Java Reference
In-Depth Information
The statement
import java.util
at the beginning of the class tells the compiler
to introduce the definitions for the
java.util
classes into the
Diagnostic
class.
You needed to do this to use the Java
Date
class in the code.
You introduced a data member named
msgSize
in the class
PrintfileErrorMsg
.
This variable hid the similarly named variable in the class
TextMessage
from
HelloWorld, that is, when HelloWorld performed the following statement:
// Print this data member in PrintfileErrorMsg.
System.out.println ("msgSize for PrintfileErrorMsg = " +
myErrorMsg3.msgSize);
Before the change, the variable in
TextMessage
was printed. After the change,
the variable in
PrintfileErrorMsg
was printed.
You constructed an interface definition (
WriteLineInterface
) and defined a
supporting class (
WriteLine
). You then used this class to simplify the logic in
PrintfileErrorMsg
. In a real system, any other class could also use the
Write-
Line
infrastructure in order to centrally manage printing functions.
You demonstrated how polymorphism can be used. You sent the same message
(
setErrorMsg (String)
) to instances of three different classes (
ErrorMsg
,
Print-
fileErrorMsg
, and
Diagnostic
). Each of these classes respond to the message in
a unique way:
ErrorMsg
simply stored the
String
and did not print any text.
PrintfileErrorMsg
printed the message after skipping some lines and
centering the
String
in the print line.
PrintfileErrorMsg
also printed the
values in the two
msgSize
variables.
Diagnostic
printed the message and then performed a standard Java
function to print the current call stack. This function would be very use-
ful in a production system for recording contextual information appro-
priate for “postmortem” analysis of application or system failures.
You used a new object in the
System
class named
err
. This object is similar to
the
System.out
object you have been using all along, except it is designed to
write error messages instead of standard, or informative messages. Both objects
will write to the display device by default.
