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