Java Reference
In-Depth Information
C
LASSES
, O
BJECTS
,
AND
M
EMBERS
R
EVIEW
This is a good time to review some important concepts:
A Java
class
is the blueprint, or specification, for all instances of its type.
A Java
object
is an instance of a class.
A calling program (a class itself) first creates a
new
object prior to using it.
In the process, the calling program stores a
reference
to the object in an
object in-
stance variable
(also known as a reference variable). This reference variable is a
handle
, or a pointer, to the object. It contains identifying information about the
object, including information such as the object's storage location (in memory).
The calling program can create many instances of a class and manages each in-
stance by managing the reference variable.
Data members
(variables) and
methods
that are defined by this class are auto-
matically members of this object. They are associated with this instance of the
object and are accessed using the object instance variable as a prefix.
A program can use these object instance (reference) variables to manage more
than one instance of a class. For example:
In this example,
myErrorMsg
is one object instance variable, and
myother-
ErrorMsg
is another. They point to different objects of type
ErrorMsg
.
The object instance variable is used to identify the object that contains the data
members or methods:
