Classes are templates for objects. Conversely, you can say that the type of an object is the same as its class. A class is an abstract description of an object. You could say that it is a set of instructions for building an object. The attributes of objects are defined by the components of the class, which describe the state and behavior of objects.
Local and Global Classes
Classes in ABAP Objects can be declared either globally or locally. You define global classes and interfaces in the Class Builder (Transaction SE24) in the ABAP Workbench. They are stored centrally in class pools in the class library in the Repository. All ABAP programs have access to these global classes. Local classes are defined within an ABAP program. Local classes and interfaces can only be used in the program in which they are defined. When you use a class in an ABAP program, the system first searches for a local class with the specified name. If it does not find one, it then looks for a global class. Apart from the visibility question, there is no difference between using a global class and using a local class.
There is, however, a significant difference in the way that local and global classes are designed. If you are defining a local class that is only used in a single program, it is usually sufficient to define the outwardly visible components so that it fits into that program. Global classes, on the other hand, must be able to be used anywhere. This means that certain restrictions apply when you define the interface of a global class, since the system must be able to guarantee that any program using an object of a global class can recognize the data type of each interface parameter.
The following sections describe how to define local classes and interfaces in an ABAP program. For information about how to define local classes and interfaces, refer to the Class Builder section of the ABAP Workbench documentation.
Defining Local Classes
Local classes consist of ABAP source code, enclosed in the ABAP statements CLASS - ENDCLASS. A complete class definition consists of a declaration part and, if required, an implementation part. The declaration part of a class names class consists of a statement block:
It contains the declaration for all components (attributes, methods, events) of the class. When you define local classes, the declaration part belongs to the global program data. You should therefore place it at the beginning of the program.
If you declare methods in the declaration part of a class, you must also write an implementation part for it. This consists of a further statement block:
The implementation part of a class contains the implementation of all methods of the class. The implementation part of a local class is a processing block. Subsequent coding that is not itself part of a processing block is therefore not accessible.
Structure of a Class
The following statements define the structure of a class:
· A class contains components
· Each component is assigned to a visibility section
· Classes implement methods
The following sections describe the structure of classes in more detail.
The components of a class make up its contents. All components are declared in the declaration part of the class. The components define the attributes of the objects in a class. When you define the class, each component is assigned to one of the three visibility sections, which define the external interface of the class. All of the components of a class are visible within the class. All components are in the same namespace. This means that all components of the class must have names that are unique within the class.
There are two kinds of components in a class - those that exist separately for each object in the class, and those that exist only once for the whole class, regardless of the number of instances. Instance-specific components are known as instance components. Components that are not instance-specific are called static components.
In ABAP Objects, classes can define the following components. Since all components that you can declare in classes can also be declared in interfaces, the following descriptions apply equally to interfaces.
Attributes are internal data fields within a class that can have any ABAP data type. The state of an object is determined by the contents of its attributes. One kind of attribute is the reference variable. Reference variables allow you to create and address objects. Reference variables can be defined in classes, allowing you to access objects from within a class.
The contents of instance attributes define the instance-specific state of an object. You declare them using the DATAstatement.
The contents of static attributes define the state of the class that is valid for all instances of the class. Static attributes exist once for each class. You declare them using the CLASS-DATA statement. They are accessible for the entire runtime of the class.
All of the objects in a class can access its static attributes. If you change a static attribute in an object, the change is visible in all other objects in the class.
The technical properties of instance attributes belong to the static properties of a class. It is therefore possible to refer in a LIKE addition to the visible attributes of a class – through the class component selector or through reference variables, without prior creation of an object.
Methods are internal procedures in a class that define the behavior of an object. They can access all of the attributes of a class. This allows them to change the data content of an object. They also have a parameter interface, with which users can supply them with values when calling them, and receive values back from them The private attributes of a class can only be changed by methods in the same class.
The definition and parameter interface of a method is similar to that of function modules. You define a method meth in the definition part of a class and implement it in the implementation part using the following processing block:
You can declare local data types and objects in methods in the same way as in other ABAP procedures (subroutines and function modules). You call methods using the CALL METHOD statement.
You declare instance methods using the METHODSstatement. They can access all of the attributes of a class, and can trigger all of the events of the class.
You declare static methods using the CLASS-METHODSstatement. They can only access static attributes and trigger static events.
As well as normal methods, which you call using CALL METHOD, there are two special methods called constructor and class_constructor that are automatically called when you create an object or when you first access the components of a class.
Objects or classes can use events to trigger event handler methods in other objects or classes. In a normal method call, one method can be called by any number of users. When an event is triggered, any number of event handler methods can be called. The link between the trigger and the handler is not established until runtime. In a normal method call, the calling program determines the methods that it wants to call. These methods must exist. With events, the handler determines the events to which it wants to react. There does not have to be a handler method registered for every event.
The events of a class can be triggered in the methods of the same class using the RAISE EVENT statement. You can declare a method of the same or a different class as an event handler method for the event evt of class class using the addition FOR EVENT evt OF class.
Events have a similar parameter interface to methods, but only have output parameters. These parameters are passed by the trigger (RAISE EVENTstatement) to the event handler method, which receives them as input parameters.
The link between trigger and handler is established dynamically in a program using the SET HANDLER statement. The trigger and handlers can be objects or classes, depending on whether you have instance or static events and event handler methods. When an event is triggered, the corresponding event handler methods are executed in all registered handling classes.
You declare instance events using the EVENTSstatement. An instance event can only be triggered in an instance method.
You declare static events using the CLASS-EVENTSstatement. All methods (instance and static methods) can trigger static events. Static events are the only type of event that can be triggered in a static method.
See also Triggering and Handling Events.
You can define your own ABAP data types within a class using the TYPESstatement. Types are not instance-specific, and exist once only for all of the objects in a class.
Constants are special static attributes. You set their values when you declare them, and they can then no longer be changed. You declare constants using the CONSTANTS statement. Constants are not instance-specific, and exist once only for all of the objects in a class.
You can divide the declaration part of a class into up to three visibility areas:
These areas define the external visibility of the class components, that is, the interface between the class and its users. Each component of a class must be assigned to one of the visibility sections.
All of the components declared in the public section are accessible to all users of the class, and to the methods of the class and any classes that inherit from it. The public components of the class form the interface between the class and its users.
All of the components declared in the protected section are accessible to all methods of the class and of classes that inherit from it. Protected components form a special interface between a class and its subclasses. Since inheritance is not active in Release 4.5B, the protected section currently has the same effect as the private section.
Components that you declare in the private section are only visible in the methods of the same class. The private components are not part of the external interface of the class.
The three visibility areas are the basis for one of the important features of object orientation - encapsulation. When you define a class, you should take great care in designing the public components, and try to declare as few public components as possible. The public components of global classes may not be changed once you have released the class.
For example, public attributes are visible externally, and form a part of the interface between an object and its users. If you want to encapsulate the state of an object fully, you cannot declare any public attributes.
As well as defining the visibility of an attribute, you can also protect it from changes using the READ-ONLY addition.
Classes with Non-Public Instantiation
Using the statement
CLASS class DEFINITON CREATE PROTECTED|PRIVATE.
you can ensure that objects of the class class can be created only in the class itself or in its subclasses. This makes for unique instance management.