Class-data versus data and methods versus class-methods in OO ABAP

Question

Hi
I was going thorugh following OO ABAP code.
CLASS vessel DEFINITION.
  PUBLIC SECTION.
    METHODS: constructor,
             drive IMPORTING speed_up TYPE i,
             get_id RETURNING value(id) TYPE i.
    CLASS-METHODS: start,
                   objects,
                   inheritance,
                   interfaces,
                   events.
  PROTECTED SECTION.
    DATA: speed TYPE i,
          max_speed TYPE i VALUE 100.
  PRIVATE SECTION.
    CLASS-DATA object_count TYPE i.
    DATA id TYPE i.
ENDCLASS.
Whats is difference between methods and class-methods ?
What is the difference between data and class-data ?

Answer 1

Hi Rajesh,
There are two types of componenets in a class
1)Static components
2) Instance components
Instance components exist for every instance of the class, while static exist only once for any number of instances of the class.
Components of the class are methods, attributes, events etc.
static attributes are represented by CLASS-DATA and instance attributes are represented by DATA.
static methods hence are done by CLASS-METHODS and can access only static attributes.
Instance methods are done by METHODS and can access any attribute.
For eg: supposing that in a class, there is a static attribute. Suppose after one instance is created, we are setting this static attribute value as 10. Now we are creating another instance of the same class. Now when you try to display the value of this attribute, it will be 10.ie. it needs to be initialized once and can be shared between instances.
Just go through this document..You will get nice info from this.
http://esnips.com/doc/5c65b0dd-eddf-4512-8e32-ecd26735f0f2/prefinalppt.ppt
http://esnips.com/doc/2c76dc57-e74a-4539-a20e-29383317e804/OO-abap.pdf
If you want to go deeper, like object persistence and all, just refer this document.
http://esnips.com/doc/92be4457-1b6e-4061-92e5-8e4b3a6e3239/Object-Oriented-ABAP.ppt
Regards,
SP.