ABAP Objects Intro / Refresher Preparation for Agile Software Engineering Course Juergen Heymann, Nena Raab CPO Software Engineering
Purposes & Format
Purpose: Prepare for AgileSE Course ABAP Fast Introduction / Refresher of Object Oriented ABAP (OO ABAP) Learn how to develop OO ABAP source code based
Format: Self-study course with set of modules with
Short video intro to concept Programming exercise Short video on sample solution Modules build on each other
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Learning Modules Module 4
Module 1
From Functions to Objects Object Basics OO Key Concept: Encapsulation
Module 5
Module 2
Local and Global Classes (SE24) Class Attributes and Methods Design Pattern: Factory Method
OO Key Concept: Polymorphism Interfaces Various Uses of Interfaces
Inheritance/Subclassing
Module 3
Introduction to UML and object modeling
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Assumptions / Prerequisites
The course is developed from the perspective of an experienced ABAP developer. Assumptions: You are familiar with ABAP programming (non-OO) and the tools You have access to a system for the exercises
Positioning The course is intended as quick refresher / fast intro to OO programming in ABAP. For deeper study, read e.g. …
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ABAP Objects Refresher Module 1: From Functions to Objects Object Basics OO Key Concept: Encapsulation
From Functions to Objects 1
Characteristics of the Procedural Programming Model Separation of data and function •
Data represents state of ‘real world objects’
•
Functions implement behavior of 'real world objects
Usually direct access to data (non-encapsulated)
function
function
function
function
Modularization is used to encapsulate functions function
form routine
form routine
form routine
Notes: You can encapsulate data (access) by functions All data is accessible to all functions data
data
data
data data © 2011 SAP AG. All rights reserved.
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From Functions to Objects 2
Multiple Instances When you want 'multiple instances' you use internal- or DB-tables Row in table holds the data for one instance e.g. order items table
Code works on 'the current instance' = a 'current row of the table' address 'current instance' by field-symbol or by taking it into a work-area hard to address multiple 'current instances' you work with hard to have deeper structures
Cars code
ID
code code
name
speed
1
Porsche 911
170
2
VW Bug 1300
120
Notes: New instance append to table Row identified by a 'key'
code
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From Functions to Objects 3
Objects – a Real World Object Representation What changes going to Objects?
method
Putting Code and Data together Instance specific Data and code that 'belongs together' is put together Instance specific Data Instance Attributes Functions/Forms Methods Methods work only on their object's data (keep their object data consistent) Data is usually hidden ('private') and methods are public
Each Object Instance is created separately 'CREATE OBJECT' instead of 'append line' Terminology
private
public
method
data
Class Car set_name
name
set_speed
speed
Object 1:
Object 2:
set_name
Porsche 911
set_name
VW Bug 1300
set_speed
170
set_speed
120
Class: Code + structure to define objects Object: Concrete instance of a class
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What the Code Looks Like Definition – Implementation - Client Definition
Definition of a class (here local in program) Definition of the methods with parameters
Definition of attributes of the class
Implementation Implementation of the methods
Client code (here: main program) DATA … TYPE REF TO are reference variables that can hold references to objects (of a certain type) the CREATE OBJECT statement creates a new instance of a class methods are called on the object to change the data of this object
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From Functions to Objects 4
Instances and References Instances / Objects A new object is created by "CREATE OBJECT objref" (objref = name of reference variable) You can have many objects/instances of a class. (they live in memory)
data car1 type ref to lcl_car. data car2 type ref to lcl_car. data car3 type ref to lcl_car. create object: car1, car2, car3.
car1
Object References Objects are 'accessed' via a reference that 'points to' the object instance References are stored in reference variables of TYPE REF TO … Reference variables are typed so that only references of objects of that specified type can be stored there Reference values can be copied and cleared car3 = car1 only copies the reference, not the object!
car2 car3
car3 = car1. clear car2.
car1
car2
Garbage Collection
"– copy reference "– clear reference
X
car3
Objects no longer reachable by any reference are 'reclaimed' (instance discarded, memory reused)
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OO Key Concept: Encapsulation Public and Private
Motivation
Definition
You want to encapsulate/hide internal details such as data and internal operations You want to provide a well-defined, simple and consumable public interface to the client The private object components (data, methods) can be restructured without the need to change clients of the object
Some Principles Keep your data 'private‘ since private data can only be modified within the class The object itself ensures the consistency of its data
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Special Method: Constructor
A special method of a class is the CONSTRUCTOR
Definition
This method is called implicitly by the system when the object is created Arguments can be defined that then have to be passed to the "CREATE OBJECT" statement Constructors are optional Constructors are usually used to 'initialize' an object to a consistent state
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Exercise 1: Basic OO Concepts and ABAP OO Syntax Exercise: Modeling different types of cars with their fuel consumption Create a program/report (in $TMP) that you will write your local classes in
Further Instructions Define a local class for car e.g. lcl_car Define attributes of car (private) – name of car (e.g. 'Porsche Carrera 911') – capacity of gas tank in liters
– fuel consumption - liters per 100 km the car consumes – current tank level in liters (e.g. 10,2 liters)
Define methods (public) – constructor: sets the attributes name, capacity and fuel consumption
Create car objects in your main program Create 3 different cars (instances) with 3 different tank capacities and gas consumptions. Call methods on the objects to drive various distances
Call methods on the objects to ask for the current tank level and output these values (use WRITE statements)
References ABAP Objects syntax reference by sample code
– refuel: fill up the gas tank returning the number of liters used
ABAP Workbench Shortcuts
– drive_distance: reduces the gas in the tank – get_current_tank_level: returns the current tank level in percentage (e.g. “35% filled”)
© 2011 SAP AG. All rights reserved.
CTRL + Space
Code Completion
SHIFT + F1
Format Source
F2
Check Source
F3
Activate Source
F9
Select all Objects in Activation List
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Review of Exercise 1
Content Look at the sample solution Show objects in debugger lcl_car - name - tank_capacity - fuel_consumption_per_km - curr_tank_level + constructor( ) + refuel( ) + drive_distance( ) + get_curr_tank_level( )
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ABAP Objects Refresher Module 2: Local and Global Classes (SE24) Class Attributes and Methods Design Pattern: Factory Method
Local and Global Classes Local Classes Can be defined within any source unit e.g. report, function, global class … Are only visible in that source unit same class name can be used in many source units Naming: Usually start with LCL_...
Prog1 lcl_car lcl_truck
Prog2 lcl_car
Global Classes Are globally visible in the system like any other function module, DDIC element, … Are edited with the Class Builder transaction (SE24) Naming: Usually start with CL_... Using global classes: like local classes
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Demo SE24
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Static (Class Level) Attributes
Class vs. Instance Level Instance Attributes exist per instance
CL_CAR no_of_instances = 6
– defined with "DATA“
name = Porsche 911
Class attributes exist only once per class, shared for all instances = 'static' – defined with "CLASS-DATA" name = VW Bug 1300
Usage
class-data:
separate instances and its data
Suppose you want to know the 'number of cars created'. Where would you do that? Increment where? How guarantee that it is incremented only once per instance? Constructor of 'create method' …
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Using Static (Class Level) Attributes
Example: Instance Counter
Definition
Counter exists only once for the class, i.e. is shared by all instances Counter is public read-only Static attribute Constructor increments the count
Access static attributes with "=>" Access class attributes using static operator "=>“ e.g. class_name=>attribute_name
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Using Static (Class Level) Methods
To access non-public class attributes you need class methods
Definition
Methods exist on class level Defined with "CLASS-METHODS" Class methods have no access to instance attributes A static method cannot be overwritten by a subclass
Call static methods using "=>" access class methods using static operator "=>“ e.g. class_name=>method_name © 2011 SAP AG. All rights reserved.
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Example: Factory Method Using Create Method instead of Constructor Definition
Motivation • The class itself wants to control the object instantiation, not the client • E.g. ensure that object exists only once or once per key
• The client wants always the same instance without holding the instance
Other Benefits
factory method
• Client and class creation are loosely coupled • You can return the object of a generic type see interfaces • You can use where-used-list to find the object creators
Example: Introduce "factory method“ If a class method is used to create an object then you must set 'create private' (no 'create object' outside class) © 2011 SAP AG. All rights reserved.
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Exercise 2: Class Methods and Data Implement Instance Management
Exercise: Implement a factory method to create an object Ensure that an object with a given ID (='key') exists only once
Further Instructions Copy the code from exercise 1 and change it in the following way: Replace the constructor by a static class method, e.g. "get_instance“ (keeping the parameter) Change class definition to “create private“ to prevent the creation of objects outside the class. Fix the appearing syntax errors (replace all CREATE OBJECTs with calls to get_instance) Remember in the creation method which cars were created (by name key): use a static internal table with row type [string, type ref to lcl_car]
If get_instance() is called again with the same key (e.g. 'VW Golf'), return the existing instance.
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Review of Exercise 2
Content Look at sample solution Show objects in debugger lcl_car - inst_table - name - tank_capacity - fuel_consumption_per_km - curr_tank_level + get_instance( ) + refuel( ) + drive_distance( ) + get_curr_tank_level( )
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ABAP Objects Refresher Module 3: Introduction to UML and object modeling
UML (Unified Modeling Language): Class Syntax
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UML: Class Diagram Static View of Classes and Their Relationships ('Associations')
Aggregation relation (= 'contains' 'is-part-of') Generalization © 2011 SAP AG. All rights reserved.
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UML: Association Syntax
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UML: Sequence Diagram Who Calls Who and in What Order
Delegation Principle Visualized
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CRC Modeling Classes – Responsibilities – Collaborations
CRC Modeling Method Similar to UML classes but more abstract Use cards with
class name responsibilities of class (bullet points: data held, methods / operations offered) collaborations (names of other classes it collaborates with)
CRC cards are usually moved around to show relationships by location and arrows
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Student has address knows professors enrolls in class drop class take exam
professor classes exam scores
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ABAP Objects Refresher Module 4: OO Key Concept: Polymorphism Interfaces Various uses of interfaces
OO Key Concept: Polymorphism Depend Upon Abstractions, not on Concrete Implementations
Motivation •
Support Polymorphism = 'having multiple forms‘
•
Several implementations for the same abstract interface examples in the non-OO world: BAdIs or Customizing with callback function modules You want to offer (future) extensibility
•
Some Examples Imagine 'generic save' method on each object Imagine an archive manager that needs data in a specific format of its objects to be archived Imagine a refuel method for all kinds of vehicles
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any_storable_obj->save_your_data_to_db( ).
archive_manager->archive( archivable_obj ).
any_vehicle>refuel( 45,2 ).
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Polymorphism: Interfaces Definition – Implementation – Client Interface Definition
… is like the definition of a class • It contains elements like types, methods, … • But all elements are public Interface Implementation
• A class can implement many interfaces ‘full name’ of element: ~
Client • Interface references allow access to all interface elements i.e. regardless of implementing class Specify class when using interface reference
Object reference can be 'converted' to an interface reference type that it implements (cast)
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Exercise 3: Interfaces for Generic Object Access
Exercise: Access different vehicle types via a common interface Further Instructions Copy the code from exercise 2 and change it in the following way: Define an interface 'lif_vehicle' with methods – Copy the methods refuel, drive_distance and get_current_tank_level into the interface (from exercise 1)
Define and implement a class ‘lcl_truck’ that implements the 'lif_vehicle' interface. Let the ‘lcl_car’ class implement the 'lif_vehicle‘ interface – Remove methods contained in the interface from the class definition – Rename the method names of the implementation
In the report: – Create several trucks in addition to the cars and drive some distances – Collect references to all vehicles in an internal table using the interface type – Loop over the table, check that the current tank level is less than 100% and then refuel all of them
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Review of Exercise 3
Content
lif_vehicle <>
Look at sample solution Show objects in debugger
get_current_tank_level( ) refuel( ) drive_distance( )
lcl_car - inst_table - name - tank_capacity - fuel_consumption_per_km - curr_tank_level
lcl_truck - name - tank_capacity - fuel_consumption_per_km - curr_tank_level …special truck methods
+ get_instance( ) …special car methods
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Various Uses of Interfaces Some Purposes
Generic interface / access to objects from frameworks
1.
Which data is required from the client in which format Other interactions with the client e.g. notifications, call backs
Example: Each class whose objects need to be archived implement the 'archiving interface'.
Decouple implementation from interface
2. •
Exchange the implementing class
Decoupling during development
3. • •
E.g. you need to 'use' / call a class that is built by another developer but is not there yet You agree together on the interface. Then the 'caller' can use the interface while the developer has time to implement the interface.
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ABAP Objects Refresher Module 5: Polymorphism: Inheritance
Polymorphism: Inheritance/Subclassing/Derivation Class Hierarchy
Motivation •
• •
Inheritance is a means to implement conceptual hierarchies and/or share common code between classes A super class implements the characteristics that are common to all its subclasses Subclasses are specializations that can do more
lcl_vehicle
+ refuel() + drive_distance() …
Inheritance •
The subclass inherits the data and behavior from its superclass: it can access/redefine all components of its super class
Technical Restriction Class/static components cans not be redefined A class can only inherit from one class
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Polymorphism: Inheritance/Subclassing/Derivation Parent Class - Subclass Parent Class • The parent class must be opened up for derivation: the final flag has to be removed • New visibility level protected: components visible in class and subclasses
Subclass • Subclass can define further attributes and methods • Subclass can access all components of its super class that are protected or public
• Subclass can redefine all methods of its super class that are protected or public • Subclass can call the implementation of the parent class
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Exercise 4: Use Inheritance lif_vehicle <>
Exercise: Move common code to a super-class Further Instructions
get_current_tank_level( ) refuel( ) drive_distance( )
Copy code from exercise 3 to new program Create class lcl_vehicle that implements the lif_vehicle interface (which contains the three methods…) Move data and implementation of interface methods from lcl_car to vehicle level and make it protected so that the subclasses can access them
lcl_vehicle <>
Define lcl_car and lcl_truck as subclasses of vehicle (note: the existing interface method implementations are no longer needed since they are implemented in the super-class vehicle)
- name - tank_capacity - fuel_consumption_per_km - curr_tank_level
The truck can do more: It has a reserve tank that has its own capacity and must be refueled as well and needs its own get tank level method – You can call the refuel() method of the vehicle to get the main tank refueled
Create a few vehicles and drive some distances using the vehicle interface (table) as in exercise 3
lcl_car - inst_table + get_instance( )
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lcl_truck reserve_tank_level … + refuel( ): redefinition + get_reserve_tank_level()
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Review of Exercise 4
Content Look at sample solution Show objects in debugger
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Polymorphism: Inheritance and Delegation Additional Notes
•
Inheritance entails a very strong coupling between classes and should not be misused just as a convenient means to share implementation code!
•
Key Rule: A subclass must do the same (or more) as its parent / super class
•
•
Liskov Substitution Principle: “Behavior 'promises' by the ‘interface’ of a class C must be satisfied by all its subtypes S’
•
Typical violation: Square class is subclass of Rectangle? NO: Subclass ≠ Subset! Rectangle client code will expect to set width and height independently
Therefore it is wise to prefer delegation over inheritance whenever possible, i.e. have a class beside that implements the common parts
Cparent S¹
S²
Inheritance
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S¹
Cany¹
S²
Cany² Delegation
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ABAP Objects Syntax
QT3: ABAP_ASE_OO_INTRO_LANG_REF
ABAP Objects Sample Code as Syntax Reference Definition
Main Program
Implementation
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Sample Solutions
Implementation
QT3: ABAP_ASE_OO_INTRO_P1_SOL
Exercise 1 (QT3 System) Definition
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Implementation
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QT3: ABAP_ASE_OO_INTRO_P1_SOL
Exercise 1 (QT3 System) Main Program
lcl_car - name - tank_capacity - fuel_consumption_per_km - curr_tank_level + constructor() + refuel( ) + drive_distance( ) + get_curr_tank_level( )
Back to Exercise © 2011 SAP AG. All rights reserved.
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QT3: ABAP_ASE_OO_INTRO_P2_SOL
Exercise 2 (QT3 System) Definition
Implementation
…
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QT3: ABAP_ASE_OO_INTRO_P2_SOL
Exercise 2 (QT3 System) lcl_car
Main Program
- inst_table - name - tank_capacity - fuel_consumption_per_km - curr_tank_level + get_instance( ) + refuel( ) + drive_distance( ) + get_curr_tank_level( )
Back to Exercise © 2011 SAP AG. All rights reserved.
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QT3: ABAP_ASE_OO_INTRO_P3_SOL
Exercise 3 (QT3 System) New Interface
New Truck Class – Implementation
New Truck Class – Definition
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QT3: ABAP_ASE_OO_INTRO_P3_SOL
Exercise 3 (QT3 System) Changed Car Class – Definition
Changed Car Class – Implementation
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QT3: ABAP_ASE_OO_INTRO_P3_SOL
Exercise 3 (QT3 System) lif_vehicle <>
Main Program get_current_tank_level( ) refuel( ) drive_distance( )
lcl_car - inst_table - name - tank_capacity - fuel_consumption_per_km - curr_tank_level
lcl_truck - name - tank_capacity - fuel_consumption_per_km - curr_tank_level …special truck methods
+ get_instance( ) …special car methods
Back to Exercise © 2011 SAP AG. All rights reserved.
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QT3: ABAP_ASE_OO_INTRO_P4_SOL
Exercise 4 (QT3 System) New Abstract Vehicle Class
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Changed Car Class
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QT3: ABAP_ASE_OO_INTRO_P4_SOL
Exercise 4 (QT3 System) lif_vehicle <>
Changed Truck Class get_current_tank_level( ) refuel( ) drive_distance( )
lcl_vehicle <> - name - tank_capacity - fuel_consumption_per_km - curr_tank_level
lcl_car - inst_table + get_instance( )
© 2011 SAP AG. All rights reserved.
lcl_truck reserve_tank_level … + refuel( ): redefinition + get_reserve_tank_level()
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QT3: ABAP_ASE_OO_INTRO_P4_SOL
Exercise 4 (QT3 System) Main Program
Back to Exercise
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