Generics Type casting?

Question

In a abstract factory pattern i am using generics. I have BaseEntity interface which extends Serializable, the Employee class implements the BaseEntity. In the abstract class i have this getJavaObj method

getJavaObj() is a method which takes a Long empId and returns a Map<String, ? extends BaseEntity>

public abstract Map<String, ? extends BaseEntity> getJavaObj(Long id);

Using generics i am trying to do this in the main class it gives;

Map<String, Employee> emp = getJavaObj(empId);

It gives this error Type safety: Unchecked cast from Map<String,capture#1-of ? extends BaseEntity> to Map

When i do the type casting like this

Map<String, Employee> emp = (Map<String, Employee>)getJavaObj(empId);

It gives this warning

Type safety: Unchecked cast from Map<String,capture#1-of ? extends Serializable> to Map Type safety: Unchecked cast from Map<String,capture#1-of ? extends BaseEntity> to Map

Is there a way to avoid type-casting or resolve the warnings even after typecasting? Since the object i am returning does extend to the Serializable via BaseEntity interface.

Answer 1

Redefine your factory method:

public abstract <T extends BaseEntity> Map<String, T> getJavaObj(Long id);

Then you can get the correct type back without a cast:

Map<String, Employee> emp = getJavaObj(empId);