What's an object? A beginner's guide to Object Oriented Programming
tl;dr
Object Oriented Programming (OOP) is an incredibly important concept that is difficult to explain to a person new to programming. This post will attempt to break down the concept of OOP into plain English using baking as an example.
Objects. How do they work?
When I started programming, the term Object Oriented Programming (OOP) was being thrown around constantly, but the internet doesn’t always take into consideration that newbies don’t know how to read technical documentation. Technical documentation, even that which is aimed at new programmers, often forgets to take into consideration the basics that one is “supposed to know”. One of these things, I found, was as simple as explaining what an object is.
For the longest time, I thought arrays and objects were the same thing. An array is an ordered map. An easy way to think about this is like a grocery list.
(
[0] => sugar
[1] => flour
[2] => butter
)
If you wanted to reference a specific item in that array, you could just print $array[0] and get “sugar”.
Objects are slightly more difficult to explain. Even when you search on Google, it tells you that there’s no way you can sum it up easily:
The Oracle article actually does a decent job at explaining it, but I’ll give an example that finally made everything click for me.
For this lesson, we will cover the following terms:
- Class
- Constructor
- Object
- Instance
- Inheritance
- Function (or Method in other languages like C++ or Java)
And because I like baking, I’ll use the class Recipe.
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<? php
class Recipe {
protected $ingredients;
protected $steps;
public function Recipe() {
$this->ingredients = array();
$this->steps = array();
}
public function addIngredient($ingredient) {
$this->ingredients[] = $ingredient;
}
public function addStep($step) {
$this->steps[] = $step;
}
public function printOutput() {
echo "Ingredients:\n";
for($i = 0; $i < count($this->ingredients); $i++) {
echo $i . " " . $this->ingredients[$i] . "\n";
}
// Prints numbered steps
echo "Steps:\n";
for($i = 0; $i < count($this->steps); $i++) {
echo $i . " " . $this->steps[$i] . "\n";
}
echo "\n";
}
}
Class
In this example, Recipe is an example of a Class. A Class is your source of truth or set of basic instructions for the thing you’re trying to make. It should be pretty vague and generic so that you can reuse it in other situations. At its most basic core concept, a recipe is a list of ingredients and a list of steps you follow with those ingredients.
Constructor
Your Constructor (line 6) contains instructions on how you want to build (or instantiate) your Class. In this case, you build your recipe by having a list of ingredients and a list of steps.
Object
Now we want to actually do something with our recipe.
//Create a new instance of Recipe called $sugarCookie
$sugarCookie = new Recipe();
$sugarCookie->addIngredient("sugar");
$sugarCookie->addIngredient("flour");
$sugarCookie->addIngredient("butter");
$sugarCookie->addStep("Mix together");
$sugarCookie->addStep("Cut out shapes");
$sugarCookie->addStep("Put in oven");
$sugarCookie->printOutput();We want to make an actual item (Object) by cloning our original recipe (Class). Here, we create a new instance of Recipe and call it $sugarCookie. We add our ingredients, run our cookie magic, and get magic tasty goodies out of it.
Instance
An Instance is an Object that is a copy of a Class. $sugarCookie is an instance of Recipe. We can make as many Instances of Recipe as we want!
//Create a new instance of Recipe called $pie
$pie = new Recipe();
$pie->addIngredient("pie crust");
$pie->addIngredient("filling");
$pie->addStep("Roll out crust");
$pie->addStep("Fill with filling");
$pie->addStep("Roll out top crust");
$pie->addStep("Put in oven");
$pie->printOutput();Here we have $pie as another Instance of our original class of Recipe. We have completely different variables (pie crust and filling) but the same basic structure is the same as $sugarCookie.
Inheritance
Remember how I said a Class should be as vague as possible? The reason is because of Inheritance.
class Brownie extends Recipe {
public function Brownie() {
parent::Recipe();
$this->ingredients[] = "chocolate";
}
}
$brownie = new Brownie();
$brownie->addIngredient("sugar");
$brownie->addIngredient("flour");
$brownie->addIngredient("eggs");
$brownie->addStep("Mix together");
$brownie->addStep("Bake");
$brownie->printOutput();Here we are extending our Recipe to create a new class of Brownie. Since all brownies have a lot of chocolate, it makes sense that our Brownie class should add it by default. So while our Brownie class inherits all of the basic ingredients and steps from our Recipe class, it doesn’t do enough to cover this specific class’s needs, so we modify it as needed.
Function
Looking through our Recipe we see things like public function addIngredients(). This is an example of a Function. I like to think of functions as verbs. They do things to the variables (in this case, ingredients). Our functions should always be small and reusable to avoid surprises. In this case, addIngredients makes a list of the ingredients we need and nothing else.
Let’s look at it all together
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<?php
//Make a new recipe class that is easy to duplicate
class Recipe {
protected $ingredients;
protected $steps;
public function Recipe() { //the constructor that builds our class
$this->ingredients = array();
$this->steps = array();
}
public function addIngredient($ingredient) {
$this->ingredients[] = $ingredient; //adds ingredients to an array
}
public function addStep($step) {
$this->steps[] = $step; //adds steps to an array
}
public function printOutput() {
// Prints ingredients
echo "Ingredients:\n";
for($i = 0; $i < count($this->ingredients); $i++) {
echo $i . " " . $this->ingredients[$i] . "\n";
}
// Prints numbered steps
echo "Steps:\n";
for($i = 0; $i < count($this->steps); $i++) {
echo $i . " " . $this->steps[$i] . "\n";
}
echo "\n";
}
}
//Create a new instance of Recipe called $sugarCookie
$sugarCookie = new Recipe();
$sugarCookie->addIngredient("sugar");
$sugarCookie->addIngredient("flour");
$sugarCookie->addIngredient("butter");
$sugarCookie->addStep("Mix together");
$sugarCookie->addStep("Cut out shapes");
$sugarCookie->addStep("Put in oven");
$sugarCookie->printOutput();
//Create a new instance of Recipe called $pie
$pie = new Recipe();
$pie->addIngredient("pie crust");
$pie->addIngredient("filling");
$pie->addStep("Roll out crust");
$pie->addStep("Fill with filling");
$pie->addStep("Roll out top crust");
$pie->addStep("Put in oven");
$pie->printOutput();
//Extend Recipe to create a new class of Brownie. Brownie inherits from Recipe
class Brownie extends Recipe {
public function Brownie() {
parent::Recipe(); //inherits the parent class's constructor
$this->ingredients[] = "chocolate";
}
}
//Create a new instance of Brownie called $brownie
$brownie = new Brownie();
$brownie->addIngredient("sugar");
$brownie->addIngredient("flour");
$brownie->addIngredient("eggs");
$brownie->addStep("Mix together");
$brownie->addStep("Bake");
$brownie->printOutput();
Scope
“But wait!” you exclaim. “If we’re using the same constructor in Recipe and Brownie (lines 7 and 42), how come we don’t end up with chocolate in our $pie? This is where something called scope comes into play. Scope is best described as the things (such as variables or functions) available to you at a given point. Looking at the graph below, variables and functions are available for inheritance (think of your parents giving you their DNA) but you’re unable to inherit “up” (you cannot affect your parents’ DNA).
Everything in our Class (all Variables and Functions) are available for use in all Objects that inherit (or extend) that class. Things within specific Objects are only available to the Object and the Functions within it. Variables used in a Function are only available to that Function. It’s like an apartment: a landlord can have the master keys and be able to get into the common area and your apartment, but your apartment key doesn’t work on anyone else’s apartment.
Conclusion
Object Oriented Programming is an incredibly important thing to learn, but it’s not always easy to understand all the different parts. People use all sorts of examples to try to explain it, but it didn’t really click with me until someone sat down and explained it to me in plain English using an example that I was intimately familiar with. This is just one way that I’ve found helped me learn. Hopefully it was helpful for you!
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