Instance variables are unique data in object-oriented programming that belong to each object. They can have different values and visibility properties, and are distinct from static variables. Inheritance affects access to instance variables, with private variables restricted to their respective classes and protected variables visible to both superclass and subclass.
An instance variable is a piece of data in object-oriented programming that has its own unique value for each object it resides in. Instance variables can also be called instance members, non-static fields, and data members. They are an important part of most objects, as they contain specific data that the object uses for calculation.
For example, if there are two objects, A and B, where object A has an instance variable called “blah” and object B also has an instance variable called “blah”, the two variables “blah” are entities distinct. Each variable is associated with the object that encapsulates it. The full name of the “blah” variable associated with object A is “A.blah”, while the full name of the “blah” variable associated with object B is “B.blah”. Each if these variables can have their own value. If objects A and B are of the same type and another object of that type, C, is created and object C will also have an instance variable “bla”, which can have its own unique value.
Instance variables are often confused with static variables, but the two are significantly different. Unlike instance variables, of which every object of a particular type has its own, static variables have only one value and this value is the same for every object of the appropriate type. Static variables are often called class variables, which are not the same as instance variables.
Depending on the visibility properties assigned to them, instance variables can be visible only within a particular object, to an object and its subclasses, or to the object in which it resides as well as to any other object. The latter type of visibility, called public visibility, is often considered a bad practice, such as variables, because it is easy to violate encapsulation principles in object-oriented programming. Despite this taboo, there are times when public instance variables are useful. For example, many standard classes in Java have public instance variables, which sometimes allow for easy property modification.
The visibility of an instance variable also comes into play with inheritance. If one class inherits from another, this does not immediately give both classes access to the other’s instance variables. Private instance variables in the superclass and subclass remain restricted to those classes only, so a private variable in the subclass cannot be seen by the superclass and vice versa. The only instance variables that both the superclass and subclass can see are the protected variables in the superclass.
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