By Barry A. Burd

When you start learning object-oriented programming, you may think that this class idea is a big hoax. Some geeks in Silicon Valley had nothing better to do, so they went to a bar and made up some confusing gibberish about classes. They don’t know what it means, but they have fun watching people struggle to understand it.

Well, that’s not what classes are all about. Classes are serious stuff. What’s more, classes are useful. Many reputable studies have shown that classes and object-oriented programming save time and money.

Even so, the notion of a class can be elusive. Even experienced programmers — the ones who are new to object-oriented programming — have trouble understanding how an object differs from a class.

Classes, objects, and tables

Because classes can be mysterious, I’ll expand your understanding with another analogy. The image below has a table of three purchases. The table’s title consists of one word (the word Purchase), and the table has three column headings: the words unitPrice,quantity, and taxable. Well, this has the same stuff — Purchase, unitPrice, quantity, and taxable.

class Purchase {

double unitPrice;

int quantity;

boolean taxable;

}

So in the image below, think of the top part of the table (the title and column headings) as a class. Like the code above, this top part of the table tells you what it means to be a Purchase. (It means having an unitPrice value, a quantity value, and a taxable value.)

creating a table with Java
A table of purchases.

A class is like the top part of a table. And what about an object? Well, an object is like a row of a table. For example, with this code, you create two objects (two instances of the Purchase class).

class ProcessPurchases {

 

public static void main(String[] args) {

Purchase purchase1 = new Purchase();

purchase1.unitPrice = 20.00;

purchase1.quantity = 3;

purchase1.taxable = true;

Purchase purchase2 = new Purchase();

purchase2.unitPrice = 20.00;

purchase2.quantity = 3;

purchase2.taxable = false;

 

double purchase1Total = purchase1.unitPrice * purchase1.quantity;

if (purchase1.taxable) {

purchase1Total *= 1.05;

}

 

double purchase2Total = purchase2.unitPrice * purchase2.quantity;

if (purchase2.taxable) {

purchase2Total *= 1.05;

}

 

if (purchase1Total == purchase2Total) {

System.out.println("No difference");

} else {

System.out.println("These purchases have different totals.");

}

}

}

The first object has unitPrice value 20.00, quantity value 3, and taxable value true. In the corresponding table, the first row has these three values — 20.00, 3, and true.

creating a purchase table in java
A purchase corresponds to a row of the table.

Some questions and answers

Here’s the world’s briefest object-oriented programming FAQ:

  • Can I have an object without having a class?

No, you can’t. In Java, every object is an instance of a class.

  • Can I have a class without having an object?

Yes, you can. In fact, many Java programs create a class without an object. That’s just fine. It’s business as usual.

  • After I’ve created a class and its instances, can I add more instances to the class?

Yes, you can. In a for loop, you could create a dozen instances and you’d have a dozen rows in the table. With no objects, three objects, four objects, or more objects, you still have the same old class.

  • Can an object come from more than one class?

Bite your tongue! Maybe other object-oriented languages allow this nasty class cross-breeding, but in Java, it’s strictly forbidden. That’s one thing that distinguishes Java from some of the languages that preceded it: Java is cleaner, more uniform, and easier to understand.