Objects

What is an object?

• Objects are meant to be a computer representation of common sense meaning of object.
• Programwise, an object consists of a group of variables and methods (subroutines).
• Variables define the state of the object.
• Methods determine what the object can do.
• Objects encapsulate a hopefully logical hunk of program.

Diagram of a general object. Lifted from The Java Tutorial (Sun Microsystems).

Diagram of an object representing a bicycle. Lifted from The Java Tutorial (Sun Microsystems).

Instantiation

• An instance of an object is constructed using the new operator.
  For the nerds:
  • Code is reusable.
  • Each instance has its own set of instance variables in the heap.
• Objects can instantiate other objects.
• During execution, there can be several objects of the same type in existence at the same time.
• The browser instantiates an Applet object.
• The Applet object instantiates other objects, and so on.
• An object is available for garbage collection when other objects no longer point to it.

The Incredible Machine demonstration

• Machines (like a programs) are made of a collection of objects.
• Moving the objects to the screen instantiates them.
• There can be several copies of the same object in a machine.