I'm new to OOP/PHP. What's the practicality of visibility and extensibility in classes?

Question

I'm obviously brand new to these concepts. I just don't understand why you would limit access to properties or methods. It seems that you would just write the code according to intended results. Why would you create a private method instead of simply not calling that method? Is it for iterative object creation (if I'm stating that correctly), a multiple developer situation (don't mess up other people's work), or just so you don't mess up your own work accidentally?

Answer 1

We create private methods so that consumers of our classes don't have to care about implementation details - they can focus on the few nifty things our classes provide for them.

Moreover, we're obligated to consider every possible use of public methods. By making methods private, we reduce the number of features a class has to support, and we have more freedom to change them.

Say you have a Queue class - every time a caller adds an item to the queue, it may be necessary to to increase the queue's capacity. Because of the underlying implementation, setting the capacity isn't trivial, so you break it out into a separate function to improve the readability of your Enqueue function. Since callers don't care about a queue's capacity (you're handling it for them), you can make the method private: callers don't get distracted by superfluous methods, you don't have to worry that callers will do ridiculous things to the capacity, and you can change the implementation any time you like without breaking code that uses your class (as long as it still sets the capacity within the limited use cases defined by your class).

Answer 2

There are several reasons for using encapsulation, one of the strongest is: Imagine using a large, complicated library written by someone else. If every object was unprotected you could unknowingly be accessing or changing values that the developer never intended to be manipulated in that way.

Hiding data makes the program easier to conceptualize and easier to implement.

Answer 3

It's all about encapsulation. Methods are private that do the inner grunt work while exposing graceful functions that make things easy. E.g. you might have an $product->insert() function that utilizes 4 inner functions to validate a singleton db object, make the query safe, etc - those are inner functions that don't need to be exposed and if called, might mess up other structures or flows you, the developer, have put in place.

Answer 4

Your last two points are quite accurate - you don't need multiple developers to have your stuff messed with. If you work on a project long enough, you'll realize you've forgotten much of what you did at the beginning.

One of the most important reasons for hiding something is so that you can safely change it later. If a field is public, and several months later you want to change it so that every time the field changes, something else happens, you're in trouble. Because it was public, there's no way to know or remember how many other places accessed that field directly. If it's private, you have a guarantee that it isn't being touched outside of this class. You likely have a public method wrapped around it, and you can easily change the behavior of that method.

In general, more you things make public, the more you have to worry about compatibility with other code.

Answer 5

For example - private/protected method may be part of some class which is called in another (public) method. If that part is called in more public methods, it makes sense. And yet you don't want these methods to be called anywhere else.

It's quite the same with class properties. Yes, you can write all-public classes, but whats the fun in that?

Answer 6

a multiple developer situation (don't mess up other people's work), or just so you don't mess up your own work accidentally?

Mainly these two things. Making a method public says "this is how the class is supposed to be used by its clients", making it private says "this is an implementation detail that may change without warning and which clients should not care about" AND forces clients to follow that advice.

A class with a few, well documented public methods is much easier to use by someone who's not familiar with it (which may well be its original author, looking at it for the first time in 6 months) than one where everything is public, including all the little implementation details that you don't care about.

Answer 7

It all comes down to encapsulation. This means hiding the insides of the class and just caring about what it does. If you want to have a credit card processing class, you don't really care 'how' it processes the credit card. You just want to be able to go: $creditCardProcessor->charge(10.99, $creditCardNumber); and expect it to work.

By making some methods public and others private or protected, we leave an entry way for others so they know where it is safe to call code from. The public methods and variables are called an 'interface'.

For any class, you have an implementation. This is how the class carries out its duty. If it is a smoothie making class, how the class adds the ingredients, what ingredients it adds, etc are all part of the implementation. The outside code shouldn't know and/or care about the implementation.

The other side of the class it its interface. The interface is the public methods that the developer of the class intended to be called by outside code. This means that you should be able to call any public method and it will work properly.

Answer 8

It makes collaboration easier, you tell the users of your classes what parts should not change so often and you can guarantee that your object will be in a meaningful state if they use only public methods.

It does not need to be so strict as distinguishing between private/public/whatever (I mean enforced by the language). For example, in Python, this is accomplished by a naming convention. You know you shouldn't mess with anything marked as not public.