have no interaction with instance fields/methods of the class, so making a call to a static method dependent on a particular instance/subclass wouldn't make sense.

It's easier to think of static methods as just utility/library methods. And they are contained within classes just for the purpose of organization.