Java Reference
In-Depth Information
Checked at compile time
Checked at runtime
a2
=
(K)
b2
Figure 16-2.
Runtime and compile-time checks made for downcasting
■
the
Object
class is at the top of every class hierarchy in Java. this allows you to assign a reference of any
class type to a variable of the
Object
type. the following type of assignment is always allowed:
Tip
Object obj = new AnyJavaClass(); // Upcasting
Whether downcasting from an
Object
type to another class type will succeed depends on the downcasting rule as
discussed above.
The instanceof Operator
How can you be sure that a downcasting will always succeed at runtime? Java has an
instanceof
operator, which
helps you determine that whether a reference variable has a reference to an object of a class or a subclass of the class
at runtime. It takes two operands and evaluates to a
boolean
value
true
or
false
. Its syntax is
<<Class Reference Variable>>instanceof <<Class Name or Interface>>
If
<<Class Reference Variable>>r
efers to an object of class
<<Class Name>>o
r any of its descendants,
instanceof
returns
true
. Otherwise, it returns
false
. Note that if
<<Class Reference Variable>>i
s
null
,
instanceof
always returns
false
.
You should use the
instanceof
operator before downcasting to check if the reference variable you are trying to
downcast is of the type you expected it to be. For example, if you want to check if a variable of
Employee
type refers to a
Manager
object at runtime, you would write
Manager mgr = new Manager();
Employee emp = mgr;
if (emp instanceof Manager) {
// The following downcast will always succeed
mgr = (Manager)emp;
}
else {
// emp is not a Manager type
}
The
instanceof
operator goes through two types of checks: compile-time check and runtime check. The
compiler checks if it is ever possible for the left-hand operand to refer to an object of the right-hand operand. This
check may not be obvious to you at this point. The purpose of using the
instanceof
operator is to compare the
runtime type of a reference variable to a type. In short, it compares two types. Does it ever make sense to compare a
