Mithat Konar (the wiki)

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======  Defining Classes in Java ======

We are going to write a simple class for implementing a counter similar to one of these:((Picture from: "Totty Clicker - Gadgets at Play.com (UK)." Play.com (UK): DVDs, Music CDs, MP3s, Video Games, Books, Electronics & Gadgets - Free Delivery. [[http://www.play.com/Gadgets/Gadgets/4-/11566684/Totty-Clicker/Product.html?ptsl=1&ob=Price&fb=0#]] (accessed January 25, 2011). 
)) \\ {{ :python:about_python:tottycounter.jpg?300 |}}

In case you've never seen one of these bits of advanced technology before, it's a tally counter or, as I prefer to call it, a clicker counter. It has two controls: a button on top you click to advance the counter by one and another button elsewhere you press to reset the count to zero. Our goal is to build one of these in software using object-orientation.

We first have to decide what a counter thing //is//. One way to start building this model for a class is to start listing the public-facing **behavior** (or the //interface//) you want an object of that class to have. A pretty comprehensive list of the things you might do with a clicker-counter is:

  * //click//: makes the count increase by one.
  * //reset//: sets the count to zero.

Next we can think about what **attributes** we'll need to keep track of the **state** of a clicker-counter. In this case, it's pretty simple: all we really need is one integer to store the //count// value.

So, a summary of what we need so far is:

  * a //click// operation
  * a //reset// operation
  * an integer to store the //count//

In Java, object attributes, which together make up the state, are defined in variables called **instance variables**. A class definition can include as many instance variables as it needs to store the state. In our case, we are getting off easy: the clicker-counter only needs one. The **operations** our object will be capable of, which make up behavior, are defined using **methods**. We call anything belonging to a class (e.g., instance variables and methods) a **member** of the class.


===== Our first Java class =====

Class names in Java traditionally use [[http://www.c2.com/cgi/wiki?CamelCase|CamelCase]] with the first letter capitalized.

Here's a Java definition for a ''ClickerCounter'' class:

<code java>
public class ClickerCounter {
    
    int count;
    
    void click(){
        count++;
    }
    
    void reset(){
        count = 0;
    }    
}
</code>

I have deliberately not used any comments in the definition so you can more easily see the code.

In the above, ''count'' is an attribute/instance variable that keeps track of the state of the object. and the ''click'' and ''reset'' methods implement the behavior.

In actuality, this is a pretty crappy class definition for reasons we'll see later. But for now it's enough to get us started.

==== Instantiation and use ====

The above is just a class definition. It doesn't actually make any objects you can use. The code below shows you how to make, or **instantiate** a ''ClickerCounter'' object and then call the object's methods to change the state of the object. 

<code java>
var myCounter = new ClickerCounter();  // instantiate a ClickerCounter

myCounter.reset();  // count is 0
myCounter.click();  // count is 1
System.out.println(myCounter.count);
myCounter.click();  // count is 2
myCounter.click();  // count is 3
System.out.println(myCounter.count);
myCounter.reset();  // count is 0
System.out.println(myCounter.count);
</code>

===== Where does the code go? =====

==== The easy-for-beginners way ====
If there's only one custom class in your program, an easy way to write your program is to incorporate the class definition and the program's ''main()'' method into the same class definition.

<file java ClickerCounter.java>
public class ClickerCounter {

    // Class definition (instance members)
    int count;
    
    void click(){
        count++;
    }
    
    void reset(){
        count = 0;
    }   

    // Program's main method that uses the class
    public static void main(String[] args) {
        var myCounter = new ClickerCounter();  // instantiate a ClickerCounter
        
        myCounter.reset();  // count is 0
        myCounter.click();  // count is 1
        System.out.println(myCounter.count);
        myCounter.click();  // count is 2
        myCounter.click();  // count is 3
        System.out.println(myCounter.count);
        myCounter.reset();  // count is 0
        System.out.println(myCounter.count);
    }    
}
</file>

This isn't generally recommended though because very often you will write more than one custom class in your program. If this is the case, the approach above won't work because Java's rules say any file can only contain at most one public class definition.((You //can// have additional non-public class definitions in a file, but that's something we can't discuss yet.))

==== The better way ====

A better way to manage your programs' custom classes is to put each custom class into its own file. When you do this, the files need to be the name of the class with the ''.java'' extension.

The following is how to break up the class definition and the "main" stuff. Here is the file that has only the class definition:

<file java ClickerCounter.java>
public class Counter {

    // Class definition
    int count;
    
    void click(){
        count++;
    }
    
    void reset(){
        count = 0;
    }
}
</file>

And here is the file that has the "main" stuff:

<file java ClickerExample.java>
public class ClickerExample {

    public static void main(String[] args) {
        var myCounter = new ClickerCounter();  // instantiate a ClickerCounter
        
        myCounter.reset();  // count is 0
        myCounter.click();  // count is 1
        System.out.println(myCounter.count);
        myCounter.click();  // count is 2
        myCounter.click();  // count is 3
        System.out.println(myCounter.count);
        myCounter.reset();  // count is 0
        System.out.println(myCounter.count);
    }    
}
</file>

This is the best way to manage your files, and the way I will use in the remainder of these tutorials. However, depending on how you are expected to submit homework, you may be required to use the "easy for beginners" way to write you code.

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