Why can't I define a static method in a Java interface?

Question

EDIT: As of Java 8, static methods are now allowed in interfaces.

Here\'s the example:

public interface IXMLizable         


        

Answer 1

Several answers have discussed the problems with the concept of overridable static methods. However sometimes you come across a pattern where it seems like that's just what you want to use.

For example, I work with an object-relational layer that has value objects, but also has commands for manipulating the value objects. For various reasons, each value object class has to define some static methods that let the framework find the command instance. For example, to create a Person you'd do:

cmd = createCmd(Person.getCreateCmdId());
Person p = cmd.execute();

and to load a Person by ID you'd do

cmd = createCmd(Person.getGetCmdId());
cmd.set(ID, id);
Person p = cmd.execute();

This is fairly convenient, however it has its problems; notably the existence of the static methods can not be enforced in the interface. An overridable static method in the interface would be exactly what we'd need, if only it could work somehow.

EJBs solve this problem by having a Home interface; each object knows how to find its Home and the Home contains the "static" methods. This way the "static" methods can be overridden as needed, and you don't clutter up the normal (it's called "Remote") interface with methods that don't apply to an instance of your bean. Just make the normal interface specify a "getHome()" method. Return an instance of the Home object (which could be a singleton, I suppose) and the caller can perform operations that affect all Person objects.

Answer 2

• "Is there a particular reason that static methods cannot be overridden".

Let me re-word that question for your by filling in the definitions.

• "Is there a particular reason that methods resolved at compile time cannot be resolved at runtime."

Or, to put in more completely, If I want to call a method without an instance, but knowing the class, how can I have it resolved based upon the instance that I don't have.

Answer 3

Normally this is done using a Factory pattern

public interface IXMLizableFactory<T extends IXMLizable> {
  public T newInstanceFromXML(Element e);
}

public interface IXMLizable {
  public Element toXMLElement();
}

Answer 4

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

Answer 5

With the advent of Java 8 it is possible now to write default and static methods in interface. docs.oracle/staticMethod

For example:

public interface Arithmetic {

    public int add(int a, int b);

    public static int multiply(int a, int b) {
        return a * b;
    }
}

public class ArithmaticImplementation implements Arithmetic {

    @Override
    public int add(int a, int b) {
        return a + b;
    }

    public static void main(String[] args) {
        int result = Arithmetic.multiply(2, 3);
        System.out.println(result);
    }
}

Result : 6

TIP : Calling an static interface method doesn't require to be implemented by any class. Surely, this happens because the same rules for static methods in superclasses applies for static methods on interfaces.

Answer 6

Let's suppose static methods were allowed in interfaces: * They would force all implementing classes to declare that method. * Interfaces would usually be used through objects, so the only effective methods on those would be the non-static ones. * Any class which knows a particular interface could invoke its static methods. Hence a implementing class' static method would be called underneath, but the invoker class does not know which. How to know it? It has no instantiation to guess that!

Interfaces were thought to be used when working with objects. This way, an object is instantiated from a particular class, so this last matter is solved. The invoking class need not know which particular class is because the instantiation may be done by a third class. So the invoking class knows only the interface.

If we want this to be extended to static methods, we should have the possibility to especify an implementing class before, then pass a reference to the invoking class. This could use the class through the static methods in the interface. But what is the differente between this reference and an object? We just need an object representing what it was the class. Now, the object represents the old class, and could implement a new interface including the old static methods - those are now non-static.

Metaclasses serve for this purpose. You may try the class Class of Java. But the problem is that Java is not flexible enough for this. You can not declare a method in the class object of an interface.

This is a meta issue - when you need to do ass

..blah blah

anyway you have an easy workaround - making the method non-static with the same logic. But then you would have to first create an object to call the method.