Java Reference
In-Depth Information
A Swing Calculator
Designing a realistic Swing calculator is the subject of Programming Project 17.3. In
this programming example, we will develop a simplified calculator to get you started
on that Programming Project. Display 17.19 contains a GUI for a calculator that keeps
a running total of numbers. The user enters a number in the text field and then clicks
either the
+
-
button. The number in the text field is then added into or subtracted
from a running total that is kept in the instance variable
result
, and then the new
total (the new value of
result
) is given in the text field. If the user clicks the
"Reset"
button, then the running total—that is, the value of
result
—is set to zero. When the
GUI is first run, the running total, that is, the value of
result
, is set to zero.
Most of the details are similar to things you have already seen, but one new
element is the use of exception handling. If the user enters a number in an incorrect
format, such as placing a comma in a number, then one of the methods throws a
NumberFormatException
. If the user enters a number in an incorrect format, such as
2,000
with a comma instead of
2000
, the method
assumingCorrectNumberFormats
invokes the method
stringToDouble
with the alleged number string
"2,000"
as
an argument. Then
stringToDouble
calls
Double.parseDouble
, but
Double.
parseDouble
throws a
NumberFormatException
because no Java number string can
contain a comma. Because the invocation of
Double.parseDouble
takes place within
an invocation of the method
stringToDouble
,
stringToDouble
in turn throws
a
NumberFormatException
. The invocation of
stringToDouble
takes place inside
the invocation of
assumingCorrectNumberFormats
, so
assumingCorrectNumber
Formats
throws the
NumberFormatException
that it received from the invocation
of
stringToDouble
. However, the invocation of
assumingCorrectNumberFormats
is inside a
try
block. The exception is caught in the following
catch
block. At this
point, the
JTextField
(named
ioField
) is set to the error message
"Error: Reenter
Number."
.
Notice that if a
NumberFormatException
is thrown, the value of the instance
variable
result
is not changed. A
NumberFormatException
can be thrown by an
invocation of
stringToDouble
in either of the following lines of code from the
method
assumingCorrectNumberFormats
:
result = result + stringToDouble(ioField.getText());
or
or
result = result − stringToDouble(ioField.getText());
If the exception is thrown, execution of the method
stringToDouble
ends immediately
and control passes to the
catch
block. Thus, control passes to the
catch
block before
the previous addition or subtraction is performed. So
result
is unchanged, and the
user can reenter the last number and proceed with the GUI as if that incorrect number
were never entered.
Uncaught Exceptions
In a Swing program, throwing an uncaught exception does not end the GUI, but it may leave
it in an unpredictable state. It is best to always catch any exception that is thrown even if all
that the
catch
block does is output an instruction to redo something, such as reentering
some input or just outputting an error message.
