I thought interfaces couldn't be instantiated in java

Question

I'm working on some JPA stuff and i'm a little confused with some of the start up code that you have to write.

EntityManagerFactory factory = Persistence.createEntityManagerFactory("sample");
EntityManager manager = factory.createEntityManager();
EntityTransaction transaction = manager.getTransaction();

Those three variables all have an interface as their type. How can we do things like

manager.persist()
transaction.commit()

etc if interfaces cannot be instantiated?

score 2 · Answer 1 · answered Jul 17 '13 at 17:49

Interface cannot be instantiated but an interface reference can hold an object of any class implementing that interface. So in your case

EntityManagerFactory factory

is a reference of interface which is holding an object of the class implenting it , returned by :

Persistence.createEntityManagerFactory("sample");

and hence this statement becomes correct:

EntityManagerFactory factory = Persistence.createEntityManagerFactory("sample");

snrlx · Answer 2 · 2013-07-17T17:57:23.620

1

You are correct. Interfaces can not be instantiated but they provide a contract to call methods on objects that implement the interfaces.

So when you for example take a look at the EntityManager, factory.createEntityManager() is returning an object that implements the interface EntityManager. Interfaces make sure that the returned object provides certain required methods.

edited Jul 17 '13 at 17:57

answered Jul 17 '13 at 17:50

snrlx

4,987
2
27
34

I don't know if I'd say it can be a _very different_ type. It _will_ be an implementation class, but that class _will_ implement `EntityManager` – Colin M Jul 17 '13 at 17:55
You are absolutely right. It is confusing to say it like that. – snrlx Jul 17 '13 at 17:56
So is this similar to how `List` is an interface but `LinkedList` and `ArrayList` can be pointed to by a variable of type `List` ? – rage Jul 17 '13 at 17:58
Yes, LinkedList and ArrayList are concrete implementations of the List interface. – snrlx Jul 17 '13 at 18:01
So how can i see which objects that implement those interfaces are being returned? So i can see how they implement the methods that the interface defines. – rage Jul 17 '13 at 18:10

score 0 · Answer 3 · answered Jul 17 '13 at 17:55

I think you are misunderstand what happened here.

  EntityManager manager = factory.createEntityManager(); // here manager is only a reference You are getting that from EntityManagerFactory.

Now Factory class returns the manager type object. There is no instantiating for interfaces in Java

score 0 · Answer 4 · answered Jul 17 '13 at 17:58

I think this is better seen than expained.

Examples:

public class AttackCommand implements Command {}...
public class DefendCommand implements Command {}...
....

let's say we wanted to add these to a common list of commands. Then you could add these to the list below.

(in a new class)

public ArrayList<Command> commands = new ArrayList();

public CommandManager() {
    commands.add(new AttackCommand());
    commands.add(new DefendCommand());
}

Now here is where that supposed reference comes in. What if we wanted to get a list of the command by name (pretending command has a getName method), or the last attacked target via AttackCommand (pretending it has a LastAttacked method)?.

public void printNames() {
    for (Command cmd : commands) {
        System.out.println(cmd.getName());
    }
}

public Entity getLastAttackTarget() {
    for (Command cmd : commands) {
        if (cmd instanceof AttackCommand) {
            return cmd.lastAttacked();
        }
    }
}

(I know that a map of the commands to just grab by name would be better, but for the sake of the example....)

In essence, it's a better general reference to all things that inherit the interface, but not the interface itself.

I thought interfaces couldn't be instantiated in java

4 Answers4