Defining Classes in Java

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:

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

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.

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.

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);

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.

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);
    }    
}

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.²⁾

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:

ClickerCounter.java

public class Counter {
 
    // Class definition
    int count;
 
    void click(){
        count++;
    }
 
    void reset(){
        count = 0;
    }
}

And here is the file that has the “main” stuff:

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);
    }    
}

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.

Copyright © 2020 Mithat Konar. All rights reserved.