Java Reference
In-Depth Information
■
java has a special reference type that is called null type. the null type does not have a name. the null type has a
literal value, which is represented by
null
. the null type is assignment compatible with all other reference types. You can
assign any reference type variable a
null
value. You can cast
null
value to any reference type. It is to be emphasized
that
null
is a literal value of “null reference type,” not a keyword.
Tip
Using Dot Notation to Access Fields of a Class
Dot notation is used to refer to instance variables. The general form of the dot notation syntax is
<<Reference Variable Name>>.<<Instance Variable Name>>
For example, you use
jack.name
to refer to the
name
instance variable of the instance to which the
jack
reference
variable is referring. If you want to assign a value to the
name
instance variable, you can use
jack.name = "Jack Parker";
The following statement assigns the value of the
name
instance variable to a
String
variable
aName
:
String aName = jack.name;
How do you refer to class variables? You have two ways to refer to a class variable using dot notation.
•
You can refer to a class variable using the name of the class.
<<Class Name>>.<<Class Variable Name>>
For example, you can use
Human.count
to refer to the
count
class variable of the
Human
class.
To assign a new value, say 101, to the
count
class variable you can write
Human.count = 101;
To read the value of the
count
class variable into a variable called
population
, you can use
long population = Human.count;
You can also use a reference variable to refer to the class variable of a class. For example, you
•
can use
jack.count
to refer to the
count
class variable of the
Human
class. You can use the
following statement to assign value, say 101, to the
count
class variable:
jack.count = 101;
The following statement reads the value of the
count
class variable into a variable called
population
:
long population = jack.count;
Both of the above statements assume that
jack
is a reference variable of
Human
type and it
refers to a valid
Human
instance.
