Object-Oriented Programming
Object-oriented programming is a Programming Paradigm, applied in Flow and the Packages built on it. In this section we will give an overview of the basic concepts of Object Orientation.
Programs have a certain purpose, which is - generally speaking - to solve a problem. “Problem” does not necessarily mean error or defect but rather an actual task. This Problem usually has a concrete counterpart in real life.
A Program could for example take care of the task of booking a cruise in the Indian Ocean. If so we obviously have a problem (a programmer that has been working to much and finally decided to go on vacation) and a program, promising recuperation by booking a coach on one of the luxury liners for him and his wife.
Object Orientation assumes that a concrete problem is to be solved by a program, and a concrete problem is caused by real objects. Therefore focus is on the object. This can be abstract of course: it will not be something as concrete as a car or a ship all the time, but can also be a reservation, an account or a graphical symbol.
objects are “containers” for data and corresponding functionality. The data of an object is stored in its Properties. The functionality is provided by Methods, which can for example alter the properties of the object. In regard to the cruise liner we can say, that it has a certain amount of coaches, a length and width and a maximum speed. Further it has methods to start the motor (and hopefully to stop it again also), change the direction as well as to increase thrust, for you can reach your holiday destination a bit faster.
Why Object Orientation after all?
Surely some users will ask themselves why they should develop object oriented in the first place. Why not (just like until now) keep on developing procedural, thus stringing together functions? Because procedural programming has some severe disadvantages:
Properties and methods belonging together with regard to content can not be united. This methodology, called Encapsulation in Object Orientation, is necessary, if only because of clear arrangement.
It is rather difficult to re-use code
All properties can be altered everywhere throughout the code. This leads to hard-to-find errors.
Procedural code gets confusing easily. This is called Spaghetti code.
Furthermore Object Orientation mirrors the real world: Real objects exist, and they all have properties and (most of them) methods. This fact is now represented in programming.
In the following we’ll talk about the object ship. We’ll invoke this object, stock it with coaches, a motor and other useful stuff. Furthermore, there will be functions, moving the ship, thus turning the motor on and off. Later we’ll even create a luxury liner based on the general ship and equip it with a golf simulator and satellite TV.
On the following pages, we’ll try to be as graphic as possible (but still semantically correct) to familiarize you with object orientation. There is a specific reason: The more you can identify with the object and its methods, the more open you’ll be for the theory behind Object Oriented Programming. Both is necessary for successful programming – even though you’ll often not be able to imagine the objects you’ll later work with as clearly as in our examples.
Classes and Objects
Let’s now take a step back and imagine there’d be a blueprint for ships
in general. We now focus not the ship but this blueprint. It is called
class, in this case it is the class Ship
. In PHP this is written as
follows;
PHP Code:
<?php
class Ship {
...
}
?>
Note
In this piece of code we kept noting the necessary PHP tags at the beginning and end. We will spare them in the following examples to make the listings a bit shorter.
The key word class
opens the class and inside the curly brackets properties
and methods are written. we’ll now add these properties and methods:
PHP Code:
class Ship {
public $name;
public $coaches;
public $engineStatus;
public $speed;
function startEngine() {}
function stopEngine() {}
function moveTo($location) {}
}
Our ship now has a name ($name
), a number of coaches ($coaches
) and a
speed ($speed
). In addition we built in a variable, containing the status
of the engine ($engineStatus
). A real ship, of course, has much more
properties, all important somehow – for our abstraction these few will be
sufficient though. We’ll focus on why every property is marked with the key
word public
further down.
Note
For methods and properties we use a notation called lowerCamelCase: The first letter is lower case and all other parts are added without blank or underscore in upper case. This is a convention used in Flow.
We can also switch on the engine (startEngine()
), travel with the ship to
the desired destination (moveTo($location)
) and switch off the engine
again (stopEngine()
). Note that all methods are empty, i.e. we have no
content at all. We’ll change this in the following examples, of course. The
line containing method name and (if available) parameters is called method
signature or method head. Everything contained by the method ist called method
body accordingly.
Now we’ll finally create an object from our class. The class ship
will be
the blueprint and $fidelio
the concrete object.
PHP Code:
$fidelio = new Ship();
// Display the object
var_dump($fidelio);
The key word new is used to create a concrete object from the class.
This object is also called Instance **and the creation process
consequentially **Instantiation. We can use the command var_dump()
to
closely examine the object. We’ll see the following
PHP Code:
object(Ship)#1 (3) {
["name"] => null
["coaches"] => null
["engineStatus"] => null
["speed"] => null
}
We can clearly see that our object has 4 properties with a concrete value, at the moment still null, for we did not yet assign anything. We can instantiate as many objects from a class as we like, and every single one will differ from the others – even if all of the properties have the same values.
PHP Code:
$fidelio1 = new Ship();
$fidelio2 = new Ship();
if ($fidelio1 === $fidelio2) {
echo 'objects are identical!'
} else {
echo 'objects are not identical!'
}
In this example the output is objects are not identical!
The arrow operator
We are able to create an object now, but of course it’s properties are still empty.We’ll hurry to change this by assigning values to the properties. For this, we use a special operator, the so called arrow operator (->). We can use it for getting access to the properties of an object or calling methods. In the following example, we set the name of the ship and call some methods:
PHP Code:
$ship = new Ship();
$ship->name = "FIDELIO";
echo "The ship's Name is ". $ship->name;
$ship->startEngine();
$ship->moveTo('Bahamas');
$ship->stopEngine();
$this
Using the arrow operator we can now comfortably access properties and methods
of an object. But what to do, if we want to do this from inside a method, e.g.
to set $speed ``inside of the method ``startEngine()
? We don’t know at this
point, how an object to be instantiated later will be called. So we need a
mechanism to do this independent from the name. This is done with the special
variable $this
.
PHP Code:
class Ship {
...
public $speed;
...
function startEngine() {
$this->speed = 200;
}
}
With $this->speed
you can access the property speed in the actual object,
independently of it’s name.
Constructor
It can be very useful to initialize an object at the Moment of
instantiating it. Surely there will be a certain number of coaches built in
right away, when a new cruise liner is created - so that the future guest will
not be forced to sleep in emergency accommodation. So we can define the number
of coaches right when instantiating. The processing of the given value is done
in a method automatically called on creation of an object, the so called
Constructor. This special method always has the name __construct()
(the
first two characters are underscores).
The values received from instantiating are now passed on to the constructor as
Argument and then assigned to the properties $coaches ``respectively ``$name
.
Inheritance of Classes
With the class we created we can already do a lot. We can create many ships and send them to the oceans of the world. But of course the shipping company always works on improving the offer of cruise liners. Increasingly big and beautiful ships are built. Also new offers for the passengers are added. FIDELIO2, for example, even has a little golf course based on deck.
If we look behind the curtain of this new luxury liner though, we find that the
shipping company only took a ship type FIDELIO and altered it a bit. The basis
is the same. Therefore it makes no sense to completely redefine the new ship –
instead we use the old definition and just add the golf course – just as the
shipping company did. Technically speaking we extend an “old” class definition
by using the key word extends
.
PHP Code:
class LuxuryLiner extends Ship {
public $luxuryCoaches;
function golfSimulatorStart() {
echo 'Golf simulator on ship ' . $this->name . '
started.';
}
function golfSimulatorStop() {
echo 'Golf simulator on ship ' . $this->name . '
stopped.';
}
}
$luxuryShip = new LuxuryLiner('FIDELIO2','600')
Our new luxury liner comes into existence as easy as that. We define, that the
luxury liner just extends the Definition of the class Ship
. The extended
class (in or example Ship
) is called parent class **or **superclass.
The class formed by Extension (in our example LuxuryLiner
) is called
child class **or **sub class.
The class LuxuryLiner
now contains the complete configuration of the base
class Ship
(including all properties and methods) and defines additional
properties (like the amount of luxury coaches in $luxuryCoaches
) and
additional methods (like golfSimulatorStart()
and golfSimulatorStop()
).
Inside these methods you can again access the properties and methods of the
parent class by using $this
.
Overriding Properties and Methods
Inside an inherited class you can not only access properties and methods of the
parent class or define new ones. It’s even possible to override the original
properties and methods. This can be very useful, e.g. for giving a method of
a child class a new functionality. Let’s have a look at the method
startEngine()
for example:
PHP Code:
class Ship {
...
$engineStatus = 'OFF';
...
function startEngine() {
$this->engineStatus = 'ON';
}
...
}
class Luxusliner extends Ship {
...
$additionalEngineStatus = 'OFF';
...
function startEngine() {
$this->engineStatus = 'ON';
$this->additionalEngineStatus = 'ON';
}
...
}
Our luxury liner (of course) has an additional motor, so this has to be
switched on also, if the method startEngine()
is called. The child class
now overrides the method of the parent class and so only the method
startEngine()
of the child class is called.
Access to the parent class through “parent”
Overriding a method comes in handy, but has a serious disadvantage. When
changing the method startEngine()
in the parent class, we’d also have to
change the method in the child class. This is not only a source for errors but
also kind of inconvenient. It would be better to just call the method of the
parent class and then add additional code before or after the call. That’s
exactly what can be done by using the key word parent
. With
parent::methodname()
the method of the parent class can be accessed
comfortably - so our former example can be re-written in a smarter way:
PHP Code:
class Ship {
...
$engineStatus = 'OFF';
...
function startEngine() {
$this->engineStatus = 'ON';
}
...
}
class Luxusliner extends Ship {
...
$additionalEngineStatus = 'OFF';
...
function startEngine() {
parent::startEngine();
$this->additionalEngineStatus = 'ON';
}
...
}
Abstract classes
Sometimes it is useful to define “placeholder methods” in the parent class
which are filled in the child class. These “placeholders” are called
abstract methods. A class containing abstract methods is called abstract
class. For our ship there could be a method setupCoaches()
. Each type of
ship is to be handled differently for each has a proper configuration. So each
ship must have such a method but the concrete implementation is to be done
separately for each ship type.
PHP Code:
abstract class Ship {
...
function __construct() {
$this->setupCoaches();
}
abstract function setupCoaches();
...
}
class Luxusliner extends Ship {
...
function setupCoaches() {
echo 'Coaches are being set up';
}
}
$luxusschiff = new Luxusliner();
In the parent class we have defined only the body of the
method setupCoaches()
. The key word abstract
makes sure that the method
must be implemented in the child class. So using abstract classes, we can
define which methods have to be present later without having to implement them
right away.
Interfaces
Interfaces are a special case of abstract classes in which all methods are
abstract. Using Interfaces, specification and implementation of functionality
can be kept apart. In our cruise example we have some ships supporting
satellite TV and some who don’t. The ships who do, have the methods
enableTV()
and disableTV()
. It is useful to define an interface
for that:
PHP Code:
interface SatelliteTV {
public function enableTV();
public function disableTV();
}
class Luxusliner extends Ship implements SatelliteTV {
protected $tvEnabled = false;
public function enableTV() {
$this->tvEnabled = true;
}
public function disableTV() {
$this->tvEnabled = false;
}
}
Using the key word implements
it is made sure, that the class implements
the given interface. All methods in the interface definition then have to be
realized. The object LuxuryLiner
now is of the type Ship
but also of
the type SatelliteTV
. It is also possible to implement not only one
interface class but multiple, separated by comma. Of course interfaces can also
be inherited by other interfaces.
Visibilities: public, private and protected
Access to properties and methods can be restricted by different visibilities to hide implementation details of a class. The meaning of a class can be communicated better like this, for implementation details in internal methods can not be accessed from outside. The following visibilities exist:
public: properties and methods with this visibility can be accessed from outside the object. If no Visibility is defined, the behavior of
public
is used.protected: properties and methods with visibility
protected
can only be accessed from inside the class and it’s child classes.private: properties and methods set to
private
can only be accessed from inside the class itself, not from child classes.
Access to Properties
This small example demonstrates how to work with protected properties:
PHP Code:
abstract class Ship {
protected $coaches;
...
abstract protected function setupCoaches();
}
class Luxusliner extends Ship {
protected function setupCoaches() {
$this->coaches = 300;
}
}
$luxusliner = new Luxusliner('Fidelio', 100);
echo 'Number of coaches: ' . $luxusliner->coaches; // Does NOT work!
The LuxuryLiner
may alter the property coaches
, for this is protected
.
If it was private
no access from inside of the child class would
be possible. Access from outside of the hierarchy of inheritance (like in the
last line of the example) is not possible. It would only be possible if the
property was public
.
We recommend to define all properties as protected
. Like that, they can not
be altered any more from outside and you should use special methods (called
getter and setter) to alter or read them. We’ll explain the use of these
methods in the following section.
Access to Methods
All methods the object makes available to the outside have to be defined as
public
. All methods containing implementation details, e.g.
setupCoaches()
in the above example, should be defined as protected
.
The visibility private
should be used most rarely, for it prevents methods
from being overwritten or extended.
Often you’ll have to read or set properties of an object from outside. So you’ll need special methods that are able to set or get a property. These methods are called setter respectively getter. See the example.
PHP Code:
class Ship {
protected $coaches;
protected $classification = 'NORMAL';
public function getCoaches() {
return $this->coaches;
}
public function setCoaches($numberOfCoaches) {
if ($numberOfCoaches > 500) {
$this->classification = 'LARGE';
} else {
$this->classification = 'NORMAL';
}
$this->coaches = $numberOfCoaches;
}
public function getClassification() {
return $this->classification;
}
...
}
We now have a method setCoaches()
which sets the number of coaches.
Furthermore it changes - depending on the number of coaches - the ship
category. You now see the advantage: When using methods to get and set the
properties, you can perform more complex operations, as e.g. setting of
dependent properties. This preserves consistency of the object. If you set
$coaches
and $classification
to public
, we could set the number of
cabins to 1000 and classification to NORMAL
- and our ship would end up
being inconsistent.
Note
In Flow you’ll find getter and setter methods all over. No property in
Flow is set to public
.
Static Methods and Properties
Until now we worked with objects, instantiated from classes. Sometimes though,
it does not make sense to generate a complete object, just to be able to use a
function of a class. For this php offers the possibility to directly access
properties and methods. These are then referred to as static properties
respectively static methods
. Take as a rule of thumb: static properties are
necessary, every time two instances of a class are to have a common property.
Static methods are often used for function libraries.
Transferred to our example this means, that all ships are constructed by the
same shipyard. in case of technical emergency, all ships need to know the
actual emergency phone number of this shipyard. So we save this number in a
static property $shipyardSupportTelephoneNumber
:
PHP Code:
class Luxusliner extends Ship {
protected static $shipyardSupportTelephoneNumber = '+49 30 123456';
public function reportTechnicalProblem() {
echo 'On the ship ' . $this->name . ' a problem has been discovered.
Please inform ' . self::$shipyardSupportTelephoneNumber;
}
public static function setShipyardSupportTelephoneNumber($newNumber) {
self::$shipyardSupportTelephoneNumber = $newNumber;
}
}
$fidelio = new Luxusliner('Fidelio', 100);
$figaro = new Luxusliner('Figaro', 200);
$fidelio->reportTechnicalProblem();
$figaro->reportTechnicalProblem();
Luxusliner::setShipyardSupportTelephoneNumber('+01 1000');
$fidelio->reportTechnicalProblem();
$figaro->reportTechnicalProblem();
// Output
On the ship Fidelio a problem has been discovered. Please inform +49 30 123456
On the ship Figaro a problem has been discovered. Please inform +49 30 123456
On the ship Fidelio a problem has been discovered. Please inform +01 1000
On the ship Figaro a problem has been discovered. Please inform +01 1000
What happens here? We instantiate two different ships, which both have a problem
and do contact the shipyard. Inside the method reportTechnicalProblem()
you
see that if you want to use static properties, you have to trigger them with the
key word self::
. If the emergency phone number now changes, the shipyard has
to tell all the ships about the new number. For this it uses the
static method setShipyardSupportTelephoneNumber($newNumber)
. For the
method is static, it is called through the scheme classname::methodname()
,
in our case LuxuryLiner::setShipyardSupportTelephoneNumber(...)
.
If you check the latter two problem reports, you see that all instances of the
class use the new phone number. So both ship objects have access to the same
static variable $shipyardSupportTelephoneNumber
.
Important design- and architectural patterns
In software engineering you’ll sooner or later stumble upon design problems that are connatural and solved in a similar way. Clever people thought about design patterns aiming to be a general solution to a problem. Each design pattern is so to speak a solution template for a specific problem. We by now have multiple design patterns that are successfully approved in practice and therefore have found there way in modern programming and especially Flow. In the following we don’t want to focus on concrete implementation of the design patterns, for this knowledge is not necessary for the usage of Flow. Nevertheless deeper knowledge in design patterns in general is indispensable for modern programming style, so it might be fruitful for you to learn about them.
Tip
Further information about design patterns can e.g. be found on http://sourcemaking.com/ or in the book PHP Design Patterns by Stephan Schmidt, published by O’Reilly.
From the big number of design patterns, we will have a closer look on two that are essential when programming with Flow: Singleton & Prototype.
Singleton
This design pattern makes sure, that only one instance of a class can exist
at a time. In Flow you can mark a class as singleton by annotating it
with @Flow\Scope("singleton")
. An example: our luxury liners are all constructed
in the same shipyard. So there is no sense in having more than one instance of
the shipyard object:
PHP Code:
/**
* @Flow\Scope("singleton")
*/
class LuxuslinerShipyard {
protected $numberOfShipsBuilt = 0;
public function getNumberOfShipsBuilt() {
return $this->numberOfShipsBuilt;
}
public function buildShip() {
$this->numberOfShipsBuilt++;
// Schiff bauen und zurückgeben
}
}
$luxuslinerShipyard = new LuxuslinerShipyard();
$luxuslinerShipyard->buildShip();
$theSameLuxuslinerShipyard = new LuxuslinerShipyard();
$theSameLuxuslinerShipyard->buildShip();
echo $luxuslinerShipyard->getNumberOfShipsBuilt(); // 2
echo $theSameLuxuslinerShipyard->getNumberOfShipsBuilt(); // 2
Prototype
Prototype is sort of the antagonist to Singleton. While for each class only one
object is instantiated when using Singleton, it is explicitly allowed to have
multiple instances when using Prototype. Each class annotated with
@Flow\Scope("prototype")
is of type Prototype. Since this is the default
scope, you can safely leave this one out.
Note
Originally for the design pattern Prototype is specified, that a new object is to be created by cloning an object prototype. We use Prototype as counterpart to Singleton, without a concrete pattern implementation in the background, though. For the functionality we experience, this does not make any difference: We invariably get back a new instance of a class.
Now that we refreshed your knowledge of object oriented programming, we can take a look at the deeper concepts of Flow: Domain Driven Design, Model View Controller and Test Driven Development. You’ll spot the basics we just talked about in the following frequently.