Java Glossary : console

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console

A window where you can view the System.out.println and other debugging data.

You normally do I/O to the console with System.out.println. For details of how, see the File IO 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 output.

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. If you use 4DOS or 4NT you can redirect stdout with 1> and stderr with 2>.

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

In NT or 4NT 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.

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

In Opera you can view it with Tools | Advanced | Java Console. You also might want to turn on the console with Start | Settings Control Panel | Java Plug-In | Basic | Show Console.
In Mozilla, you enable it with: Edit | Preferences | Advanced | Enable Java. You also have to turn on the console with Start | Settings Control Panel | Java Plug-In | Basic | Show Console. If you dismiss it, it is gone until you restart.
In Netscape, you enable it with: Edit | Preferences | Advanced | Enable Java. You also have to turn on the console with Start | Settings | Control Panel | Java Plug-In | Basic | Show Console. You view it with Communicator | Tools | Java Console.
In Internet Explorer, you enable it with: Tools | Internet Options | Advanced | Java | Java Console. You also have to turn on the console with Start | Settings | Control Panel | Java Plug-In | Basic Show Console. You view it with: View | Java Console.
In Java.exe it is enabled by default; you turn it off by using javaw.exe instead. For the plug-in you also have to turn on the console with Start | Settings | Control Panel | Java Plug-In | Basic Show Console.
In javaws.exe, Java WebstStart, you must start javaws.exe. Then click file | preferences | advanced | show console.

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.

exec ¤ File I/O Amanuensis ¤ File

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