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Started working on the methods section of chapter 4, changed readme to reflect being on github.
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4 - Object Oriented Programming/1.1 - Classes and Objects.tex
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| Classes \emph{AND} Objects? | |||
| What is the difference? | |||
| \\ | |||
| Well I am glad you asked. | |||
| A class is the definition or blueprint of an object. | |||
| A class tells a program what to expect when coming across an object of the given class. | |||
| What methods and properties to expect and even how to create and destroy objects. | |||
| \par | |||
| \emph{An object refers to a single instance of a class} | |||
| \\ | |||
| Objects are refered to as being instances of a class. | |||
| When you deine a class you are not creating a usable object that you can then call methods on | |||
| or access properties of. | |||
| You must then create an instance of that class (object) to be able to use it throughout your program. | |||
| \subsection{Classes} | |||
| Ok, so as I mentioned before we need to first define a class before we can start creating objects | |||
| and using them in our program. | |||
| How do we do this? | |||
| \par | |||
| \begin{lstlisting}[caption={Class Definition}] | |||
| class Person | |||
| \end{lstlisting} | |||
| Ok...? | |||
| That seems too easy? | |||
| \\ | |||
| Yes creating classes is usually fairly easy, just make sure to check how to create a class in your language of choice. | |||
| \subsection{Objects} | |||
| Ok, so we have our class definition from above, but how do we create an instance of this class so we can use it in our program? | |||
| \par | |||
| \begin{lstlisting}[caption={Object Declaration}] | |||
| class Person | |||
| p = new Person() | |||
| \end{lstlisting} | |||
| That is it. | |||
| We can create an instance of our \pigVar{Person} class by using the \pigVar{new} keyword and calling \pigVar{Person()}. | |||
| We can assign this instance to a variable, \pigVar{p}, and then use \pigVar{p} as an alias for our object throughout | |||
| our program. | |||
| \par | |||
| Can we only have one object? | |||
| No, you can have as many instances as your would like. | |||
| \par | |||
| \begin{lstlisting}[caption={Multiple Object Instances}] | |||
| class Person | |||
| p1 = new Person() | |||
| p2 = new Person() | |||
| p3 = new Person() | |||
| \end{lstlisting} | |||
| This then allows us to act on each of these instances as though they are separate. | |||
| What does that mean? | |||
| It means that if we were to modify a property of \pigVar{p1} then it would not have any effect on | |||
| the same properties in \pigVar{p2} and \pigVar{p3}. | |||
| \subsection{Properties} | |||
| We are able to store variables inside of a class, these are called properties. | |||
| To define a property we must define its name, access modifier and default value (if any). | |||
| \par | |||
| An access modifier can either be \emph{public}, \emph{private} or \emph{protected} (some languages do not support | |||
| access modifiers). | |||
| The \emph{public} modifier means that anyone who has access to the object can read and modify that property. | |||
| The \emph{private} modifier means that no one outside of the object can read and modify the property, meaning that | |||
| only the object itself has acess to the given property. | |||
| The \emph{protected} modifier means that the given object and its children (we will get to this later in the chapter) | |||
| will have access to read and modify the property. Lets look at an example. | |||
| \par | |||
| \begin{lstlisting}[caption={Class Properties}] | |||
| class Person | |||
| public name | |||
| private age = 22 | |||
| p = new Person() | |||
| p.name = ``Brett Langdon'' | |||
| p.age = 23 //this will cause an error | |||
| \end{lstlisting} | |||
| In this example we are creating a class with two properties, one is public (\pigVar{name}) and the other is private (\pigVar{age}). | |||
| We then create a new instance of our class assigning it to the variable \pigVar{p}. | |||
| Then we set the public property \pigVar{name} to \pigVal{``Brett Langdon''}. | |||
| In line 8 there is the comment ``this will cause an error'' this is because the property \pigVar{age} is private and cannot be accessed | |||
| from outside of the class. | |||
| \subsection{Methods} | |||
| So what is a Method? | |||
| A method, simply put, is a function that belongs to a class. | |||
| We use methods for the same reasons that we use functions for, to provide code reuse within our applications. | |||
| Ok, so we know how to use functions, but how do we use them from within a class? | |||
| \par | |||
| \begin{lstlisting}[caption={Class Methods}] | |||
| class Person | |||
| public name | |||
| private age | |||
| function printName() | |||
| print this.name | |||
| p = new Person() | |||
| p.name = ``brett'' | |||
| p.printName() | |||
| \end{lstlisting} | |||
| The output of this code would be \pigOut{brett}. | |||
| \subsection{Special Methods} | |||