Object

A particular instance of a class. The set of values of the attributes of a particular object is called its state. The object consists of state and the behavior that's defined in the object's class. Objects are key to understanding object-oriented technology. Examples of real-world objects: your dog, your desk, your television set, your bicycle.

Real-world objects have two characteristics: state and behavior. Bicycles also have state (current gear, current pedal cadence, current speed) and behavior (changing gear, changing pedal cadence, applying brakes).

A software object.

Software objects are conceptually similar to real-world objects: they too consist of state and related behavior. An object stores its state in fields and exposes its behavior through methods . Methods operate on an object's internal state and serve as the primary mechanism for object-to-object communication. Hiding internal state and requiring all interaction to be performed through an object's methods is known as data encapsulation — a fundamental principle of object-oriented programming.

Consider a bicycle, for example:

A bicycle modeled as a software object.

By attributing state (current speed, current pedal cadence, and current gear) and providing methods for changing that state, the object remains in control of how the outside world is allowed to use it. For example, if the bicycle only has 6 gears, a method to change gears could reject any value that is less than 1 or greater than 6.

Bundling code into individual software objects provides a number of benefits, including:

• Modularity: The source code for an object can be written and maintained independently of the source code for other objects. Once created, an object can be easily passed around inside the system.

• Information-hiding: By interacting only with an object's methods, the details of its internal implementation remain hidden from the outside world.

• Code re-use: If an object already exists (perhaps written by another software developer), you can use that object in your program. This allows specialists to implement/test/debug complex, task-specific objects, which you can then trust to run in your own code.

• Plug-ability and debugging ease: If a particular object turns out to be problematic, you can simply remove it from your application and plug in a different object as its replacement. This is analogous to fixing mechanical problems in the real world. If a bolt breaks, you replace it, not the entire machine.