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General Java Questions II
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
implement one or more interfaces. Interfaces are Java's way to support multiple
inheritance.
Does anyone know how could I get the size of an Enumeration object? The API
for Enumeration only contains getNext() and next().
Answer 1: You can't. Theoretically, some classes that implement Enumeration may
also provide some way to get a size, but you'd have to know about the more specific
run-time type and cast to it... and none of the standard java.util Collections classes
nor Vector or such provide these methods in their Enumeration implementations.
Answer2:
you can make your own class like this:
import java.util.*;
public class MyEnumeration{
int size;
int index = 0;
Enumeration e;
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