This section discusses the structure and special features of class pools.

Classes and interfaces are both object types. You can define them either globally in the R/3 Repository or locally in an ABAP program. If you define classes and interfaces globally, they are stored in special ABAP programs called class pools (type K) or interface pools (type J), which serve as containers for the respective object types. Each class or interface pool contains the definition of a single class or interface. The programs are automatically generated by the Class Builder when you create a class or interface.

A class pool is comparable to a module pool or function group. It contains both declarative and executable ABAP statements, but cannot be started on its own. Instead, the system can only execute the statements in the class pool on request, that is, when the CREATE OBJECT statement occurs to create instances of the class.

Interface pools do not contain any executable statements. Instead, they are used as containers for interface definitions. When you implement an interface in a class, the interface definition is implicitly included in the class definition.

Class pools are structured as follows:

Class pools contain a definition part for type declarations, and the declaration and implementation parts of the class.

Class pools are different from other ABAP programs for the following reasons:

• ABAP programs such as executable programs, module pools, or function modules, usually have a declaration part in which the global data for the program is defined. This data is visible in all of the processing blocks in the program. Class pools, on the other hand, have a definition part, in which you can define data and object types, but no data objects or field symbols. The types that you define in a class pool are only visible in the implementation part of the global class.
• The only processing blocks that you can use are the declaration part and implementation part of the global class. The implementation part may only implement the methods declared in the global class. You cannot use any of the other ABAP processing blocks (dialog modules, event blocks, subroutines, function modules).
• The processing blocks of class pools are not controlled by the ABAP runtime environment. No events occur, and you cannot call any dialog modules or procedures. Class pools serve exclusively for class programming. You can only access the data and functions of a class using its interface.
• Since events and dialog modules are not permitted in classes, you cannot process screens in classes. You cannot program lists and selection screens in classes, since they cannot react to the appropriate events. It is intended to make screens available in classes. Instead of dialog modules, it will be possible to call methods of the class from the screen flow logic.

The classes and interfaces that you define in the definition part of a class pool are not visible externally. Within the class pool, they have a similar function to local classes and interfaces in other ABAP programs. Local classes can only be instantiated in the methods of the global class. Since subroutines are not allowed in class pools, local classes are the only possible modularization unit in global classes. Local classes have roughly the same function for global classes as subroutines in function groups, but with the significant exception that they are not visible externally.