Java Reference
In-Depth Information
Although an upcast is always safe (the superclass's interface is a subset of the sub-
class'sinterface),thesamecannotbesaidofadowncast.
Listing2-38
showsyouwhat
kind of trouble you can get into when downcasting is used incorrectly.
Listing 2-38.
The trouble with downcasting
class A
{
}
class B extends A
{
void m() {}
}
class DowncastDemo
{
public static void main(String[] args)
{
A a = new A();
B b = (B) a;
b.m();
}
}
class named
B
. Although
A
does not declare any members,
B
declares a single
m()
method.
Athirdclassnamed
DowncastDemo
providesa
main()
methodthatfirstinstanti-
ates
A
,andthentriestodowncastthisinstanceto
B
andassigntheresulttovariable
b
.
Thecompilerwillnotcomplainbecausedowncastingfromasuperclasstoasubclassin
the same type hierarchy is legal.
However,iftheassignmentisallowed,theapplicationwillundoubtedlycrashwhen
it tries toexecute
b.m();
.The crash happens because the JVMwill attempt tocall a
method that does not exist—class
A
does not have an
m()
method.
Fortunately,thisscenariowillneverhappenbecausetheJVMverifiesthatthecastis
legal.Becauseitdetectsthat
A
doesnothavean
m()
method,itdoesnotpermitthecast
by throwing an instance of the
ClassCastException
class.
