We are going to write a simple class for implementing a counter similar to one of these:¹⁾

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.

Class names in Java traditionally use CamelCase with the first letter capitalized.

Here's a Java definition for a ClickerCounter class:

ClickerCounter.java

public class ClickerCounter {
 
    int count;
 
    void click(){
        count++;
    }
 
    void reset(){
        count = 0;
    }    
}

In Java, class definitions need to go in their own files, and the files need to be called the name of the class with the .java extension.²⁾ 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. 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

The above is just a class definition. It doesn't actually make an object we can use.

The program below shows you how to make, or instantiate a ClickerCounter and then call the object's methods to change the state of the object.

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);
    }    
}

The ClickerCounter class definition above effectively encapsulates the state and behavior of a clicker counter. But it has issues. For example, if we change our example as follows:

ClickerExample.java

public class ClickerExample {
 
    public static void main(String[] args) {
        var myCounter = new ClickerCounter();
 
        myCounter.reset();
        myCounter.count = 492341;  // <- LOOK HERE!
        System.out.println(myCounter.count);
        myCounter.click();
        myCounter.click();
        System.out.println(myCounter.count);
    }    
}

the object will happily oblige the user's wish to set the count to some arbitrary value. That's not something you can typically do with a clicker counter. In other words, the object offers no protection of its state. This is easy enough to solve with Java's access modifiers. Let's look at a modified version of our class definition that gives us some protection using access modifiers:

ClickerCounter.java

public class ClickerCounter {
 
    private int count;
 
    public void click(){
        count++;
    }
 
    public void reset(){
        count = 0;
    }    
}

Access modifiers are placed before the start of a member. Java offers four levels of protection, and they just so happen to all start with the letter 'p':

• package: this is what you get by default (i.e., when you don't specify anything, as in our first definition of the class). It means anything within the same package as the class definition is able to access the member.
• public: anyone anywhere can access the member. This is a good choice for methods that are part of the class' interface.
• private: only those of the same class can access the member.
• protected: only those of the same _or a derived_ class can access the member. ³⁾

With the changes made above, if we try to run ClickerExample we will see that the code will refuse to compile. When the compiler hits the statement

myCounter.count = 492341;

it cannot access the count member of myCounter because we said it's private. Remember, we want our objects to be self-governing, and this helps make sure this is the case.

Copyright © 2020 Mithat Konar. All rights reserved.