Question

I have an interface and a class implementing the interface like this:

public interface DocumentDAO<T>{
    public long insert (T doc);
}

public class PostgreSQLDocumentDAO implements DocumentDAO<Document>{
    public long insert(Document doc){}
    protected void insertDocument(Document doc){}
}

I want to override the insert method of PostgreSQLDocumentDAO changing the parameter from Document to Page, where Page is a class that extends Document.

I want something like this:

public class PostgreSQLPageDAO extends PostgreSQLDocumentDAO{
    public long insert(Page doc){
        super.insertDocument(page);
        //specific implementation goes here
    }
}

How I can get this?

---

UPDATE: The code above isn’t right because a class called Inserter uses the interface DocumentDAO. When it calls the insert method, even if is a PostgreSQLPageDAO object, it will call the insert method to insert Document because this method is inherited.

I want to do a way to when the Inserterclass call the insert method, it uses the method that inserts the page. In this way, I think I need override the insert (Document doc) from PostgreSQLDocumentDAO.

Answer 1

Two options here.

1. You can modify PostgreSQLDocumentDAO

Then you change it to

public class PostgreSQLDocumentDAO<D extends Document> implements DocumentDAO<D>

and

public class PostgreSQLPageDAO extends PostgreSQLDocumentDAO<Page>

2.You cannot modify PostgreSQLDocumentDAO

Then you need to overload and override the insert method in PostgreSQLPageDAO with:

// Override
@Override
public long insert(Document doc){
    if (!(doc instanceof Page))
         throw new IlegalArgumentException("insert only accepts Page");
    super.insert(doc);
}

// Overload
public long insert(Page page){
    super.insert(page);
}