ABAP OO refresher for AgileSE Course (1).pdf

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

© 2011 SAP AG. All rights reserved.

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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


3

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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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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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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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”)


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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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


25

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


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


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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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( )


lcl_truck reserve_tank_level … + refuel( ): redefinition + get_reserve_tank_level()

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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


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


43

Sample Solutions

Implementation

QT3: ABAP_ASE_OO_INTRO_P1_SOL

Exercise 1 (QT3 System) Definition


Implementation

45


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.

46


Exercise 2 (QT3 System) Definition

Implementation

…


47


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( )


48


Exercise 3 (QT3 System) New Interface

New Truck Class – Implementation

New Truck Class – Definition


49


Exercise 3 (QT3 System) Changed Car Class – Definition

Changed Car Class – Implementation


50


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


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Exercise 4 (QT3 System) New Abstract Vehicle Class


Changed Car Class

52


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( )


lcl_truck reserve_tank_level … + refuel( ): redefinition + get_reserve_tank_level()

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Exercise 4 (QT3 System) Main Program

Back to Exercise


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ABAP OO refresher for AgileSE Course (1).pdf

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