1. OBJECTS & DATABASES
Arnaud Sahuguet – CIS-550

2. OO terminology is very loose. Don’t be surprised
Contents
Why objects
Basics of OO
class
inheritance
encapsulation
object identity
Object-Oriented Database
what’s a schema
ODL
OQL
Examples
Final remarks
OO terminology is very loose. Don’t be surprised
Misc..
ORDBMS
Arnaud Sahuguet – CIS-550

3. Why objects? Better modeling DB & programming languages
objects seem to be a “friendly” paradigm
“Jack” project at PENN (virtual reality)
relational modeling
1 table for people’s heads
1 table for people’s arms
etc.
object-oriented modeling
1 object per person
1 object is composed of sub-objects
DB & programming languages
embedded query languages
C++, Java prefer to speak directly to objects
Arnaud Sahuguet – CIS-550

4. functional programming has no state
What’s an object?
Object = a human abstraction
Object = a black box
from the object point of view
1 state
some associated methods
from the outside point of view
1 hidden state
1 interface (some methods that can be called)
functional programming has no state
Example
a Win’95 window
Arnaud Sahuguet – CIS-550

5. How objects are defined
An object is an instance of a CLASS
constructor/destructor
CLASS
type definition (attributes)
method signature & implementation
abstract class
when some implementation is missing
an abstract class cannot be instantiated
interface
only signatures
Abstract classes and interfaces are extremely useful.
Arnaud Sahuguet – CIS-550

6. Class Person & Subclass Patient
public class Person {
String first_name;
public String _get_first_name(){return first_name;}
public void _set_first_name(String target){first_name = target;}
String last_name;
public String _get_last_name(){ return last_name;}
public void _set_last_name(String target){last_name = target;}
/* missing code */ };
public class Patient extends Person {
Date dob;
char sex;
long mrn;
String insurance_carrier = new String();
long insurance_number;
Provider primary_care_provider; }
Arnaud Sahuguet – CIS-550

7. Encapsulation The inside of the object can be hidden Visibility
to make the object appear simple
to prevent attempt to change the object’s state
Visibility
attribute and methods can be set to
public
full visibility from the outside
private
cannot be accessed from the outside
“restricted” access granted through methods
getXXX()
setXXX()
Arnaud Sahuguet – CIS-550

8. !!! Multiple Inheritance can create AMBIGUITY !!!
Inheritance (I)
Rationale
better modeling
“factoring” common features
specialization
code reuse
Inheritance of
attributes
methods
Single/Multiple inheritance
!!! Multiple Inheritance can create AMBIGUITY !!!
Arnaud Sahuguet – CIS-550

9. Inheritance (II) Class hierarchy Method overriding super-class(es)
sub-class(es)
 casting/coercion
Method overriding
methods can be inherited
a class can rely on a method form its super-class
a class can redefine inherited methods (overriding)
e.g. toString() in Java, display for o2
Arnaud Sahuguet – CIS-550

10. Object identity Objects  type + method OID objects have a state
objects have a unique object identifier (OID)
objects have a life time
from their construction to their destruction
OID
Person p1 = new Person(“Peter”, “Buneman”);
Person p2 = new Person(“Peter”, “Buneman”);
p1 and p2 are NOT the same
Each time a constructor is called, a new OID is created.
Because of the OID,
there is no need for keys, or is there?
Arnaud Sahuguet – CIS-550

11. What is a schema Entities Relations Constraints Documentation
1-1, 1-many, many-1, many-many,
Constraints
referential integrity (key, foreign keys)
null
unique
cardinality
other rules
Documentation
Arnaud Sahuguet – CIS-550

12. How to define a schema? using words using diagrams
it’s always a good start
using diagrams
standard representation (see Derby)
can be used with computer tools (CASE,etc.)
e.g ER diagrams
using Data Definition Languages (DDL)
ODL for object-oriented schema
schema description should be
independent of any implementation
easily mappable to any implementation
easily mappable to any programming language (binding)
Arnaud Sahuguet – CIS-550

13. ODMG (www.odmg.org) Object Model Object Definition Language (ODL)
The ODMG Object Model provides the unifying basis for the entire ODMG standard. The ODMG Object Model extends the OMG Object Model with capabilities like relationships and transactions to support database functionality.
Object Definition Language (ODL)
ODL creates a layer of abstraction such that an ODL-generated schema is independent of both the programming language and the particular ODMG-compliant DBMS.
ODL considers only object type definitions and ignores the actual implementation of methods. An ODL-generated schema can be freely moved between compliant DBMSs, different language implementations, or even translated into other DDLs, such as those proposed by SQL3.
Object Query Language (OQL)
OQL is an SQL-like declarative language that provides a rich environment for efficient querying of database objects, including high-level primitives for object sets and structures.
Language bindings
Arnaud Sahuguet – CIS-550

14. ODL The key concepts of the object model include:
Attributes and relationships object properties
Object operations (behavior) and exceptions
Multiple inheritance
Extents and keys
Object naming, lifetime and identity
Atomic, structured and collection literals
List, set, bag and array collection classes
Concurrency control and object locking
Database operations
Once again, there are different flavors of ODL
Arnaud Sahuguet – CIS-550

15. Some ODL features class attribute struct relationship/inverse
“collection”
set, bag, list, array, dictionary
enum
keys
exception
extents
they define object containers for each class
used by queries
Arnaud Sahuguet – CIS-550

16. OQL OQL syntax (previously presented) Use of extents Advantages of OQL
various terminologies: names, containers
Advantages of OQL
OQL take advantage of object-oriented features
methods
path-expressions (“->”)
de-referenciation
queries are more elegant than in SQL
Examples
movie database
Arnaud Sahuguet – CIS-550

17. Final Remarks OO-DBMS are not studied in the book about ORDBMS
they are not really OO, whatever you can read about it
data is still stored into tables
oo is something like a macro-layer
Bibliography
“A First Course in Database Systems”, by Ullman & Widow Prentice-Hall 1997
“The ODS standard: ODMG-2.0” Morgan Kaufmann 1997
Questions
Arnaud Sahuguet – CIS-550