Form Routines

Question

Hi  ABAP Gurus,
                         i am new to abap.i Have some small doubts.please explain me some doubts.please do explain me in simple words dont send links.
1. what is a FORM Routine?
2. what is importing ,Exporting?
3. what is the diff. b/w exporting & Returning?
4.what is changing?
5. what is returning ,receiving?
6.what is PERFORM Statement?
7.explain me about public,protected and private?
note:please do explain me these with simple coding so that i can be familiar to know where to use them.

Answer 1

Check this link -
http://www.geocities.com/rmtiwari/Resources/Utilities/ABAPReference/ABAPReference.html
PERFORM
Variants:
1. PERFORM form.
2. PERFORM form IN PROGRAM prog.
3. PERFORM n OF form1 form2 form3 ... .
4. PERFORM n ON COMMIT.
5. PERFORM n ON ROLLBACK.
6. PERFORM form(prog).
Variant 1
PERFORM form.
Extras:
1. ... USING    p1 p2 p3 ... 2. ... CHANGING p1 p2 p3 ...
3. ... TABLES   itab1 itab2 ...
Effect
Calls the subroutine form defined usng a FORM statement. After the subroutine has finished, processing continues after the PERFORM statement. The parameters of the subroutine are position parameters, and must be passed in the call according to the definition of the formal parameters in the corresponding FORM statement.
Example
PERFORM HELP_ME.
FORM HELP_ME.
ENDFORM.
The PERFORM statement calls the subroutine HELP_ME.
Note
Non-Catchable Exceptions:
PERFORM_PARAMETER_MISSING: The subroutine called has more parameters than you passed to it.
PERFORM_TOO_MANY_PARAMETERS: You passed more parameters to the subroutine than it was expecting.
PERFORM_CONFLICT_TYPE,
PERFORM_CONFLICT_TAB_TYPE,
PERFORM_STD_TAB_REQUIRED,
PERFORM_CONFLICT_GENERIC_TYPE,
PERFORM_BASE_WRONG_ALIGNMENT,
PERFORM_PARAMETER_TOO_SHORT,
PERFORM_CAST_DEEP_MISMATCH,
PERFORM_TABLE_REQUIRED: The type of a parameter did not correspond to the type specified in the FORM statement.
Addition 1
... USING    p1 p2 p3 ...
Addition 2
... CHANGING p1 p2 p3 ...
Effect
These two additions have an identical function. However, you should always use the same addition as is used in the corresponding FORM definition (for documentary reasons).
The statement passes the parameters p1 p2 p3 ... to the subroutine. A subroutine may have any number of parameters.
The order in which you list the parameters is crucial. The first parameter in the PERFORM statement is passed to the first formal parameter in the FORM defintion, the second to the second, and so on.
You can specify offset and length of a parameter as variables. If you use the addition ' ...USING p1+off(*)', the parameter p1 will be passed with the offset off, but the length will not exceed the total length of the field.
Example
DATA: NUMBER_I TYPE I VALUE 5,
      NUMBER_P TYPE P VALUE 4,
      BEGIN OF PERSON,
        NAME(10)      VALUE 'Paul',
        AGE TYPE I    VALUE 28,
      END   OF PERSON,
      ALPHA(10)       VALUE 'abcdefghij'.
FIELD-SYMBOLS <POINTER> TYPE ANY.
ASSIGN NUMBER_P TO <POINTER>.
PERFORM CHANGE USING 1
                     NUMBER_I
                     NUMBER_P
                     <POINTER>
                     PERSON
                     ALPHA+NUMBER_I(<POINTER>).
FORM CHANGE USING VALUE(PAR_1)
                  PAR_NUMBER_I
                  PAR_NUMBER_P
                  PAR_POINTER
                  PAR_PERSON STRUCTURE PERSON
                  PAR_PART_OF_ALPHA.
  ADD PAR_1 TO PAR_NUMBER_I.
  PAR_NUMBER_P = 0.
  PAR_PERSON-NAME+4(1) = ALPHA.
  PAR_PERSON-AGE = NUMBER_P + 25.
  ADD NUMBER_I TO PAR_POINTER.
  PAR_PART_OF_ALPHA = SPACE.
ENDFORM.
Field contents after the PERFORM statement:
NUMBER_I    = 6
NUMBER_P    = 6
<POINTER>   = 6
PERSON-NAME = 'Paula'
PERSON-AGE  = 25
ALPHA       = 'abcde    j'
Notes
   1. If you want to pass the body of an internal table itab that has a header line, you must use the notation itab[] (see Data Objects). If you do not use the brackets, the header line of the tabel is passed.
   2. The field types and lengths of the parameters remain the same. If a parameter is changed within the subroutine, it will still have the changed value after the subroutine has finished. This does not apply to parameters passed using VALUE. werden.
   3. If you pass literals, they may not be changed unless you pass them to a formal parameter defined with USING VALUE.
Addition 3
... TABLES itab1 itab2 ...
Effect
TABLES allows you to pass internal tables to a subroutine.
Example
TYPES: BEGIN OF ITAB_TYPE,
         TEXT(50),
         NUMBER TYPE I,
       END OF ITAB_TYPE.
DATA:  ITAB TYPE STANDARD TABLE OF ITAB_TYPE WITH
                 NON-UNIQUE DEFAULT KEY INITIAL SIZE 100,
       BEGIN OF ITAB_LINE,
         TEXT(50),
         NUMBER TYPE I,
       END OF ITAB_LINE,
       STRUC like T005T.
PERFORM DISPLAY TABLES ITAB
                USING  STRUC.
FORM DISPLAY TABLES PAR_ITAB STRUCTURE ITAB_LINE
             USING  PAR      like      T005T.
  DATA: LOC_COMPARE LIKE PAR_ITAB-TEXT.
  WRITE: / PAR-LAND1, PAR-LANDX.
  LOOP AT PAR_ITAB WHERE TEXT = LOC_COMPARE.
  ENDLOOP.
ENDFORM.
Within the subroutine DISPLAY, you can use any internal table operation to work with the internal table that you passed to it.
Note
If you use TABLES, it must always be the first addition in a PERFORM statement.
Variant 2
PERFORM form IN PROGRAM prog.
Extras:
1. ... USING    p1 p2 p3 ... 2. ... CHANGING p1 p2 p3 ...
3. ... TABLES   itab1 itab2 ...
4. ... IF FOUND
Passsing SY-REPID not allowed and Receiving SY-SUBRC not allowed.
Effect
This variant is similar to variant 2 (external perform). However, here you can specify the names of both the subroutine and the program in which it occurs dynamically at runtime. If you do this, you should place the variables form and prog in parentheses. The names in form and prog must be entered in uppercase, otherwise a runtime error occurs. If you do not specify any additions (such as USING) you do not need to specify the program after IN PROGRAM. In this case, the system looks for the subroutine in the current program.
Example
DATA: RNAME(30) VALUE 'WRITE_STATISTIC',   "Form and program
                                           "names must
      PNAME(8)  VALUE 'ZYX_STAT'.          "be written in
                                           "upper case
PERFORM (RNAME)         IN PROGRAM ZYX_STAT.
PERFORM WRITE_STATISTIC IN PROGRAM (PNAME).
PERFORM (RNAME)         IN PROGRAM (PNAME).
All three PERFORM statements have the same effect, that is, they call the subroutine 'WRITE_STATISTIC', which is defined in the program 'ZYX_STAT'.
Notes
Dynamic PERFORM statements require more CPU time, since the system has to locate the subroutine each time.
Note
Non-Catchable Exceptions:
PERFORM_NOT_FOUND: Unable to find the specified subroutine.
LOAD_PROGRAM_NOT_FOUND: Unable to find the specified main program.
PERFORM_PROGRAM_NAME_TOO_LONG: The specified program cannot exist because the program name is longer than 40 characters.
Addition 1
... USING p1 p2 p3 ...
Effect
See variant 1, addition 1.
Addition 2
... CHANGING p1 p2 p3 ...
Effect
See variant 1, addition 2.
Addition 3
... TABLES itab1 itab2 ...
Effect
See variant 1, addition 3.
Addition 4
... IF FOUND
Effect
The system only calls the subroutine if it and its main program exist. If this is not the case, the statement is ignored.
Variant 3
PERFORM n OF form1 form2 form3 ... .
Effect
Calls the subroutine with the index n from the list of subroutines in the statement. At runtime, n must contain a value between 1 (first name) and the total number of subroutines listed in the PERFORM statement (last name). The list can contain up to 256 subroutines.
Note
Non-Catchable Exceptions:
PERFORM_INDEX_0: The specified index was too small.
PERFORM_INDEX_NEGATIVE: The specified index was negative.
PERFORM_INDEX_TOO_LARGE: The specified index was too big.
Variant 4
PERFORM n ON COMMIT.
Extras:
1. ... LEVEL idx
Effect
Calls the specified subroutine at the next COMMIT WORK statement. This allows you to delay the subroutine until the logical transaction is finished. Even if you register the same subroutine more than once, it is only executed once. For further information, refer to COMMIT WORK. The ROLLBACK WORK statement deregisters all subroutines registered using this addition.
Notes
   1. You cannot use USING or CHANGING with the ... ON COMMIT addition. If you need to pass data to the subroutine, you must place it in global variables or use the EXPORT ... TO MEMORYstatement.
   2. The PERFORM ... ON COMMIT statement can also be used during update. The corresponding subroutine is called at the end of the update task.
Addition 1
... LEVEL idx
Effect
The LEVEL addition, followed by a field, determines the sequence in which the specified subroutines will be executed at the COMMIT WORK statement. The subroutines are called in ascending order of their level. If you do not use the LEVEL addition, the subroutine assumes the level zero. If two or more subroutines have the same level, they are executed in the sequence in which they are called in the program. You assign levels by defining constants in an include program. The level must have type I.
Variant 5
PERFORM n ON ROLLBACK.
Effect
The specified subroutine is executed if a ROLLBACK WORK occurs. This allows you, for example, to delete data, such as an internal table or data in memory, that was intended for use in PERFORM...ON COMMIT. If you register the same subroutine more than once, it will still only be executed once per ROLLBACK WORK.
Notes
   1. Subroutines registered using PERFORM... ON COMMIT cannot have USING or CHANGING parameters. Any data you want to pass to them must be contained in global variables or buffered using EXPORT ... TO MEMORY.
   2. If you catch a type A message ( MESSAGE) using the EXCEPTIONS..ERROR_MESSAGE addition in the CALL FUNCTION statement, a ROLLBACK WORK occurs, in which the subroutines registered using PERFORM ... ON ROLLBACK are executed.
Variant 6
PERFORM form(prog).
Extras:
1. ... USING    p1 p2 p3 ... 2. ... CHANGING p1 p2 p3 ...
3. ... TABLES   itab1 itab2 ...
4. ... IF FOUND
PERFORM form(prog) not allowed.
Effect
Calls the subroutine form defined in program prog ("external PERFORM").
Notes
   1. You pass parameters to the external subroutine as described in variant 1.
   2. However, you can also do it using a shared data area ( DATA BEGIN OF COMMON PART).
   3. Please consult Data Area and Modularization Unit Organization documentation as well.
   4. You can use nested calls, including several different subroutines in different programs.
   5. When you call a subroutine that belongs to a program prog, prog is loaded into the PXA buffer (if it has not been loaded already). To minimize the possibility of memory problems, try not to use too many external subroutines from a large number of different programs.
Hope this helps.
ashish