console : Java Glossary

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console

A window where you can view the System.out.println and other debugging data from a Java application or Applet. Sometimes it will be covered over by the Applet or Java Web Start app. Sometimes you have to enable it.

In Java 1.5, if an Applet fails, it will sometimes display a broken icon. Right click on it and that will give you the option to look at the Java console to learn why. The stack trace might mean nothing to you, but it will to those who wrote the program. Copy the entire contents to help in diagnosing the problem.

Engaging the Java Console in Your Browser Redirection
java.exe vs javaw.exe Console Window Size
Writing to the Console Reading From The Console
Import Static Shortcut Learning More
Merging System.out and System.err Links

Engaging the Java Console in Your Browser

If you are in a browser you have to enable to console before you can see it.

Opera logo Opera Engaging the Console

Last revised/verified: 2006-02-07
In Opera you can enable the console with:
  1. Click “Tools”.
  2. Click “Advanced”.
  3. Click “Java Console”.
You also have to enable the console on the Java Control Panel.
  1. Click “task bar Start”.
  2. Click “Settings”.
  3. Click “Control Panel”.
  4. On Vista, click “Programsl”.
  5. Double click “Java”.
  6. Click “Advanced”.
  7. Click “+ Java console”.
  8. Click “Show Console”.
  9. Click “OK”.
  10. Click “File”.
  11. Click “Close”.
Finally, to make the console visible:
  1. Click “Tools”.
  2. Click “Advannced”.
  3. Click “Java console”.

Netscape logo Netscape Engaging the Console

Last revised/verified: 2008-01-07
In Netscape, you enable the console with:
  1. Click Netscape ! shield icon 
  2. Click “Advanced”.
  3. Click “Enable Java”.
  4. Click “Done”.
You also have to enable the console on the Java Control Panel.
  1. Click “task bar Start”.
  2. Click “Settings”.
  3. Click “Control Panel”.
  4. On Vista, click “Programsl”.
  5. Double click “Java”.
  6. Click “Advanced”.
  7. Click “+ Java console”.
  8. Click “Show Console”.
  9. Click “OK”.
  10. Click “File”.
  11. Click “Close”.

firefox firefox Engaging the Console

Last revised/verified: 2006-02-07
In Firefox, you enable the console with:
  1. Click “Tools”.
  2. Click “Options”.
  3. Click “Content”.
  4. click “Enable Java”.
  5. click “OK”.
You also have to enable the console on the Java Control Panel.
  1. Click “task bar Start”.
  2. Click “Settings”.
  3. Click “Control Panel”.
  4. On Vista, click “Programsl”.
  5. Double click “Java”.
  6. Click “Advanced”.
  7. Click “+ Java console”.
  8. Click “Show Console”.
  9. Click “OK”.
  10. Click “File”.
  11. Click “Close”.

Mozilla logo Mozilla Engaging the Console

Last revised/verified: 2006-02-07
In Mozilla, you enable the console with:
  1. Click “Edit”.
  2. Click “Preferences”.
  3. Click “Advanced”.
  4. Click “Enable Java”.
  5. Click “OK”.
You also have to enable the console on the Java Control Panel.
  1. Click “task bar Start”.
  2. Click “Settings”.
  3. Click “Control Panel”.
  4. On Vista, click “Programsl”.
  5. Double click “Java”.
  6. Click “Advanced”.
  7. Click “+ Java console”.
  8. Click “Show Console”.
  9. Click “OK”.
  10. Click “File”.
  11. Click “Close”.
Finally, to make the console visible:
  1. Click “Tools”.
  2. Click “Web Development”.
  3. Click “Java console”.

Sea Monkey logo Sea Monkey Engaging the Console

Last revised/verified: 2006-03-19
In Sea Monkey, you enable the console with:
  1. Click “Edit”.
  2. Click “Preferences”.
  3. Click “Advanced”.
  4. Click “Enable Java”.
  5. Click “OK”.
You also have to enable the console on the Java Control Panel.
  1. Click “task bar Start”.
  2. Click “Settings”.
  3. Click “Control Panel”.
  4. On Vista, click “Programsl”.
  5. Double click “Java”.
  6. Click “Advanced”.
  7. Click “+ Java console”.
  8. Click “Show Console”.
  9. Click “OK”.
  10. Click “File”.
  11. Click “Close”.
Finally, to make the console visible:
  1. Click “Tools”.
  2. Click “Web Development”.
  3. Click “Java console”.

Internet Explorer logo Internet Explorer Engaging the Console

Last revised/verified: 2006-02-07
In Internet Explorer, you enable the console with:
  1. Click “Tools”.
  2. Click “Internet Options”.
  3. Click “Advanced”.
  4. Click “Java Sun”.
  5. Select Use Java (pick latest). Do not use the MS JVM.
In Windows, you also have to enable the console on the Java Control Panel.
  1. Click “task bar Start”.
  2. Click “Settings”.
  3. Click “Control Panel”.
  4. On Vista, click “Programsl”.
  5. Double click “Java”.
  6. Click “Advanced”.
  7. Click “+ Java console”.
  8. Click “Show Console”.
  9. Click “OK”.
  10. Click “File”.
  11. Click “Close”.
  12. Restart your browser.
In Mac OS Leopard, you also have to enable the console on the Java Preferences Application.
  1. Applications
  2. Utilities
  3. Java Preferences Application.
  4. Click “Advanced”.
  5. Click “Show Console”.
  6. Restart your browser.
Finally, to make the console visible:
  1. Click “Tools”.
  2. Click “Sun Java console”.

Java logo Java.exe Engaging the Console

Last revised/verified: 2006-02-07
In java.exe the console is enabled by default; you turn it off by using javaw.exe instead. You also have to enable the console on the Java Control Panel.
  1. Click “task bar Start”.
  2. Click “Settings”.
  3. Click “Control Panel”.
  4. On Vista, click “Programsl”.
  5. Double click “Java”.
  6. Click “Advanced”.
  7. Click “+ Java console”.
  8. Click “Show Console”.

Java Web Start logo  javaws.exe Engaging the Console

Last revised/verified: 2006-02-07
In jawaws.exe (Java Web Start), you must:
  1. start javaws.exe
  2. Click “edit”.
  3. Click “preferences”.
  4. Click “advanced”.
  5. Click “+ Java console”.
  6. Click “Show Console”.
  7. Click “OK”.

java.exe vs javaw.exe

You can hide the console by using javaw.exe (java without) instead of using java.exe.

One of the most useful associations to set up is to make *.jar files executable with java.exe. Then you can just type the name of the jar on the command line to start it executing. If you use javaw.exe rather than java.exe you won’t see the console output. Watch out, Java installers often associate *.jar files with javaw.exe instead of java.exe, overriding your setting.

Writing to the Console

You normally do I/O to the console with System.out.println. For details of how, see the File I/O Amanuensis. Mainly the console is used for debugging or crude programs like HelloWorld. There are no cursor positioning or colour commands. In theory you could insert your own ANSI control sequences, but Windows/NT just ignores them, even if you install ANSI.SYS. If your console supports ANSI sequences you could use: 
System.out.print( "\033[H\033[2J" );
You can effectively clear the console with a println loop of about 80 lines. This won’t clear a log file of course. You can also write to the less-redirectable error log with System.err.println.

Some consoles don’t display System.out data, just System.err. It is thus probably best to get in the habit of using System.err.println for debugging or error message output.

Import Static Shortcut

The JDK 1.5+ import static feature can be used to abbreviate code to write to the console.

Merging System.err and System. out

To merge System.err and System. out you could use setErr and setOut to point them to the same PrintStream, e.g. System.setErr( System. out );

Redirection

You can redirect any output to the console with the usual DOS piping commands, namely > and |. You can also internally redirect System.out to a file with System.setOut, and System.err to a file with System.setErr. You can also, of course, write directly to files using a PrintWriter. If you use 4DOS or tcc/TakeCommand you can redirect stdout with 1> and stderr with 2>. You can also use the tee utility to get the output both in a file and on the console. See tcc/TakeCommand for details. You can find similar features in other script languages.

Console Window Size

In NT or tcc you can expand the size of the DOS window and make it scrollable. Right click the DOS box shortcut ⇒ properties ⇒ layout ⇒ screen buffer size ⇒ height That way you can scroll back and see stuff that has rolled off the screen.

Reading From the Console

When you use the console for input, you use Ctrl-Z to signal EOF in Windows and Ctrl-D in most Unices.

You can’t read a single character from the console, the way you can with getC in C. The operating system delivers I/O a line at a time. The program can’t read the characters in a line until the user hits Enter. This allows the user to correct mistakes with BS before Java sees them.

How do you get around this? AWT keystroke events are registered immediately. You can thus accept data in a component. You need to fake a console with a TextArea or similar component.

Peter van der Linden created a set of simple console routines for reading character data in various formats called EasyIn. I seem to recall him also writing a keystroke by keystroke class, but I can find no record of it.

Learning More

Sun’s Javadoc on System.err : available:
Sun’s Javadoc on System.out : available:
Sun’s Javadoc on System.in : available:
Sun’s Javadoc on System.setErr : available:
Sun’s Javadoc on System.setOut : available:
Sun’s Javadoc on System.setIn : available:
4DOS
associations
browser
console I/O Amanuensis
exec
Files & directories
Firefox
flipping JVMs
Flock
IE: Internet Explorer
jar file
java.exe
javaw.exe
JNI console classes
Mozilla
Netscape
Opera
tcc/TakeCommand
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