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:

You create maps using the CREATE TYPE statement.

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;
/

Map methods are invoked in the same manner as other member methods.

DECLARE
  po rectangle_typ;
 
BEGIN
  po :=NEW rectangle_typ(10,5);
 
DBMS_OUTPUT.PUT_LINE('AREA:' || po.area()); -- prints AREA:50
  END;
/

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:

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;
/

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).

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.