How to initialize an object with a list of different types?

Question

Hey, I'm initializing a ListRepository with two different types of initialization lists. The best way would be something like this.

public ListRepository(String id, List<PrimaryKey> initilizationList)
{
   // Load objects from data source via primary key.
}

public ListRepository(String id, List<DomainObject> initilizationList)
{
   // Store objects directly
}

Unfortunately this is not possible due to runtime type erasure. I don't like a constructor approach with List<?> as an argument, this leads to an ugly instanceof check of the first entry, to determine the list type and handle it.

How do you solve such a problem with an intuitive and clean API?

Answer 1

The constructor is doing far more than initialising the object. It is best to keep constructors simple.

I would use a static method to perform each query.

public static ListRepository<PrimaryKey> loadFromPrimaryKey(String id, List<PrimaryKey> initilizationList) {
   // Load objects from data source via primary key.
}

public static ListRepository<PrimaryKey> loadFromDomainObject(String id, List<DomainObject> initilizationList) {
   // Store objects directly
}

You would have one constructor which just takes the resulting data. This would make it much clearer as to what the methods will build.

Answer 2

Use factory method

class ListRepository {     
    public static ListRepository createPrimaryKeysRepository(String id, List<PrimaryKey> initilizationList){}
    public static ListRepository createDomainObjectsRepository(String id, List<DomainObject> initilizationList){}
}