Prev
Java Listener Experts Index
Headers
Your browser does not support iframes.
Re: applets, applications and static declarations
From:
=?ISO-8859-1?Q?Arne_Vajh=F8j?= <arne@vajhoej.dk>
Newsgroups:
comp.lang.java.programmer
Date:
Wed, 20 Jan 2010 20:03:19 -0500
Message-ID:
<4b57a7d1$0$283$14726298@news.sunsite.dk>
On 20-01-2010 09:38, LC's No-Spam Newsreading account wrote:
If I understand correctly (from my point of view of a Fortran
programmer), a "static" variable is something global to all instances of
a class.
Yes.
This makes sense for classes which are real objects, of which a
plurality of instances exist. But what about classes which do exist in a
single instance ?
1 is also all, so ...
I recently wanted to convert a working applet into a standalone
application. Both of them are things which do exist in a single instance.
It is not given that only one instance of a sub class of JFrame will
exist.
Many applications is coded that way though. But that is logic not
language.
The applet has (should have ? shall have ? must have ?) among others an
init method and a start method. According to SWING standards (ok?), the
init method has a construct
try {
SwingUtilities.invokeAndWait(new Runnable() {
public void run() {
realMain() ;
}
});
} catch (Exception e) { ...
where realMain() instantiates the (single) instance of the applet GUI,
while start() in my case establishes communication with a servlet.
An application (whatever it is) shall have a public static void
main(String[] args) method, which does whatever it should do (could be a
traditional computation reading/writing from stdin/stdout or from files).
A SWING application has in the main a construct of the form
javax.swing.SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGUI();
}
});
where createAndShowGUI instantiates the single instance of the
overarching GUI.
To convert the applet into an application what I did was essentially to
move the code from the applet init and start methods into the
application main method (actually init and main were already very
similar as shown above) and rename realMain as createAndShowGUI. Of
course I also changed the name of the top class and the related
statement from
public class myApplet26 extends JApplet implements ListSelectionListener
to
public class alone26 extends JPanel implements ListSelectionListener
I assumed the application alone26 did not need a constructor (as the
applet didn't, and as it will by definition exist in a single instance)
When compiling I found complaints about lots of class variables not
being static, so I declared them static. There were also complaints on
some the inner classes I used (all in the same source file) not being
static, so I declared them static as well.
At the end I was left with a single error message on a statement in one
of the methods
table.getSelectionModel().addListSelectionListener(this);
complaining that "this" was not static
If I commented out such statement, the application compiled and ran. Of
course one of the components of the GUI, a JTable (table) showing some
results, had lost the interactivity associated to the commented statement.
To overcome this I replaced the offending statement with
table.getSelectionModel().addListSelectionListener(myGui.fake);
where in the prologue of the topmost class among the static variables I
define
static myGui myGui;
and in createAndShowGUI() I instantiate it
myGui = new myGui();
(all these things I did already before)
In the myGui class prologue I added a class variable
alone26 fake;
and in the constructor myGui(), I added as first statement this
fake = new alone26();
I also added a dummy explicit constructor to the topmost class
public alone26() {
}
This makes the standalone application compile and work as expected, but
I'm not sure about why what I did works, whether it was the correct and
simplest thing to do, or whether it was somehow redundant instead.
I think you will be a lot better of by moving the content of
the init method to the constructor of the class extending
JFrame.
I must admit that I am somewhat assuming that you code
applets and GUI apps like I do, but you have not posted
a full example, so I have to guess.
Arne
Generated by PreciseInfo ™
"The pressure for war is mounting [again]. The people are opposed
to it, but the Administration seems hellbent on its way to war.
Most of the Jewish interests in the country are behind the war."
(Wartime Journals, Charles Lindberg, 5/1/41)