Object Type Declaration

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. For information on the CREATE TYPE SQL statement, see Oracle Database SQL Reference. For information on the CREATE TYPE BODY SQL statement, see Oracle Database SQL Reference.

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.

For more information on object types, see Oracle Database Application Developer's Guide - Object-Relational Features. For information on using PL/SQL with object types, see Chapter 12, "Using PL/SQL With Object Types".

Usage Notes

Once an object type is created in your schema, you can use it to declare objects in any PL/SQL block, subprogram, or package. For example, you can use the object type to specify the datatype of an object attribute, table column, PL/SQL variable, bind variable, record field, collection element, formal procedure parameter, or function result.

Like a package, an object type has two parts: a specification and a body. The specification (spec for short) is the interface to your applications; it declares a data structure (set of attributes) along with the operations (methods) needed to manipulate the data. The body fully defines the methods, and so implements the spec.

All the information a client program needs to use the methods is in the spec. Think of the spec as an operational interface and of the body as a black box. You can debug, enhance, or replace the body without changing the spec.

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. At least one attribute is required (the maximum is 1000); methods are optional.

In an object type spec, all attributes must be declared before any methods. Only subprograms have an underlying implementation. If an object type spec declares only attributes and/or call specs, the object type body is unnecessary. You cannot declare attributes in the body. All declarations in the object type spec are public (visible outside the object type).

You can refer to an attribute only by name (not by its position in the object type). To access or change the value of an attribute, you use dot notation. Attribute names can be chained, which lets you access the attributes of a nested object type.

In an object type, methods can reference attributes and other methods without a qualifier. In SQL statements, calls to a parameterless method require an empty parameter list. In procedural statements, an empty parameter list is optional unless you chain calls, in which case it is required for all but the last call.

From a SQL statement, if you call a MEMBER method on a null instance (that is, SELF is null), the method is not invoked and a null is returned. From a procedural statement, if you call a MEMBER method on a null instance, PL/SQL raises the predefined exception SELF_IS_NULL before the method is invoked.

You can declare a map method or an order method but not both. If you declare either method, you can compare objects in SQL and procedural statements. However, if you declare neither method, you can compare objects only in SQL statements and only for equality or inequality. Two objects of the same type are equal only if the values of their corresponding attributes are equal.

Like packaged subprograms, methods of the same kind (functions or procedures) can be overloaded. That is, you can use the same name for different methods if their formal parameters differ in number, order, or datatype family.

Every object type has a default constructor method (constructor for short), which is a system-defined function with the same name as the object type. You use the constructor to initialize and return an instance of that object type. You can also define your own constructor methods that accept different sets of parameters. PL/SQL never calls a constructor implicitly, so you must call it explicitly. Constructor calls are allowed wherever function calls are allowed.

Related Topics

"Function Declaration"
"Package Declaration"
"Procedure Declaration"