"Casting" from a Superclass to a subclass

[Sankar Sarma]

Hello All

I have a design question regarding the above:

I have a generic data class that has a variable list of attributes - each representing a column in a table (name, type etc.). I have a class that reads data from the database and populates instances of the data class (e.g., there is a load(ID, tableName) method that returns an instance of the data for the given table).

I also have a bunch of classes that extend the above class - these represent actual tables and have the actual attributes of each table. these are used as POJOs with getters and setters.


e.g

  
 public interface DataIF { 
public boolean isDirty(); 
public String getAttribute(String attribute); 
public void setAttribute(String attrName, String Attribute)
}
 

    
public Data implements DataIF{

public DataIF load(String ID, String tableName) {

        private List attrList = new ArrayList();
      ...connect to database
      ...get data for the id
     ... load individual column values
}

public User extends DataIF{

private String userID;
private String userName;

}

 

What I want to be able to do is to "cast" the generic data class as a POJO - e.g. when my Data instance is returned, I'd like to map the data values for each column to the actual instance variables in the subclass.


What would be an ideal design for something like this.

Thanks in advance.

[Ryan Beckett]

I don't see a class named POJO nor a generic class. I thought you meant Java generics.

What I want to be able to do is to "cast" the generic data class as a POJO

You can downcast, but there's no compile-time error checking. So, you may get a ClassCastException.

 public class Test
{
	static public void main(String... args)
	{
		
		Object obj = new Object();
		String s = (String)obj; //throws ClassCastException
		System.out.println(s);
		
		Object obj2 = new String("This would print if an exception wasn't thrown");
		String s2 = (String)obj2; //OK since the instance type is a String
		System.out.println(s2);
	}
}
   

[Campbell Ritchie]

If those objects all implement the same interface, try to declare them of that interface type, then maybe you can avoid the class cast.

[Sankar Sarma]

Campbell Ritchie wrote:If those objects all implement the same interface, try to declare them of that interface type, then maybe you can avoid the class cast.

Thank you all for the reply (Sorry for the late ack, but I am new and I kept looking at 'My Posts' and it only shows my version )

The objects return a POJO that represents a 'table' with getters and setters for each field - the generic object has a map that holds reference to each field... I'll probably use a 'translator' service to map the mapped objects to actual attributes.