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Object Types

An object type is a user-defined composite datatype that encapsulates a data structure along with the functions and procedures needed to manipulate the data. The variables that form the data structure are called attributes. The functions and procedures that characterize the behavior of the object type are called methods. A special kind of method called the constructor creates a new instance of the object type and fills in its attributes.

Object types must be created through SQL and stored in an Oracle database, where they can be shared by many programs. When you define an object type using the CREATE TYPE statement, you create an abstract template for some real-world object. The template specifies the attributes and behaviors the object needs in the application environment.

The data structure formed by the set of attributes is public (visible to client programs). However, well-behaved programs do not manipulate it directly. Instead, they use the set of methods provided, so that the data is kept in a proper state.

An object type encapsulates data and operations. You can declare attributes and methods in an object type spec, but not constants, exceptions, cursors, or types. You must declare at least one attribute (the maximum is 1000). Methods are optional.

Attributes

Like a variable, an attribute is declared with a name and datatype. The name must be unique within the object type (but can be reused in other object types). The datatype can be any Oracle type except:

  • LONG and LONG RAW
  • ROWID and UROWID
  • The PL/SQL-specific types BINARY_INTEGER (and its subtypes), BOOLEAN, PLS_INTEGER, RECORD, REF CURSOR, %TYPE, and %ROWTYPE
  • Types defined inside a PL/SQL package

You cannot initialize an attribute in its declaration using the assignment operator or DEFAULT clause. Also, you cannot impose the NOT NULL constraint on an attribute. However, objects can be stored in database tables on which you can impose constraints.

The kind of data structure formed by a set of attributes depends on the real-world object being modeled. For example, to represent a rational number, which has a numerator and a denominator, you need only two INTEGER variables. On the other hand, to represent a college student, you need several VARCHAR2 variables to hold a name, address, phone number, status, and so on, plus a VARRAY variable to hold courses and grades.

The data structure can be very complex. For example, the datatype of an attribute can be another object type (called a nested object type). That lets you build a complex object type from simpler object types. Some object types such as queues, lists, and trees are dynamic, meaning that they can grow as they are used. Recursive object types, which contain direct or indirect references to themselves, allow for highly sophisticated data models.

Methods

In general, a method is a subprogram declared in an object type spec using the keyword MEMBER or STATIC. The method cannot have the same name as the object type or any of its attributes. MEMBER methods are invoked on instances, as in

instance_expression.method()

However, STATIC methods are invoked on the object type, not its instances, as in

object_type_name.method()

Like packaged subprograms, methods have two parts: a specification and a body. The specification (spec for short) consists of a method name, an optional parameter list, and, for functions, a return type. The body is the code that executes to perform a specific task.

For each method spec in an object type spec, there must either be a corresponding method body in the object type body, or the method must be declared NOT INSTANTIABLE to indicate that the body is only present in subtypes of this type. To match method specs and bodies, the PL/SQL compiler does a token-by-token comparison of their headers. The headers must match exactly.

Like an attribute, a formal parameter is declared with a name and datatype. However, the datatype of a parameter cannot be size-constrained. The datatype can be any Oracle type except those disallowed for attributes. (See “Attributes”.) The same restrictions apply to return types.


Published at :
Written By
Evaristus Didik
Concentration & Content Coordinator | School of Information Systems http://sis.binus.ac.id/
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