Object Oriented Programming - 1
Introducing Object Oriented
Object definition: a tangible thing that we can sense, feel, and manipulate;
In programming, an object is a collection of data and associated behaviors.
Oriented Programming definition: So object-oriented means functionally directed toward modeling objects. This is one of many techniques used for modeling complex systems. It is defined by describing a collection of interacting objects via their data and behavior.
Object-oriented analysis (OOA) is the process of looking at a problem, system, or task (that somebody wants to turn into an application) and identifying the objects and interactions between those objects. The analysis stage is all about what needs to be done
For instance, in the following scenario:
Review our history
Apply for jobs
Browse, compare, and order products
A better turn of phrase might be object-oriented exploration. In software development, the initial stages of analysis include interviewing customers, studying their processes, and eliminating possibilities.
Object-oriented design (OOD) is the process of converting such requirements into an implementation specification. The designer must name the objects, define the behaviors, and formally specify which objects can activate specific behaviors on other objects. The design stage is all about how things should be done.
The output of the design stage is an implementation specification. If we were to complete the design stage in a single step, we would have turned the requirements defined during object-oriented analysis into a set of classes and interfaces that could be implemented in (ideally) any object-oriented programming language.
Object-oriented programming (OOP) is the process of converting this perfectly-defined design into a working program that does exactly what the CEO originally requested.
No matter how hard we try to separate these stages, we'll always find things that need further analysis while we're designing. When we're programming, we find features that need clarification in the design.
Most twenty-first century development happens in an iterative development model.
Objects and Classes
So, an object is a collection of data with associated behaviors. How do we differentiate between types of objects? Apples and oranges are both objects, but it is a common adage that they cannot be compared. Apples and oranges aren't modeled very often in computer programming, but let's pretend we're doing an inventory application for a fruit farm. To facilitate the example, we can assume that apples go in barrels and oranges go in baskets.
[This is very important]
Now, we have four kinds of objects: apples, oranges, baskets, and barrels. In object-oriented modeling, the term used for a kind of object is class. So, in technical terms, we now have four classes of objects.
It's important to understand the difference between an object and a class. Classes describe objects. They are like blueprints for creating an object. You might have three oranges sitting on the table in front of you. Each orange is a distinct object, but all three have the attributes and behaviors associated with one class: the general class of oranges.
The relationship between the four classes of objects in our inventory system can be described using a Unified Modeling Language (invariably referred to as UML, because three-letter acronyms never go out of style) class diagram. Here is our first class diagram:
This diagram shows that an Orange is somehow associated with a Basket and that an Apple is also somehow associated with a Barrel. Association is the most basic way for two classes to be related.
UML covers far more than class and object diagrams; it also has a syntax for use cases, deployment, state changes, and activities.
Our initial diagram, while correct, does not remind us that apples go in barrels or how many barrels a single apple can go in. It only tells us that apples are somehow associated with barrels. The association between classes is often obvious and needs no further explanation, but we have the option to add further clarification as needed.
This diagram tells us that oranges go in baskets, with a little arrow showing what goes in what. It also tells us the number of that object that can be used in the association on both sides of the relationship. One Basket can hold many (represented by a *) Orange objects. Any one Orange can go in exactly one Basket. This number is referred to as the multiplicity of the object. You may also hear it described as the cardinality. These are actually slightly distinct terms. Cardinality refers to the actual number of items in the set, whereas multiplicity specifies how small or how large the set could be.
I sometimes forget which end of the relationship line is supposed to have which multiplicity number. The multiplicity nearest to a class is the number of objects of that class that can be associated with any one object at the other end of the association. For the apple goes in barrel association, reading from left to right, many instances of the Apple class (that is many Apple objects) can go in any one Barrel. Reading from right to left, exactly one Barrel can be associated with any one Apple.
Specifying attributes and behaviors
We now have a grasp of some basic object-oriented terminology. Objects are instances of classes that can be associated with each other. An object instance is a specific object with its own set of data and behaviors; a specific orange on the table in front of us is said to be an instance of the general class of oranges. That's simple enough, but let's dive into the meaning of those two words, data and behaviors.
Data describes objects
Data represents the individual characteristics of a certain object. A class can define specific sets of characteristics that are shared by all objects from that class. The orange class could have a weight attribute to represent that datum. All instances of the orange class have a weight attribute, but each orange has a different value for this attribute. As a more realistic example, two objects representing different customers might have the same value for a first name attribute.
Attributes are frequently referred to as members or properties.
In our fruit inventory application, the fruit farmer may want to know what orchard the orange came from, when it was picked, and how much it weighs.
They might also want to keep track of where each Basket is stored. Apples might have a color attribute, and barrels might come in different sizes. Some of these properties may also belong to multiple classes (we may want to know when apples are picked, too), but for this first example, let's just add a few different attributes to our class diagram:
Depending on how detailed our design needs to be, we can also specify the type for each attribute.
Behaviors are Actions
Behaviors are actions that can occur on an object. The behaviors that can be performed on a specific class of object are called methods.
At the programming level, methods are like functions in structured programming, but they magically have access to all the data associated with this object. Like functions, methods can also accept parameters and return values.
A method's parameters are provided to it as a list of objects that need to be passed into that method. The actual object instances that are passed into a method during a specific invocation are usually referred to as arguments. These objects are used by the method to perform whatever behavior or task it is meant to do. Returned values are the results of that task.
We've stretched our comparing apples and oranges example into a basic (if far-fetched) inventory application. Let's stretch it a little further and see whether it breaks. One action that can be associated with oranges is the pick action. If you think about implementation, pick would need to do two things:
Place the orange in a basket by updating the Basket attribute of the orange Add the orange to the Orange list on the given Basket.
So, pick needs to know what basket it is dealing with. We do this by giving the pick method a Basket parameter. Since our fruit farmer also sells juice, we can add a squeeze method to the Orange class. When called, the squeeze method might return the amount of juice retrieved, while also removing the Orange from the Basket it was in.
The class Basket can have a sell action. When a basket is sold, our inventory system might update some data on as-yet unspecified objects for accounting and profit calculations. Alternatively, our basket of oranges might go bad before we can sell them, so we add a discard method. Let's add these methods to our diagram:
Adding attributes and methods to individual objects allows us to create a system of interacting objects.
Each object in the system is a member of a certain class. These classes specify what types of data the object can hold and what methods can be invoked on it. The data in each object can be in a different state from other instances of the same class; each object may react to method calls differently because of the differences in state.
Object-oriented analysis and design is all about figuring out what those objects are and how they should interact. The next section describes principles that can be used to make those interactions as simple and intuitive as possible.
Hiding details and creating the public interface
The key purpose of modeling an object in object-oriented design is to determine what the public interface of that object will be.