Java Newsletters Archive: 8

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* Issue No: 8 24 Oktober 2000 *
* http://www.javafaq.nu/java *
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Table of Contents

> How to get the size of an Enumeration object..
> How does BufferedReader work and how to use it to get input
from a keyboard?
> Do exist Compound Files in Java?
> About Vector initializing...
> RMI versus Socket communication-1
> RMI versus Socket communication-2
> Bitwise operations change the 0/1 bits of a number. Why?
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Tip 1

Does anyone know how could I get the size of an Enumeration
object? The API for Enumeration only contains getNext() and next().

A1: 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.

A2: you can make your own class like this:

import java.util.*;

public class MyEnumeration{
int size;
int index = 0;
Enumeration e;

public MyEnumeration(Vector v){
size = v.size();
e = v.elements();
index = 0;
}

public boolean hasMoreElements(){
return e.hasMoreElements();
}

public Object nextElement(){
index++;
return e.nextElement();
}

public int size(){
return size;
}

public int getIndex(){
return index;
}
}

by Nicolas Delbing lambs@mail1.stofanet.dk
and Victor Vishnyakov mailto:tcch_@mail.ru

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Tip 2

Can anyone please explain clearly how BufferedReader works and
how to use it to get input from a keyboard?
Q: Can anyone please explain clearly how BufferedReader works and
how to use it to get input from a keyboard?

A: BufferedReader is a filter reader class.
That is, it wraps another reader and reading from it is like
reading from the reader it wraps, except that it changes something.
In the case of BufferedReader, it reads in large chunks and then
you can retrieve its data in smaller bits. To use it to read from
System.in, you first need a reader to wrap. You can bridge from an
input stream (which System.in is) to a reader by using an
InputStreamReader.
Then wrap that in a BufferedReader as follows:

BufferedReader input =
new BufferedReader(new InputStreamReader(System.in));

Now you can call methods of BufferedReader to read from standard
input. Generally, you create a BufferedReader to be able to call
the readLine() method. That isn't BufferedReader's main intended
use -- the main intended use is performance -- but you don't
generally care too awfully much about performance of reads from
the console. So call readLine to get a line of input, which will
be null on end of stream (user presses Ctrl-D on UNIX or a file
was redirected in and is done).



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Tip 3

Do exist Compound Files in Java?

Microsoft has made an API where you can have a whole "virtual"
filesystem inside a single file on the real filesystem. I think
they call it Compound Files - I have also heard about the concept
under the name of Structured Storage.

Do any of you know if something like that exists in a Java
library?
I have to work for all platforms! (It can of cause be implemented
with the use of Compound Files in its Windows implementation)
Answer: jar files are essentially the same.

Have a look at java.util.jar.JarFile





Tip 4

Is there a way to provide values for a Vector in the source code,
analogous to array initializers?

Answer: The Vector class constuctors take no arguments other
than Collection (since JDK 1.2), which is abstract, and since a
Vector is a structure whose size can change dynamically, its
contents can only be initialaized through member methods.

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Tip 5

RMI versus Socket communication

Q: I wish to get Java talking to C++ across a network.
Does anyone have any thoughts in terms of performance, ease of
development etc.
in :
Wrapping the C++ side with JNI and using RMI for the
communications.
versus
Writing sockets code and communicating via http?

A: It depends of what kind of application you're writing but l
think about the following:

- with RMI you can have remote REFERENCE instead of having to
transfer all the object through the network. The object has just
to implement Remote. So it spare bandwith and is good for
performance. This is impossible to do if you do through a socket
connection, you've to send the all object.
- You've not to take in charge the serialization (which could be
not so easy depending of your object structure), neither the
connections, etc... All of that is taken in charge by RMI.
- the performance are GOOD (even a bit more than that)
three good points to use RMI, isn't it?

The difficulty added by RMI is the configuration of both client
and server (distribution of stubs, rmiregistry, what's happen if
firewall). Depending of the environment all of that can be either
easy or complicate.
But once that all of that is in place you can extend your
application easily, so it's much more flexible and scalable.

If your needs are small perhaps that you could do your own
connection system (but for me it's less scalable and more
bandwith consuming and so less performant).


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Tip 6

RMI versus Socket communication

Q: I wish to get Java talking to C++ across a network.
Does anyone have any thoughts in terms of performance, ease of
development etc.
in :
Wrapping the C++ side with JNI and using RMI for the
communications.
versus
Writing sockets code and communicating via http?

see please first part of answer in yesterday's tip.

Answer2: I have done both. If your communication scenarios are
diverse and could keep changing, using a remote technology like
RMI can help. If the operations are few and/or not likely to
change you can save the JNI complexity. Not that it is really
hard it just can be fun keeping the JNI code in sinc with the
C++ code.



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Tip 7

I understand that bitwise operations change the 0/1 bits of a
number. Question is why?
I suppose it's interesting that you can manipulate numbers this
way, but I can't think of a practical use for doing that.
Can anyone help me understand when are bitwise operations used and
why you would use them?

Answer: Bitwise manipulation is often used where memory consumption
is critical, and a piece of information may be encoded in less
that one byte, for instance. In communication software and
protocols, information may be interpreted as a stream of bits
where the information is encoded at the bit-level, and you use
bitwise manipulation to extract the pieces of information encoded
in the bytes. There are other situations where bitwise
manipulation is used, as well.



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