General Java Questions I

I recently learned a bit about "inner classes" but this seems to be different...

Q: I m a bit new to Java programming so bear with me. My employer bought a

package of java graphics library programs to support some chart applets

we want to create. We have the source code. I m trying to create a jar

file with all the files I need to run the applet. When I currently run

the applet, the browser java tool says that it can t find

"TextComponent$1.class". I recently learned a bit about "inner classes"

but this seems to be different. The "TextComponent.java" file does

contain some inner classes, but not a class called "1". I m confused.

Is this an inner class? Or is it something else. Any help would be

appreciated. Thanks...

Answer: The TextComponent$1.class is the first anonymous class defined in

TextComponent.java. Since nested (inner) classes are compiled to their own

.class file, they needed unique names. The javac compiler is just creating a

unique file name for an anonymous nested class.

Hi there, does anybody know a good source of design patterns written in JAVA ?

Answer: A pretty good (free to download) book.

http://www.patterndepot.com/put/8/JavaPatterns.htm

Q: Whats the difference between the two: System.err. and System.out? When

should we use System.err?

Answer1: System.out leads the output to the standard output stream (normally

mapped to your console screen), System.err leads the output to the standard error

stream (by default the console, too). the standard output should be used for regular

program output, the standard error for errormessages. If you start your console

program regularly both message types will appear on your screen.

But you may redirect both streams to different destinations (e.g. files), e.g. if you want

to create an error log file where you don t want to be the regualr output in.

On an UNIX you may redirect the output as follows:

java yourprog.class >output.log 2>error.log

this causes your regular output (using System.out) to be stored in output.log and your

error messages (using System.err) to be stored in error.log

Answer2: System.err is a "special" pipe that usually is directed to the standard

consolle. You can redirect the System.out with the normal pipe control (| or >), but

System.err no. If you want to put both the "normal" output and the "error" output to a

file you must use the special redirect 2>.

This allow you to send normal messages into a file or in the /null black hole, but still

receive the error messages on the console.

What is the essential difference between an abstract class and an interface?

What dictates the choice of one over the other?

Answer: You can only extend one class (abstract or not) whereas you can always

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