Java，通用和getAll

Question

I'm trying to modify my code to avoid all the duplicate code, using generic.

I have a class DBBaseClass wich will be extended by some other class. I'm trying to create a getAll method that return all the object of the subclass stored in a database.

 class abstract DBBaseClass {

  abstract readDataBase(int _id);

  public  <T extends DBBaseClass> List<T> getAll()
  {

 List<T> array = new ArrayList<T>();    

 ArrayList<Integer> myints = DBManager.request_database_for_ids(getIdField(), getTableName());

 T myTObject = new T();    // NOT WORKING  *************************

 for (int i=0;i<myints.size();i++) {        
        myTObject.read(myints.get(i));
        array.add(myTObject);
    }               

return array;

I have the errors : - Cannot instantiate the type T - The type T is not generic; it cannot be parameterized with arguments <>

How can i instanciate my T subclass here ??

What's the trick ??

Thanks.

EDIT :

To explain more in details my goal : I have two classes SUbClassA and SubClassB that extends DBBaseClass.

I want to a method : SubClassA.getAll() that returns an arraylist of SUbClassA objects. SubClassB.getAll() that returns an arraylist of SUbClassB objects.

At first, i created getAll in all my subClasses ... So, I'm trying now to put this getAll() in the DBBaseclass.

It's easy to do if i want them to return List but i don't know how to tell the method "return me a list of this particular kind of subclass".

Answer 1

您想要使DBBaseClass通用，为此声明它是这样的：

abstract class DBBaseClass<T extends DBBaseClass<?>>

Answer 2

You only can instantiate a concret class but not T as it is only a type and not DBBaseClass as it is abstract.

What could you do:

• Define a concrete class extending DBBaseClass such as SubDBBaseClass

• Create a factory method for each sub-class that returns a concrete instance such as

   protected T create() { return new SubDBBaseClass(); } 

• Invoke this method in your DBBaseClass.getAll() method such as

   T myTObject = create(); 

Perhaps you didn't know, but your are using the active record pattern that has its ups and downs. I don't like the pattern and I would redesign the architecture. But these are only my 5 cents.