Swing Hacks

478

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Chapter 12, Miscellany

#95 Debug Your GUI

HACK

Notice the odd for loop—you start at the second-to-last element and decre-

ment by two each time. On each pass, you take the first object of the pair; if

it’s the right class to fire the event to, you get the second object of the pair

and call its event-handling method. Presumably, you could use this to mix

up different kinds of listeners in the same list, calling different methods for

different classes.

When run, this test produces the following output. Since the event firing

counts down through the listeners, it’s functionally the same as the simpler

backward

for

loop shown earlier:

[tonberry] cadamson% java EventListenerListEventSource

E called

D called

C called

B called

A called

H A C K

#95

Debug Your GUI Hack #95

Standard out and err aren’t just for log files anymore.

Debugging GUIs often means keeping one or two console windows open, so

you can see the debugging messages you print to standard out (via

System.

out.println( )

and the like), as well as stack traces printed to standard err

when exceptions are caught. In the field, you might want to log these to a

file with something like

java.util.logging, but at design time, or when

investigating a bug, you want to see exactly when the exceptions happen,

and running

tail -f mylog.txt in multiple terminal windows may not be

practical, especially if you’re trying to get a customer on the phone to do it.

An alternative is for your own application to have debugging windows that

collect everything printed to standard out and err, something that you or a

user can bring up with a keypress or menu item.

Hijacking Output Streams

Fortunately, taking control of the standard output and error streams is

pretty easy. The trick is to repoint it into your own

JTextAreas, as shown in

Example 12-16.

Example 12-16. Redirecting System.out and System.err to Swing windows

import java.awt.*;

import java.awt.event.*;

import javax.swing.*;

import java.io.*;

Debug Your GUI #95

Chapter 12, Miscellany

|

479

HACK

public class StdErrOutWindows extends Object {

JTextArea outArea, errArea;

public StdErrOutWindows ( ) {

// out

outArea = new JTextArea (20, 50);

JScrollPane pain =

new JScrollPane (outArea,

ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS,

ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);

JFrame outFrame = new JFrame ("out");

outFrame.getContentPane( ).add (pain);

outFrame.pack( );

outFrame.setVisible(true);

// err

errArea = new JTextArea (20, 50);

pain =

new JScrollPane (errArea,

ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS,

ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);

JFrame errFrame = new JFrame ("err");

errFrame.getContentPane( ).add (pain);

errFrame.pack( );

errFrame.setLocation (errFrame.getLocation( ).x + 20,

errFrame.getLocation( ).y + 20);

errFrame.setVisible (true);

// set up streams

System.setOut (new PrintStream (new JTextAreaOutputStream (outArea)));

System.setErr (new PrintStream (new JTextAreaOutputStream (errArea)));

}

public static void main (String[] args) {

new StdErrOutWindows( );

// test

System.out.println ("test to out");

System.out.println ("another test to out");

try {

throw new Exception ("Test exception");

} catch (Exception e) {

e.printStackTrace( );

}

}

public class JTextAreaOutputStream extends OutputStream {

JTextArea ta;

public JTextAreaOutputStream (JTextArea t) {

super( );

ta = t;

}

Example 12-16. Redirecting System.out and System.err to Swing windows (continued)