attributes_methods.md

Attributes (state)

The attributes describe the list of variables that can be valuated in the instance of a class

There are 3 level of attributes

• INSTANCE attributes
• STATIC attributes
• CONSTANT attributes

Instance attributes are defined using DATA keyword

Static attributes are defined using CLASS-DATA keyword

Constant attributes are defined using CONSTANTS keyword

CLASS lcl_flight DEFINITION.

  PUBLIC SECTION.

    DATA : mv_plane_number type string.
    CLASS-DATA: mv_nb_of_planes type i.
    CONSTANTS: mv_manufacturer type string value 'Airbus'.

  PRIVATE SECTION.

    DATA : mv_serial_number type string.
    CLASS-DATA: mv_cockpit_firmware type string value 'EPX-123'.
    CONSTANTS: mv_airbus_key type string value 'AIB01'.

ENDCLASS

Properties

• Constant
  • It exists ONLY ONCE for a class
  • It shares its value to every instance of the same class
  • Cannot be modified

• Static
  • It exists ONLY ONCE for a class
  • It shares its value to every instance of the same class
  • Can be modified

• Instance
  • It exists for every instance of an object of our class BUT it doesn't share it's value with other instances
  • Can be modified

• It can be specified that attribute is in READ-ONLY mode

DATA : mv_instance_readonly_attr type string value 'Instance attribute read-only' READ-ONLY.

Access

• Static and constant attributes are accessible through <class_name> => <class_attribute>

DATA(lv_nb_of_planes) = lcl_flight=>mv_nb_of_planes
DATA(lv_manufacturer) = lcl_flight=>mv_manufacturer

• Instance attributes are accessible through <reference> -> <instance_attribute>

DATA(lv_plane_number) = lo_flight->mv_plane_number

Methods (behavior)

The methods describe the collection of methods that can be used to alter the state of our class or instance itself.

A method has a SIGNATURE describe by:

• none to many IMPORTING parameters
• none to many EXPORTING parameters
• none to many CHANGING parameters
• none to ONE RETURNING parameter
• none to many EXCEPTIONS

  my_first_method IMPORTING iX_param1 TYPE c [OPTIONAL/DEFAULT]
                  EXPORTING eX_param1 TYPE c
                  CHANGING  cX_param1 TYPE c [OPTIONAL/DEFAULT]
                  RETURNING VALUE( rX_param ) TYPE c
                  EXECPTIONS ex_exception.

There are 2 level of methods

• INSTANCE method define below keyword CLASS-METHODS
• STATIC method define below keyword METHODS

Please note the S at the end of the keyword. It's important to understand that the keyword refers to a COLLECTION even if only one method is defined below the keyword.

Properties

• Static

  • It can interact only with static attributes.
  • It can be called without any instance

• Instance

  • It can interact with static attributes AND instance attributes
  • It has to be called in relation to an instance

CLASS lcl_flight DEFINITION.

  PUBLIC SECTION.

    DATA : mv_plane_number type string.
    CLASS-DATA: mv_nb_of_planes type i.
    CONSTANTS: mv_manufacturer type string value 'Airbus'.

    METHODS:  get_plane_number RETURNING VALUE( rv_plane_number ) TYPE c
               give_name IMPORTING iv_name TYPE c.

    CLASS-METHODS:  change_firmware IMPORTING iv_new_firmware TYPE c,
                    get_number_of_planes RETURNING VALUE(rv_nb_of_planes) TYPE i.

  PRIVATE SECTION.

    DATA : mv_serial_number type string.
    CLASS-DATA: mv_cockpit_firmware type string value 'EPX-123'.
    CONSTANTS: mv_airbus_key type string value 'AIB01'.

ENDCLASS

Now we need also to implement these methods, associate a code to it.

This is achieve in the IMPLEMENTATION SECTION

CLASS lcl_flight IMPLEMENTATION.

  METHOD get_plane_number.
    rv_plane_number = mv_plane_number.
  ENDMETHOD.

  METHOD give_name.
    mv_name = iv_name.
  ENDMETHOD.

  METHOD change_firmware.
    mv_cockpit_firmware = iv_new_firmware.
  ENDMETHOD.
  
  METHOD get_number_of_planes.
    rv_nb_of_planes = mv_nb_of_planes.
  ENDMETHOD.
ENDCLASS

Access

• Static methods are callable through <class_name> => <class_method>

lcl_flight=>get_number_of_planes( ).
get_number_of_planes( ). "within the context of an instance or static method

• Instance attributes are callable through <reference> -> <instance_method>

lo_flight->give_name( ).
get_plane_number( ) "within the context of an instance method <=> me->my_first_method( ).

Special methods (constructors)

Class constructor

Properties

• Name is CLASS_CONSTRUCTOR
• PUBLIC
• NO PARAMETER
• UNIQUE for a class
• CALLED ONCE for a L.U.W (local unit of work)

Advantages

In a performance aspect, this constructor can be used to load (SELECT) only once common/shared data and store them as static data which will be accessible by any instance requiring information (READ TABLE)

Constructor

Properties

• Name is CONSTRUCTOR
• PUBLIC
• takes IMPORTING parameters and EXCEPTIONS
• UNIQUE for a class
• CALLED AT EACH reference instanciation

Advantages

A instance constructor allows to give an initial state to the instance.

It's all about referential

When you define a method, you define it as follow :

  my_first_method IMPORTING iX_param1 TYPE c [OPTIONAL/DEFAULT]
                  EXPORTING eX_param1 TYPE c
                  CHANGING  cX_param1 TYPE c [OPTIONAL/DEFAULT]
                  RETURNING VALUE( rX_param ) TYPE c
                  EXECPTIONS ex_exception.

BUT when you call it, you'll use it as follow :

  my_first_method EXPORTING iX_param1 
                  IMPORTING eX_param1
                  CHANGING  cX_param1
                  RECEIVING VALUE( rX_param )
                  EXECPTIONS ex_exception.

It's all about the referential you're considering. Let's take a look at this little diagram

When the observer is from an external referential, the caller is exporting parameters to the method and then he's importing/receiving results from the method.

But when the observer is inside the method referential, the method is importing parameters from outside and then it's exporting/returning value to the outside