In Module Pool (ABAP Dynpro) programming, validations are usually added in:

1. PBO (Process Before Output) – usually for initial field checks or setting default values.

2. PAI (Process After Input) – most common place for validations after the user inputs data and presses a button.

The PAI module is triggered when a user interacts with a screen element (like pressing Enter, or a function key).

Step 2: Define Field in Screen

Suppose you have a screen (0100) with a field S_MATNR (Material Number).

1. Go to Screen Painter (SE51).

2. Add your input field (S_MATNR) and assign a name.

3. Assign a Module in the PAI Flow Logic (we’ll validate in this module):

Step 4: Optional – Validate on Field Exit

If you want validation when leaving a field, you can use Field Exit or MODULE … AT EXIT-COMMAND:

MODULE validate_matnr_onexit INPUT.
IF s_matnr IS INITIAL.
MESSAGE ‘Material cannot be empty’ TYPE ‘E’.
ENDIF.
ENDMODULE.

Step 5: Common Validation Techniques

1. Check for mandatory fields → IS INITIAL.

2. Check for valid values in DB → SELECT from tables (MARA, KNA1, etc.).

3. Cross-field validation → compare values of multiple fields.

4. Custom messages → Use MESSAGE statement with message class.

MODULE validate_data INPUT.
IF s_qty <= 0.
MESSAGE ‘Quantity must be greater than zero’ TYPE ‘E’.
ENDIF.

IF s_matnr IS INITIAL AND s_kunnr IS INITIAL.
MESSAGE ‘Material or Customer must be entered’ TYPE ‘E’.
ENDIF.
ENDMODULE.