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I defined an interface IPersistent that has two methods load and save. Using these methods I can load or save the implementing class using whatever method they want.

public interface IPersistent { void save(); void load(int id); }

What I want now, is to implement a deleteAll method, but it would have to be a static method, as it acts on class level, rather than object level. I tried using a static method in the interface, but that is not possible, as I would need to implement it in the interface, instead of any implementing class.

Typical usage would be something like this:

class Foo implements IPersistent { void load(int id) { ... } void save() { ... } static void deleteAll() { ... } } List<foo> fooList = ...; Foo.deleteAll(); for (Foo f: fooList) { f.save(); }

How would I implement this?

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  • Why not use an abstract class instead of interface? Commented Apr 9, 2015 at 14:58
  • 2
    So you have a problem like this ? Commented Apr 9, 2015 at 14:59
  • @Nizil I have Java 8, but it would have only one implementation. I want an implementation per class. Commented Apr 9, 2015 at 15:00
  • @Naveed abstract static seems not allowed Commented Apr 9, 2015 at 15:00
  • How about simply create a new abstract class and put the static method in that class and keep the interface? Commented Apr 9, 2015 at 15:01

3 Answers 3

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You should have an abstract class instead of an interface, and make all of the classes utilizing deleteAll() extend that class.

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4 Comments

abstract static seems not allowed
static methods are allowed in abstract classes.
@Zarathustra yes, but I want to override the static method in each subclass.
@BartFriederichs you can't override static methods, but you can shadow one. but don't do that ;) I prefer a mix of both ways, the one by Nizil and Lareence Aiello. An abstract class with one static generic method.
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As static method can't be overridden, your idea is quite complex to implement in Java.
I think about an other way, using generic static method and annotations.

public final class PersitenceTools { public static <T extends IPersistent> void deleteAll(Class<T> clazz) { // stuff } }

You could use it like this: PersistenceTools.deleteAll(Foo.class).
Note: Since Java 8, you can put this method in the IPersistent interface, if you want to avoid creating a tools class.

You will probably need some information about Foo (or other IPersistent object) in deleteAll. And you can use the power of annotations for this purpose !

@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.TYPE) public @interface PersitenceInfo { String info1(); }

Finally, you will be able to decorate Foo

@PeristenceInfo(info1="Foo") public class Foo implements IPersistent { // stuff }

And get this information in deleteAll through clazz.getAnnotation(PersistenceInfo.class).info1().

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I am trying to implement this, but I get a null on clazz.getAnnotation(PersistenceInfo.class)
@BartFriederichs Oh sorry ! I forget the @Retention(RententionPolicy.RUNTIME) on the annotation, to make it available at runtime ! I also added @Target to ensure this annotation is only available for class, interface or enum.
Well this would work, but its not that OOP like, Avoid Util/Tools classes. @Lawrenace Aiello s answer
@Zarathustra which was my first incling as well, but abstract static is not allowed.
@Zarathustra Actually, OP want a static function (because instance.deleteAll() has no sense), so using an abstract class isn't a good solution for this pupose (moreover, I always promote interfaces over abstract classes). I also think about a solution using a template method pattern, but still hard to implements with static function...
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Follow the java SE convention as seen in:

Path path = Paths.get("..."); File file; ... Files.copy(...); Collection colxn = ...; Collections.sort(colxn);

In your case that would be a utility class with static methods:

public class Persistents { private Persistens() {} public static void deleteAll() { ... } }
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