Class Variables vs Instance Variables

This is a post somewhat superficial only to take some doubts that sometimes people have between these two types of variables.

Under the object oriented paradigm, classes that we created may have variables with several states (protected, public and private) and two different types: class variables and instance variables.

Instance Variables

They are those variables that for each new instance of class (every new object that is created) they start with different values.

Java sample:

public class Boat

{

       // Instance variables 

       private String registration;

       private String flag;

       private String captainName;

       private float lenght;

       private float tonnage;

}

All these variables, for each new boat that is created will have different values, so it is justified to be instance variables.

Class Variables

They are those variables that will have a fixed value for all instances of that class. When you change a value in a class all instances of that class will have that value changed.

Java sample:

public class Boat

{

       // Class variables

       private static float boatFee; 

       // Instance variables 

       private String registration;

       private String flag;

       private String captainName;

       private float lenght;

       private float tonnage;

}

As the rate is a fixed amount, it is fair to become a variable of class so all instances of Boat created will have the same rate applied to the boat.

This post is on my personal blog too, in portuguese languange, here.