2.2 Object Methods
Object methods implement behavior that objects of that type perform.
Topics:
2.2.1 About Object Methods
Object methods, also known as subprograms, are functions or procedures that you can declare in an object type definition to implement behavior that you want objects of that type to perform. An application calls the subprograms to invoke the behavior.
Subprograms can be written in PL/SQL or virtually any other programming language. Methods written in PL/SQL or Java are stored in the database. Methods written in other languages, such as C, are stored externally.
Note:
SQL requires parentheses for all subprogram calls, even those that do not have arguments. This is not true for PL/SQL.
See Also:
See "Calling Object Constructors and Methods" for more information on invoking methods in PL/SQL.
2.2.2 Member Methods
Member methods provide an application with access to the data of an object instance.
You define a member method in the object type for each operation that you want an object of that type to be able to perform. Non-comparison member methods are declared as either MEMBER FUNCTION or MEMBER PROCEDURE. Comparison methods use MAP MEMBER FUNCTION or ORDER MEMBER FUNCTION as described in "Member Methods for Comparing Objects".
As an example of a member method, you might declare a function get_sum() that sums the total cost of a purchase order's line items. The following line of code calls this function for purchase order po and returns the amount into sum_line_items.
sum_line_items:= po.get_sum();abo
Dot notation specifies the current object and the method it calls. Parentheses are required even if there are no parameters.
Topics:
2.2.2.1 Declaring SELF Parameters in Member Methods
Member methods have a built-in parameter named SELF that denotes the object instance currently invoking the method.
SELF can be explicitly declared, but that is not necessary. It is simpler to write member methods that reference the attributes and methods of SELF implicitly without the SELF qualifier. In Example 2-8, the code and comments demonstrate method invocations that use an implicit SELF parameter rather than qualify the attributes hgt, len, and wth.
Example 2-8 Creating a Member Method
-- Ex. 2-8 Creating a Member Method
CREATE OR REPLACE TYPE solid_typ AS OBJECT (
len INTEGER,
wth INTEGER,
hgt INTEGER,
MEMBER FUNCTION surface RETURN INTEGER,
MEMBER FUNCTION volume RETURN INTEGER,
MEMBER PROCEDURE display (SELF IN OUT NOCOPY solid_typ) );
/
CREATE OR REPLACE TYPE BODY solid_typ AS
MEMBER FUNCTION volume RETURN INTEGER IS
BEGIN
RETURN len * wth * hgt;
-- RETURN SELF.len * SELF.wth * SELF.hgt; -- equivalent to previous line
END;
MEMBER FUNCTION surface RETURN INTEGER IS
BEGIN -- not necessary to include SELF in following line
RETURN 2 * (len * wth + len * hgt + wth * hgt);
END;
MEMBER PROCEDURE display (SELF IN OUT NOCOPY solid_typ) IS
BEGIN
DBMS_OUTPUT.PUT_LINE('Length: ' || len || ' - ' || 'Width: ' || wth
|| ' - ' || 'Height: ' || hgt);
DBMS_OUTPUT.PUT_LINE('Volume: ' || volume || ' - ' || 'Surface area: '
|| surface);
END;
END;
/
CREATE TABLE solids of solid_typ;
INSERT INTO solids VALUES(10, 10, 10);
INSERT INTO solids VALUES(3, 4, 5);
SELECT * FROM solids;
SELECT s.volume(), s.surface() FROM solids s WHERE s.len = 10;
DECLARE
solid solid_typ;
BEGIN -- PL/SQL block for selecting a solid and displaying details
SELECT VALUE(s) INTO solid FROM solids s WHERE s.len = 10;
solid.display();
END;
/
SELF is always the first parameter passed to the method.
-
In member functions, if
SELFis not declared, its parameter mode defaults toIN. -
In member procedures, if
SELFis not declared, its parameter mode defaults toINOUT. The default behavior does not include theNOCOPYcompiler hint.
2.2.2.2 Member Methods for Comparing Objects
To compare and order variables of an object type, you must specify a basis for comparing them.
The values of a scalar data type such as CHAR or REAL have a predefined order, which allows them to be compared. But an object type, such as a person_typ, which can have multiple attributes of various data types, has no predefined axis of comparison. You have the option to define a map method or an order method for comparing objects, but not both.
A map method maps object return values to scalar values and can order multiple values by their position on the scalar axis. An order method directly compares values for two particular objects.
2.2.2.2.1 About Map Methods
Map methods return values that can be used for comparing and sorting.
Return values can be any Oracle built-in data types (except LOBs and BFILEs) and ANSI SQL types such as CHARACTER or REAL. See the specific sections in Oracle Database SQL Language Quick Reference.
Generally, map methods perform calculations on the attributes of the object to produce the return value.
Map methods are called automatically to evaluate such comparisons as obj_1 > obj_2 and comparisons implied by the DISTINCT, GROUP BY, UNION, and ORDER BY clauses which require sorting by rows.
Where obj_1 and obj_2 are two object variables that can be compared using a map method map(), the comparison:
obj_1 > obj_2
is equivalent to:
obj_1.map() > obj_2.map()
Comparisons are similar for other relational operators.
Creating a Map Method defines a map method area() that provides a basis for comparing rectangle objects by their area:
2.2.2.2.2 Creating a Map Method
You create maps using the CREATE TYPE statement.
Example 2-9 Creating a Map Method
CREATE OR REPLACE TYPE rectangle_typ AS OBJECT (
len NUMBER,
wid NUMBER,
MAP MEMBER FUNCTION area RETURN NUMBER);
/
CREATE OR REPLACE TYPE BODY rectangle_typ AS
MAP MEMBER FUNCTION area RETURN NUMBER IS
BEGIN
RETURN len * wid;
END area;
END;
/2.2.2.2.3 Invoking a Map Method
Map methods are invoked in the same manner as other member methods.
Example 2-10 Invoking a Map Method
DECLARE
po rectangle_typ;
BEGIN
po :=NEW rectangle_typ(10,5);
DBMS_OUTPUT.PUT_LINE('AREA:' || po.area()); -- prints AREA:50
END;
/
A subtype can declare a map method only if its root supertype declares one.
See "Comparing Equal and Not Equal Conditions " for the use of map methods when comparing collections that contain object types.
2.2.2.2.4 Order Methods
Order methods make direct one-to-one object comparisons.
Unlike map methods, order methods cannot determine the order of a number of objects. They simply tell you that the current object is less than, equal to, or greater than the object that it is being compared to, based on the criterion used.
An order method is a function for an object (SELF), with one declared parameter that is an object of the same type. The method must return either a negative number, zero, or a positive number. This value signifies that the object (the implicit undeclared SELF parameter) is less than, equal to, or greater than the declared parameter object.
As with map methods, an order method, if one is defined, is called automatically whenever two objects of that type need to be compared.
Order methods are useful where comparison semantics may be too complex to use a map method.
Example 2-11 shows an order method that compares locations by building number:
Example 2-11 Creating and Invoking an Order Method
DROP TYPE location_typ FORCE;
-- above necessary if you have previously created object
CREATE OR REPLACE TYPE location_typ AS OBJECT (
building_no NUMBER,
city VARCHAR2(40),
ORDER MEMBER FUNCTION match (l location_typ) RETURN INTEGER );/
CREATE OR REPLACE TYPE BODY location_typ AS
ORDER MEMBER FUNCTION match (l location_typ) RETURN INTEGER IS
BEGIN
IF building_no < l.building_no THEN
RETURN -1; -- any negative number will do
ELSIF building_no > l.building_no THEN
RETURN 1; -- any positive number will do
ELSE
RETURN 0;
END IF;
END;
END;/
-- invoking match method
DECLARE
loc location_typ;
secloc location_typ;
a number;
BEGIN
loc :=NEW location_typ(300, 'San Francisco');
secloc :=NEW location_typ(200, 'Redwood Shores');
a := loc.match(secloc);
DBMS_OUTPUT.PUT_LINE('order (1 is greater, -1 is lesser):' ||a); -- prints order:1
END;
/Only a type that is not derived from another type can declare an order method; a subtype cannot define one.
2.2.2.2.5 Guidelines for Comparison Methods
You can declare a map method or an order method but not both.
For map and order type methods, you can compare objects using SQL statements and PL/SQL procedural statements. However, if you do not declare one of these methods, you can only compare objects in SQL statements, and only for equality or inequality. Two objects of the same type are considered equal only if the values of their corresponding attributes are equal.
When sorting or merging a large number of objects, use a map method, which maps all the objects into scalars, then sorts the scalars. An order method is less efficient because it must be called repeatedly (it can compare only two objects at a time).
See Also:
2.2.2.2.6 Comparison Methods in Type Hierarchies
In a type hierarchy, if the root type (supertype) does not specify a map or an order method, neither can the subtypes.
-
Map Method in a Type Hierarchy
If the root type specifies a map method, any of its subtypes can override it. If the root type does not specify a map method, no subtype can specify one either.
-
Order Method in a Type Hierarchy
Only the root type can define an order method. If the root type does not define one, its subtypes cannot add one.
2.2.3 Declaring and Invoking Static Methods
Static methods are invoked on the object type, not its instances. You use a static method for operations that are global to the type and do not need to reference the data of a particular object instance. A static method has no SELF parameter.
Static methods are declared using STATIC FUNCTION or STATIC PROCEDURE.
You invoke a static method by using dot notation to qualify the method call with the name of the object type, for example:
type_name.method()
See Also:
For information on design considerations, see "Static Methods".
2.2.4 Constructor Methods
A constructor method is a function that returns a new instance of the user-defined type and sets up the values of its attributes.
Constructor methods are either system-defined or user-defined.
To invoke a constructor, the keyword NEW can be used, but is not required.
See Also:
See Example 2-10 and "Calling Object Constructors and Methods"
2.2.4.1 System-Defined Constructors
By default, the system implicitly defines a constructor function for all object types that have attributes.
A system-defined constructor is sometimes known as the attribute value constructor. For the person_typ object type defined in Example 2-1 the name of the constructor method is the name of the object type, as shown in the following invocation:
person_typ (1, 'John Smith', '1-650-555-0135'),
2.2.4.2 Defining User-Defined Constructors
You can define constructor functions of your own to create and initialize user-defined types.
Default system-defined constructors (or attribute value constructors) are convenient to use because they already exist, but user-defined constructors have some important advantages with respect to type evolution.
See Also:
-
For information on user-defined constructors for collections, see "Using the Constructor Method to Insert Values into a Nested Table".
2.2.4.3 Literal Invocation of a Constructor Method
A literal invocation of a constructor method is a call to the constructor method in which arguments are either literals (as opposed to bind variables), or further literal invocations of constructor methods. For example:
CREATE TABLE people_tab OF person_typ;
INSERT INTO people_tab VALUES (
person_typ(101, 'John Smith', '1-650-555-0135') );