Instance Variables

• An instance variable stores the values that are tied to an object. Creating an instance variable follows the same basic syntax as declaring a local variable:

  Class	variableName;
  

  The syntactical difference is that an instance variable is not declared inside of a method, it is just inside the class.

• We will also make all of our instance variables private, which means that they cannot be directly accessed from outside of the class. This will allow us to hide the internal details of our class from anyone who uses it and allow us to write our class to ensure the integrity of the data. We will see more about this later; for now just follow this syntax.

  At this point, the actual code would make it clearer than just trying to discuss the proper syntax. So, let's start writing a class that represents a test score. A test score has two attributes tied to it - the number of points possible and the number of points earned.

  Start of a TestScore Class

  
  public class TestScore
  {
  private int pointsEarned;
  private int pointsPossible;
  } 

  Quick Review Let't have a quick review of what you already know about the code above: What is the name of the class? What is the name of the source file? How did you know this? What is the data type for the variable named pointsPossible? What is the data type for the variable named pointsEarned? How would an application use this class once it was completed? What would the code look like?

  Now, lets go over it. Here is the code again so you don't have to scroll up to see it.

  TestScore Class (repeated)

  
  public class TestScore
  {
  private int pointsEarned;
  private int pointsPossible;
  } 

• Notice that the statements for the instance variables are just inside the class, not inside a method. This means that the variables will be created when an object of the TestScore class is declared.

• Remember that an object of a particular class is called an instance of the class, and when we declare an object, it is called instantiating the class. This is where the term instance variable comes from.

• Because they are private, the instance variables will only be accessable from a method that belongs to the TestScore class. (No, we don't have any methods yet, but we will soon.)

• At this point in our example, we cannot actually create a TestScore object because we need constructors, so let's learn how to write them and the other basic methods we need..