Instantiable Classes

A class can serve many functions, the most basic of which is to serve as a blueprint for the data and methods used to access and manipulate that data. We will refer to these as “object” or “instance” classes. An object class is a class with the primary purpose of encapsulating and manipulating related data.

When you instantiate an object of a class, you create a variable of that type. The class definition is only a only the specification for each of those items. So, let’s look at the next step, which is to create objects based on the classes we’ve defined.

Objects

Once we’ve created a class, we can then use it to instantiate an object based on that class. Let’s break that statement down a bit.

The word instantiate comes from the word instance, which means “an example or single occurrence of something.” So, we’re creating a single example of a class, which we call an object.

Most high level programming languages create an object by calling a special function generically called a constructor, which usually has the same name as the class from which you are trying to create an instance. For example, we can define our Ingredient class:

public class Ingredient {
  String name;
  double amount;
  String unit;

  public Ingredient (String aName, double anAmount, String aUnit){
    this.name = aName;
    this.amount = anAmount;
    this.unit = aUnit;
  }
}

Then, elsewhere in our code, we can instantiate Ingredient objects by calling the constructor - effectively, we just call the data type itself as if it were a function along with the new keyword, which will return a new instance of that object!

Ingredient flour = new Ingredient("Flour", "1.0", "cups");
Ingredient sugar = new Ingredient("Sugar", "2.0", "cups");

Here flour and sugar are both variables of type Ingredient, but they represent different things and would each contain their own name, amount and unit.

Some Vocabulary

Object-oriented programming introduces a large number of new terms, each with very specific uses. Here is a quick overview of some of the new terms we’ve learned so far:

• Object-Oriented Programming: A programming paradigm that uses objects as the basis of the program’s structure
• Class: A specification of a new data type in a program, including fields and methods, that are used to describe a single part of a program
• Object: A unique instance of a class, representing a single concrete item in the program
• Instantiation: The process of creating an object based on a class
• Attribute: A single variable defined in a class or stored in an object; also called a field
• Method: An action that can be performed by a class or object; typically to access or change an Attribute; also called an action