1) Initilization Package Means Caching
1@@@@ The Package Specification
It is easy to understand that, the code in package body with begin..end is initilized code
LIKE:
    CREATE OR REPLACE PACKAGE BODY pkg_name
    IS
    BEGIN
      --Here codes would be runed one time only.
    END;

  Here are some rules to keep in mind for package specification construction:
  ~ You can declare elements of almost any datatype, such as numbers, exceptions,
    types, and collections, at the package level (i.e., not within a particular procedure
    or function in the package). This is referred to as package-level data; generally, you
    should avoid declaring variables in the package specification, although constants
    are always ¡°safe.¡±
    You cannot declare cursor variables (variables defined from a REF CURSOR type)
    in a package specification (or body). Cursor variables are not allowed to persist at
    the session level (see ¡°Working with Package Data¡± on page 633 for more information
    about package data persistence).

  ~ You can declare almost any type of data structure, such as a collection type, a record
    type, or a REF CURSOR type.

  ~ You can declare procedures and functions in a package specification, but you can
    include only the header of the program (everything up to but not including the IS
    or AS keyword). The header must end with a semicolon.

  ~ You can include explicit cursors in the package specification. An explicit cursor
    can take one of two forms: it can include the SQL query as a part of the cursor
    declaration, or you can ¡°hide¡± the query inside the package body and provide only
    a RETURN clause in the cursor declaration. This topic is covered in more detail in
    the section, ¡°Packaged Cursors¡± on page 635.

  ~ If you declare any procedures or functions in the package specification or if you
    declare a CURSOR without its query, then you must provide a package body in
    order to implement those code elements.

  ~ You can include an AUTHID clause in a package specification, which determines
    whether any references to data objects will be resolved according to the privileges
    of the owner of the package (AUTHID DEFINER) or of the invoker of the package
    (AUTHID CURRENT_USER). See Chapter 24 for more information on this
    feature.

  ~ You can include an optional package name label after the END statement of the
    package, as in:
        END my_package;

   @@@Example: Note the initilization between begin and end;
    SYS@ocm> !cat tmp.sql
    DROP TABLE favorites
    /
    CREATE TABLE favorites
            (  name VARCHAR2(100)
             , code INTEGER   )
    /
    BEGIN
      DELETE FROM favorites;
      INSERT INTO favorites VALUES('fatpander_small_one',1);
      INSERT INTO favorites VALUES('fatpander_big_one',2);
      COMMIT;
    END;
    /
    CREATE OR REPLACE PACKAGE favorites_pkg
      AUTHID CURRENT_USER
    IS
        --Two constants;
        c_chocolate   CONSTANT  PLS_INTEGER := 16;
        c_strawberry  CONSTANT  PLS_INTEGER := 29;
   
        --A nested table Type declaration
        --A nested table declared from the generic type.
        TYPE my_favorites_ntt IS TABLE OF INTEGER;
        my_favorites_nt   my_favorites_ntt;
   
        --A REF CURSOR returning favorites information
        TYPE fav_info_rct IS REF CURSOR
           RETURN favorites%ROWTYPE;
   
        --A procedure that accepts a list of favorites(nest table type)
        --and display the favorites information from that list.
        PROCEDURE show_favorites (  list_in IN  my_favorites_ntt);
   
        --A function that returns all the information in the favorites
        --table about the most popular item.
        FUNCTION most_popular RETURN fav_info_rct;
   
    END;
    /
    SYS@ocm> @tmp.sql
    Table dropped.
    Table created.
    PL/SQL procedure successfully completed.
    Package created.
   
    SYS@ocm> !cat tmpx.sql
    CREATE OR REPLACE PACKAGE BODY favorites_pkg
    IS
        --A private vairable
        g_most_popular   PLS_INTEGER;
       
        --Implementation of procedure
        PROCEDURE show_favorites
            ( list_in  IN  my_favorites_ntt )
        IS
        BEGIN
           FOR i IN list_in.FIRST .. list_in.LAST
           LOOP
              DBMS_OUTPUT.put_line( list_in(i) );
           END LOOP;
        END;
       
        --Implementation of procedure
        FUNCTION most_popular
          RETURN fav_info_rct
        IS
           retval  fav_info_rct;
           null_cv fav_info_rct;
        BEGIN
           OPEN retval FOR
              SELECT * FROM favorites
               WHERE code = g_most_popular;
           RETURN retval;
        EXCEPTION
           WHEN NO_DATA_FOUND THEN
              RETURN null_cv;
        END;
   
        PROCEDURE analyze_favorites
                (  year_in  IN  INTEGER )
        IS
        BEGIN
           DBMS_OUTPUT.put_line('This year is '||year_in);
        END;
    BEGIN
       --Initilization, one session one time
       g_most_popular := c_chocolate;
       analyze_favorites ( EXTRACT(YEAR FROM sysdate) );
    END;
    /
    SYS@ocm> @tmpx.sql
    Package body created.
   
   
    @@@
    SYS@ocm> !cat tmpxx.sql
    DECLARE
       show_nt    favorites_pkg.my_favorites_ntt
            := favorites_pkg.my_favorites_ntt(1,2,3);
    BEGIN
      favorites_pkg.show_favorites(show_nt);
    END;
    /
   
    SYS@ocm> @tmpxx.sql
    This year is 2013  --One session one time!!! initilization.
    1
    2
    3
    PL/SQL procedure successfully completed.
   
    SYS@ocm> @tmpxx.sql
    1
    2
    3
    PL/SQL procedure successfully completed.