Java Reference
In-Depth Information
Self-Test Exercises
27. What is a reference type? Are class types reference types? Are primitive types
(such as
int
) reference types?
28. When comparing two objects of a class type to see if they are “equal” or not,
should you use
==
or the method
equals
?
29. When comparing two objects of a primitive type (such as
int
) to see if they are
“equal” or not, should you use
==
or the method
equals
?
30. Can a method with an argument of a class type change the values of the
instance variables in the object named by the argument? For example, if the
argument is of type
ToyClass
defi ned in Display 5.11, can the method change
the
name
of its argument?
31. Suppose a method has a parameter of type
int
and the method is given an
int
variable as an argument. Could the method have been defi ned so that it changes
the value of the variable given as an argument?
The Constant
null
The constant
null
is a special constant that may be assigned to a variable of any class
type. It is used to indicate that the variable has no “real value.” If the compiler insists
that you initialize a variable of a class type and there is no suitable object with which to
initialize it, you can use the value
null
, as in the following example:
null
YourClass yourObject =
null
;
It is also common to use
null
in constructors to initialize instance variables of a class
type when there is no obvious object to use. We will eventually see other uses for the
constant
null
.
Note that
null
is not an object. It is like a reference (memory address) that does
not refer to any object (does not name any memory location). So if you want to test
whether a class variable contains
null
, you use
==
or
!=
; you do not use an
equals
method. For example, the following correctly tests for
null
:
if
(yourObject ==
null
)
System.out.println("No real object here.");
null
null
is a special constant that can be used to give a value to any variable of any class type.
The constant
null
is not an object but a sort of placeholder for a reference to an object.
Because it is like a reference (memory address), use
==
and
!=
rather than the method
equals
when you test to see whether a variable contains
null
.
