Java Reference
In-Depth Information
}
protected Port createSerialPort() {
return new EnhancedSerialPort();
}
}
Now when a
HighSpeedPrinter
is constructed and the initializers in
Printer
execute, we invoke the overriding
createSerialPort
method that returns
an instance of
EnhancedSerialPort
.
This is one example of a situation in which a method of the subclass
is invoked before the subclass object has been fully constructed and so
care must be taken to ensure that things work correctly. For example,
if
EnhancedSerialPort
initializes a field using a field from the enclosing
HighSpeedPrinter
instance, then at the time the
EnhancedSerialPort
object
is constructed, the fields of the enclosing object will have the default
"zero" values for their type.
An alternative design would have the constructor for
HighSpeedPrinter
simply reassign
serial
to refer to an
EnhancedSerialPort
object. However,
that approach causes the unnecessary construction of the original
Seri-
alPort
object, and that construction may be non-trivial and undesirablein
this example it may involve configuring hardware.
