Java Reference
In-Depth Information
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 us 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
System.exit
in Display 17.19, but the following ensures that a
System.exit
that is in some library class will be invoked:
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
Programming Projects
Visit
www.myprogramminglab.com
to complete select exercises online and get instant
feedback.
1. Design and code a Swing GUI to translate text that is input in English into Pig
Latin. You can assume that the sentence contains no punctuation. The rules for
Pig Latin are as follows:
VideoNote
Solution to
Programming
Project 17.1
a. For words that begin with consonants, move the leading consonant to the end
of the word and add “ay.” Thus, “ball” becomes “allbay”; “button” becomes
“uttonbay”; and so forth.
b. For words that begin with vowels, add “way” to the end of the word. Thus,
“all” becomes “allway”; “one” becomes “oneway”; and so forth.
