Java, declare variable with multiple interfaces?

Question

In Java, is it possible to declare a field/variable whose type is multiple interfaces? For example, I need to declare a Map that is also Serializable. I want to make sure the variable references a serializable map. The Map interface does not extend Serializable, but most of Map's implementations are Serializable.

I'm pretty sure the answer is no.

Follow up: I'm fully aware of creating a new interface that extends both Map and Serializable. This will not work as existing implementations (such as HashMap) do not implement my new interface.

Answer 1

You can do it with generics, but it's not pretty:

class MyClass<T,K,V extends Serializable & Map<K,V>> {

   T myVar;

}

Answer 2

There is no need to declare the field/variable like that. Especially since it can only be tested runtime and not compile time. Create a setter and report an error should the passed Map not implement Serializable.

The answers recommending that you create your own interface are of course not very practical as they will actively prohibit sending in things that are Maps and Serializable but not your special interface.

Answer 3

It's possible to do this using some generics tricks:

    public <T extends Map<?,?> & Serializable> void setMap(T map)

The above code uses generics to force you to pass a map which implements both interfaces. However, note that a consequence of this is that when you actually pass it the maps, they will probably need to be either marked as serializable or of a map type which is already serializable. It also is quite a bit more difficult to read. I would document that the map must be serializable and perform the test for it.