CODEWITHSUNDEEP
javagenerics
LppEdd
LppEdd

Reputation: 21124

Java generics - compile error on method parameter and method return type

Say I've got two interfaces:

public interface IObjectOne<T>
{
    List<T> digest(final List<T> myList);
}

public interface IObjectTwo<T>
{
    List<T> create();
}

And an object which manages concrete implementations of these two interfaces, which must work over the same type.

public class Entity<T>
{
  private IObjectOne<T> objectOne;
  private IObjectTwo<T> objectTwo;

  ... setters

  public IObjectOne<T> getObjectOne() {
     return objectOne;
  }

  public IObjectTwo<T> getObjectTwo() {
     return objectTwo;
  }
}

These Entitys are stored in a Map, and I retrive one of them by a key:

final Entity<?> entity = entities.get(key);

Why cannot I do that?

entity.getObjectOne().digest(entity.getObjectTwo().create());

Upvotes: 0

Views: 182

Answers (3)

GhostCat
GhostCat

Reputation: 140427

Here:

final Entity<?> entity = entities.get(key);

you are saying: you absolutely do not care about the exact type of an entity. Reminder: wildcard means "I don't know" about the exact type.

And one consequence of using the wildcard for generics, as described here:

If you want to enforce some relationship on the different types of method arguments, you can't do that with wildcards, you have to use type parameters.

In other words: by using the wildcard like this, the compiler is unable to correlate the generic types.

Upvotes: 2

xtratic
xtratic

Reputation: 4699

How are you creating the Map<String, Entity<YOUR_TYPE>>

You should parameterize your Entity<> since it is generic.

Map<String, Entity<YOUR_TYPE>> entities = new HashMap<>();
final Entity<YourType> entity = entities.get(key);

Upvotes: 0

Andy Turner
Andy Turner

Reputation: 140319

To do this, you need to do your operation in a method with a type variable:

<T> void doSomething(Entity<T> entity) {
  entity.getObjectOne().digest(entity.getObjectTwo().create());
}

You can invoke this even for an Entity<?>, because that wildcard has some type, you just don't know it.

final Entity<?> entity = entities.get(key);
doSomething(entity);

Upvotes: 2

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