Question

I was just wondering can a generic variable be reused. For example if I had a generic class that saves a database model supplied to it:

 DataManager<ModelObject1> manager = new DataManager<ModelObject1>();
 ModelObject1 object = new ModelObject1();
 object.setSomeProperty();
 manager.save(object);

Could I then reuse that same object to save a different type of model without creating a new generic variable. How could I do something like this:

 manager = null;
 manager = new DataManager<ModelObject2>();
 ModelObject2 object2 = new ModelObject2();
 object2.setSomeProperty();
 manager.save(object2);

Answer 1

Due to type erasure, a DataManager<ModelObject1>() is actually exactly the same as a DataManager<ModelObject2>() at creation time so from that perspective there is no reason why it should not be reused – after the correct casting.

However, there are some caveats. If you modify the object to contain a reference to something that is specific to the defining class then things get complicated:

Something like this would mean you cannot re-use the object safely:

DataManager<ModelObject1> manager = new DataManager<ModelObject1>();
manager.setObject(new ModelObject1());

I would strongly recommend against it without a very good reason however. Perhaps if creating the manager is a very heavy process you could justify reusing them.

This code compiles fine but I would stongly recommend an ss.clear() before the cast or an is.clear() after or you will end up with a Set<integer> containing a String :

public static void main(String[] args) {
    Set<String> ss = new HashSet<String> ();
    ss.add("Hello");
    Set<Integer> is = unsecureCast(ss);
}

static Set<Integer> unsecureCast(Set s) {
    return (Set<Integer>) s;
}