Java Reference
In-Depth Information
}
else if
(actionCommand.equals("Clear"))
story.setText("");
else
story.setText("Unexpected error.");
}
private int
getLineCount()
{
String storyString = story.getText();
int
count = 0;
for
(
int
i = 0; i < storyString.length(); i++)
if
(storyString.charAt(i) == '\n')
count++;
return
count + 1;
//The last line has no '\n'.
}
}
39.
We made the text field an instance variable because we needed to refer to it in
the definition of the method
actionPerformed
. On the other hand, the only
direct reference we had to the buttons was in the constructor. So, we need names
for the buttons only in the constructor definition.
40.
The GUI would try to add the string
"Enter numbers here."
as if it were a
string for a number. This will cause a
NumberFormatException
to be thrown and
the string
"Error: Reenter Number."
would be displayed in the text field.
41.
Every time the user clicks the addition button, the following assignment is executed:
result = result + stringToDouble(ioField.getText());
So, the number in the text field is added to the total as many times as the user
clicks the addition button. But, the value in the text field is the running total, so
the running total is added to itself. Thus, the running total is added to the total
as many times as the user clicks the addition button.
Let's say that the user starts the GUI, types in
10
, and clicks the addition button. That
adds
10
to
result
, so the value of
result
is then
0
plus
10
, which is
10
, and
10
is
displayed. Now the user clicks the addition button a second time. That adds
10
to
result
again, so the value of
result
is
10
plus
10
, which is
20
, and
20
is displayed.
Next the user clicks the addition button a third time. This time,
20
is in the text field,
and so it is added to
result
, which is also
20
. Thus, the value of
result
is now
40
,
and
40
is displayed. Note that it is always the number in the text field that is added in.
42.
The two calculator windows are completely independent. Each has its own instance
variable
result
, which has no effect on the other's instance variable
result
.
43.
If you click the close-window button in either calculator window, the entire program
ends because that causes an invocation of
System.exit
. There is no invocation of
