What Are Objects?

Anything can be thought of as an object. Objects are all around you. You can consider Tree to be a particular class of objects: trees in general. The notion of a Tree in general is a rather abstract concept - although any tree fits the description, it is more useful to think of more specific types of tree. Hence, the Oak tree in my yard which I call myOak, the Ash tree in your yard which you call thatDarnedTree, and a generalSherman, the well-known redwood, are actual instances of specific types of tree, subclasses of Tree that in this case happen to be Oak, Ash, and Redwood. Note how we drop into the jargon here - class is a term that describes a specification for a collection of objects with common properties.

A class is a specification, or blueprint - expressed as a piece of program code - that defines what goes to make up a particular sort of object. A subclass is a class that inherits all the properties of the parent class, but that also includes extra specialization. Particular classes of Tree, such as Oak or Ash, have all the characteristics of the most general type, Tree; otherwise, they could not be considered to be such. However, each subclass of Tree, such as Oak, has its own characteristics that differentiate Oak objects from other types of Tree.

Of course, you define a class specification to fit what you want to do in your application context. There are no absolutes here. For my trivial problem, the specification of a Tree class might just consist of its species name and its height. If you are an arboriculturalist, then your problem with trees may require a much more complex class, or more likely a set of classes, that involves a mass of arboreal characteristics.

Every object that your program will use will have a corresponding class definition somewhere for objects of that type. This is true in Java as well as in other object-oriented languages. The basic idea of a class in programming parallels that of classifying things in the real world. It is a convenient and welldefined way to group things together.

An instance of a class is a technical term for an existing object of that class. Ash is a specification for a type of object, and yourAsh is an object constructed to that specification. So, yourAsh would be an instance of the class Ash. Once you have a class defined, then you can come up with objects, or instances, of that class. This raises the question of what differentiates an object of a given class from an object of another class, an Ash class object, say, from a Redwood object.