Interesting question in Java!!!!!!!!

Hi,
I am in need of getting some help on the following Java problem,
I have a String variable with a value of,
"if (flag) i=i+1 else i=i-1;" or "...some other arthmetic operation.."
and at one poing of time i need to execute the code inside this String. Is there anyway? Pls do help me. Thanks in advance.
Crao

I don't know of anything that will handle both expression evaluation and program logic...
http://www.beanshell.org/intro.html
http://jep.sourceforge.net/

Similar Messages

Question about Java's HttpServer: Threading? Backlog?

Hello,
I have two questions about Java's HttpServer (com.sun.net.httpserver). From the JavaDoc:
>
Management of threads can be done external to this object by providing a Executor object. If none is provided a default implementation is used.
>
How can I get information about the default implementation in 1.6.0_13? Do you know the behavior? From my observations, the default implementation uses no Threads, meaning every request is handled in the same Thread, this results in handling the requests to the HttpServer one after another.
Is this right?
The second question is about this, also from the JavaDoc:
>
When binding to an address and port number, the application can also specify an integer backlog parameter. This represents the maximum number of incoming TCP connections which the system will queue internally. [...]
>
When setting the backlog to -1, it uses the systems default backlog. How can I determine the systems default backlog? Can some lines of Java code reveal it (there is no getBeacklog() method)? Or is it up to the Operating System (we use Redhat Linux)?
Thanks a lot for your help!
Regards,
Timo

How can I determine the systems default backlog?You can't. There is no API for that even at the C level.
Can some lines of Java code reveal itNo.
Or is it up to the Operating System (we use Redhat Linux)?Yes. Linux provides a large default. It seems to be at least 50 on most platforms. This is not something you should be worrying about.

Three questions about Java and Ftp

Hello, i've the following questions about Java and Ftp:
1- .netrc file is in $HOME directory but i can't access to this directory from java code. The following line producesan Exception (directory doesn't exists)
FileWriter file = new FileWriter ("$HOME/.netrc");
2- .netrc file must have the following permissions: -rw- --- --- but when i create the .netrc file the following permissions are on default: -rw- r-- r--, how can i change this permissions? (In java code, i can't use chmod.....)
3- Are there any way to pass parameters to a .netrc file? If i get to do this i needn't change the permissions because i can't modify or create/destroy this file.
Thanks in advanced!!!
Kike

1- .netrc file is in $HOME directory but i can't
access to this directory from java code. The
following line producesan Exception (directory
doesn't exists)
FileWriter file = new FileWriter ("$HOME/.netrc");$HOME would have to be replaced by a shell, I don't
think you can use it as part of a legal path.
Instead, use System.getProperty("user.home");
Ok, thanks
2- .netrc file must have the followingpermissions:
-rw- --- --- but when i create the .netrc file the
following permissions are on default: -rw- r--r--,
how can i change this permissions? (In java code,i
can't use chmod.....)Yes, you can: Runtime.exec("chmod ...");
I need to use estrictly the .netrc with -rw- --- --- permissions
Yes, i can use Runtime.exec ("chmod ..."); but i don't like very much this solution because is a slow solution, am i right?
3- Are there any way to pass parameters to a.netrc
file? If i get to do this i needn't change the
permissions because i can't modify orcreate/destroy
this file.I don't think so. Why do you need the .netrc file in
Java at all? Writing a GUI frontend?I want to use automatic ftp in a java program and FTP server, the files and path are not always the same, so i can:
- modify .netrc (for me is the complex option)
- destroy and create a new .netrc (is easier but i have permissions problem)
- use .netrc with parameters but i haven't found any help about it
Thanks for your prompt reply!!!!
Kike

Interested in Learning Java Programming.

Hello All,
I a college student and I am interested in learning Java programming. Where would be a good place for me to start?
Thanks

Sun's basic Java tutorial
Sun's New To Java Center. Includes an overview of what Java is, instructions for setting up Java, an intro to programming (that includes links to the above tutorial or to parts of it), quizzes, a list of resources, and info on certification and courses.
http://javaalmanac.com. A couple dozen code examples that supplement The Java Developers Almanac.
jGuru. A general Java resource site. Includes FAQs, forums, courses, more.
JavaRanch. To quote the tagline on their homepage: "a friendly place for Java greenhorns." FAQs, forums (moderated, I believe), sample code, all kinds of goodies for newbies. From what I've heard, they live up to the "friendly" claim.
Bruce Eckel's Thinking in Java (Available online.)
Joshua Bloch's Effective Java
Bert Bates and Kathy Sierra's Head First Java.

Leor Zahavi: Interested in learning Java Mobile

Myself Leor Zahavi and I am interested in learning java mobile so that I can develop mobile applications. I have some basic knowledge of core java. Please suggest me some tutorial website to learn mobile java mobile.
Thanks
Leor Zahavi

958263 wrote:
Myself Leor Zahavi and I am interested in learning java mobile so that I can develop mobile applications. I'd investigate Android and iOS app development then. Android is based on the Java language at least, so through that path you could get yourself reacquainted with it. But you'll need a good book, not a tutorial. This is no easy subject that you learn from a few dinky paragraphs and some pseudo code.
EDIT: hm, pretty old thread.

Java reflection (interesting question)

hi folks,
class A {
void foo() {
Class B overrides method foo() in A
class B extends A {
void foo() {
Now i create a object of class B and assign to A
A aref = new B();
aref.foo() will call method foo()of class B. //polymorphism
Using reflection, is it possible to call method foo() of class A using the handle aref.
thanks
venkat

hi bondvenky,
What abt the answer for my original question. How to
access the base class methods using the handle for
child class object using reflection ?as far as i know, this isn't possible - your next question is probably going to be "why". It certainly seems slightly surprising that you can't do this, but you can access private methods. Unless you consider the latter a weaker way of breaking encapsulation (!?).
what was the sun's purpose behind allowing access to
the private methods of an object using Java
Reflection? good question.. its very useful but on the other hand i can't think of a time i've used it that couldn't be classed as a hack.
Is it not a security threat to java security model?it doesn't break anything - ie its not a security loophole. It links in with your question above though - would it have been possible/useful to not allow it period?
sorry for the vague answers :(
asjf

Question about Java Micro Edition

Hello,
I am interested about writing a program for PDAs. These seem to be running the Java Micro Edition, so I guess this is a good choice. But what about programming the GUI with the j2me? Afaik, it does not support swing, and the specifications are so full of abbreviations that I do not undertand that I really don't know other than to simply ask here about the possibilities of GUI programming with j2me, is there a "standard" gui library for j2me, or is it device-dependant, and is it possible to install the j2me under windows/linux for developing and testing purposes?
Help please.. ;-)
Thanks
Jan

thx :) J2ME seems to be very interesting. But zeb is right with the post in the discussion thread for this article, "where to start" is an important questions. So please, sun, make it easier for us to us (hobby-)programmers to adopt J2ME..
I found this one alternatively:
www.ewesoft.com
does anyone have experiences with it?
Jan

Question about java thread implementation

Hi All,
I am comparing the performance of the Dining philosopher's problem
implemented in Java, Ada, C/Pthread, and the experimental language I
have been working on.
The algorithm is very simple: let a butler to restrict entry to the
eating table, so that deadlock is prevented.
It turns out that the java code is the winner and it is 2 times faster than C!
The comparison result really surprised me, and raised a big
question mark : why java runs so fast? I did not use high-level synchronization
constructs like Semaphores, atomic variables, etc.
I vaguely recall that Java thread is actually implemented by the underlying
system thread library(so on linux, the default would be NPTL, on windows NT threads,
and on Mac OSX it would be Cthread?). Can no longer remember where I read that.
Does anyone here have some notions about the Java thread
implementations(where is this formally explained)? or Does anyone know where
I can possibly find relevant literature or the answer?
thanks a lot.
cheers,
Tony

Peter__Lawrey wrote:
google has lots of information on java threaded.
One thing java does is support biased locking. i.e. the thread which last locked an object can lock that object again faster.
This may explain the difference. [http://www.google.co.uk/search?q=java+usebiasedlocking]
Note: you can turn this option off and other locking options to see if a feature is giving you the performance advantage.
Personally I have found that real world multi-threaded applications are easier to write in Java, esp. when you have a team of developers. For this reason, alot of C/C++ libraries are single threaded, even when there would be a performance advantage in being multi-threaded (because its just too hard in reality to make it thread safe and faster because of it)I didn't know that, very interesting :-)

Simple question about JAVA path

Hi. I have a simple question , but I spent several hours trying to find an answer.
How do I know the location of the java executable which is used by Concurrent Manager.
I already know that running which java under application user will not give me the correct answer

Maybe you can try like this:
     public String getPath(){
          Class cls=getClass();
          String className=cls.getName();
          String packageName=cls.getPackage().getName();
          System.out.println(className);
          System.out.println(packageName);
          String newClassName=className.replaceAll("\\.", "/");
          String resouceName=newClassName+".class";
          System.out.println(resouceName);
          URL url;
          url=cls.getClassLoader().getResource(resouceName);
          String path="";
          path=url.getFile();
          return path;
     }

Efficiency Question: Simple Java DC vs Used WD Component

Hello guys,
The scenario is - I want to call a function that returns a FileInputStream object so I can process it on my WD view. I can think of two approaches - one is to simply write a simple Java DC that does so and just use it as a used DC so I can call the functionality from there. Or create another WD DC that exposes the value (as binary) via component interface.
I'm leaning on the simple Java DC approach - its easier to create. But I'm just curious on what would be the Pro-side (if there's any) if I use the WD Component interface approach? Is there a plus for the efficiency? (Though I doubt) Or is it just a 'best/recommended practice' approach?
Thanks!
Regards,
Jan

Hi Jan
>one is to simply write a simple Java DC that does so and just use it as a used DC so I can call the functionality from there
Implemented Java API for the purpose mentioned in your question is the right way, I think. The Java API can be even located in the same WebDynpro DC as your WebDynpro components. There is not strongly necessity to put it into separate Java DC.
>Or create another WD DC that exposes the value (as binary) via component interface
You should not do this because in general WD components' purpose is UI, not API. Implementing WD component without any UI, but with some component interface has very-very restricted use cases. Such WD component shall only be a choice in the cases when the API is WebDynpro based and if you have to strongly use WebDynpro Runtime API in order to implement your own API.
If your API does not require WebDynpro Runtime API invocations or anything else related to WebDynpro then your choice is simple Java API.
>But I'm just curious on what would be the Pro-side (if there's any) if I use the WD Component interface approach? Is there a plus for the efficiency?
No. Performance will be better in simple Java API then in WebDynpro component interface. The reason is the API will not pass thought heavy WebDynpro engine.
BR, Siarhei

Question About Java Threads and Blocking

I'm helping someone rehost a tool from the PC to the Sun. We're using the Netbeans IDE and the Java programming language. I took a Java course several years ago, but need some help with something now. We're developing a front-end GUI using Swing to allow users to select different options to perform their tasks. I have a general question that will apply to all cases where we run an external process from the GUI. We have a "CommandProcessor" class that will call an external process using the "ProcessBuilder" class. I'm including the snippet of code below where this happens. We pass in a string which is the command we want to run. We also instantiate a class called "StreamGobbler" my coworker got off the Internet for redirecting I/O to a message window. I'm also including the "StreamGobbler" class below for reference. Here's the "CommandProcessor" class:
// Test ProcessBuilder
public class CommandProcessor {
public static void Run(String[] cmd) throws Exception {
System.out.println("inside CommandProcessor.Run function...");
Process p = new ProcessBuilder(cmd).start();
StreamGobbler s1 = new StreamGobbler("stdin", p.getInputStream());
StreamGobbler s2 = new StreamGobbler("stderr", p.getErrorStream());
s1.start();
s2.start();
//p.waitFor();
System.out.println("Process Returned");
Here's the "StreamGobbler" class:
import java.lang.*;
import java.io.*;
// Attempt to make the output of the process go to the message window
// as it is produced rather that waiting for the process to finish
public class StreamGobbler implements Runnable {
String name;
InputStream is;
Thread thread;
public StreamGobbler (String name, InputStream is){
this.name = name;
this.is = is;
public void start(){
thread = new Thread (this);
thread.start();
public void run(){
try{
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
while (true){
String s = br.readLine();
if (s == null) break;
System.out.println(s);
//messageWindow.writeToMessageArea("[" + name + "]" + s);
is.close();
catch(Exception ex){
System.out.println("Problem reading stream" + name + "...:" + ex);
ex.printStackTrace();
The "CommandProcessor" class calls two (2) instances of the "StreamGobbler" class, one for "stdin" and one for "stderr". My coworker discovered these are the 2 I/O descriptors that are needed for the external command we're running in this case. We're actually called the Concurrent Versions System (cvs) command from the GUI. Here's what we need it to do:
We want to display the output real-time as the external process is executing, but we want to block any actions being performed on the GUI itself until the process finishes. In other words, we want to show the user any generated output from the process, but don't want to alllow them to perform any other actions on the GUI until this process has finished. If we use the "waitFor()" function associated with a process, it blocks all external process output until the process has completed and then spews all the output to the screen all at once. That's NOT what we want. Also, if we don't use the "waitFor()" function, the code just continues on as it should, but we don't know how to block any actions on the GUI until this process has finished. My coworker tried the following code, but it also blocked any output until the process had finished:
while (s1.thread.isAlive() || s2.thread.isAlive())
// We really don't do anything here
I'm pretty sure we have to use threads for the output, but how do we instantly show all output and block any GUI actions?
Thank you in advance for your help!

You're talking about a GUI, but there's nothing in that code which is putting events into the GUI update thread. You also say that nothing happens to the GUI until the CommandProcessor.Run() method returns if you wait for the process.
This implies that you're calling CommandProcessor.Run() in an ActionListener. This will block the GUI thread until it completes.
I was going to explain what to do, but a quick Google informed me that there's a new class which is designed to help in these situations SwingWorker (or as a [separate library|https://swingworker.dev.java.net/] if you're not up-to-date yet).

Question on java

I have a Java program ( not mine) with import statements like the following:
import com.brainysoftware.tassie.ejb.Search;
import com.brainysoftware.tassie.ejb.SearchHome;
I want to know if Search here corresponds to "Search.class" and SearchHome corresponds to SearchHome.class" respectively. I was confused and for example, thought that Search and SearchHome were directories in the package tree.
Actually I also two other Java files ( actually EJBs) called SearchHome.java and Search.java and their corresponding class files in com.brainysoftware.tassie.ejb
So my question is: is Search in the import statement to be interpreted as Search.class..
I know my question is downright simple and silly. All help is appreciated
thanks in advance

actually, this would recursively import any
subpackages (or rather their classes), so
technically, you can (at least syntactically) import
a package this way!Uhm ... no, it wouldn't ... there is no such thing as a sub-package.
xxx.yyy and xxx.yyy.zzz have no special relationship whatsoever in Java. They are as related as xxx.yyy and xxx.zzz are ... the similarity in naming has no meaning in Java, it's there only to logically group APIs which belong together but you can't automatically import classes from all "sub-packages".
Ever tried to write "import java.*"? It doesn't do anything usefull.

Questions about Java Servlets and JSP

Hi,
I'm a confident Java Programmer (and really enjoy using this language) but am very new to Java servlets and Java Server Pages.
I have previously worked with Perl on my web projects (simple 'league' style voting pages). I read in my 'Core Java' book that I should no longer use perl or even cgi.
I need to know more about Java servlets and Java Server Pages so I can make the switch to a 'real' programming language.
I have a few questions:
How should I start to learn JS and JSP?
How applicable will the java knowlegdge I have already be?
Are JSP common on the world wide web?
What tools do I need to start? (I currently develop in JBuilder and have Java 1.4.1 Standard Edition)
Is it likey my web host (and others) will support JSP?
Thank-you very much for helping a novice get started,
Regards,
Paul

Hi, Steve ...has to be frustrating! But do not despair.
Let's suppose the servlet it's named MyServlet on package org.servlets
WEB-INF should look:
WEB-INF
classes
org
servlets
MyServlet.class
web.xml
web.xml file should have this two declarations:
<web-app>
<servlet>
    <servlet-name>MyServlet</servlet-name>
    <servlet-class>org.servlets.MyServlet</servlet-class>
</servlet>

<servlet-mapping>
    <servlet-name>MyServlet</servlet-name>
    <url-pattern>/MyServlet</url-pattern>
</servlet-mapping>

</web-app>Now, once the container starts (Tomcat?), you should be able to see that servlet in:
http://localhost:8080/[my-context/]MyServletAnd what my-context is? The web application context. This string should be empty if your're deploying to the root context, otherwise should the context name. In Tomcat, deploying to root context defaults to using webapps/ROOT.
Sorry for my English, but I felt the need to answer your request. I hope it helps despite my writing.

Newbie question concerning java chat

Hi everyone,
I'm going to program at chat-service in java at school. Since I'm still rather new at this game, I was wondering if anyone knew any good tutorials to start with. I'm also very interested in recommendations as to which java techniques would be best.
My initial thoughts on the project are:
1) it's going to be controlled by a central server (using servlets and JSP).
2) I wish to overcome problems concerning firewalls etc.
3) I took a peek in the book "Java Servlet Programming". It said that there were 3 possible implementations: HTTP, Sockets and RMI. I wonder if there are any other possibilities? JMS for instance?
4) I wish to implement the service in such a way, that different users can approach the service differently (eg. user A with HTTP, and user B with RMI). this should be completely transparent. But is that possible?
Well, thats basically all. I hope someone have some tips or experiences to share. I doubt that I'm the first (or the last) one embarking on a project like this.

I've found some links in previous posts.
http://www.javaworld.com/javaworld/jw-01-1997/jw-01-chat_p.html
And the section on "Java Shared Data Toolkit"

Newbie question to java web services....

hi,
I used to be a .net programmer for a few years, and am have done web services in .net.... (using IIS, of course)
And i moved onto Java side recently - and am pretty new to tomcat / axis..etc..
I looked at the netbeans ide tutorial, and it covered on how to create a web services and a client using the IDE - with that is with the sun app server. Since I want to stick with Tomcat (which is a servlet container, i believe), the compilation complained that the HelloWSImpl.java is not a servlet, and will not let me run the web app. I have tried the WSDP 2.0 tutorial as well, but I first got the FastSetsource.jar not found? Then I got some other classes not found..etc... so, until now - i still am not able to create a web service, and a cliet to talk back and forth.
I have the following questions..
1. I am confused that how come tomcat itself is already a webserver, why would people use it on top of apache http server, which is another web server?
2. To create a web services, (I am thinking of the .net web services that i have done - that once you deploy it with IIS, and you browse to that service, if the function takes 2 parameter, there will be 2 textboxes waiting for you to put data to, then once you post - the xml response will come back...), does java allow front end interface automatically like that in .net? Or you have to code the JSP pages to include those textboxes yourself, and do a manual post to the web services address?
3. Are there any book out there, explains how to use tomcat for webservices? I have looked thru some of the java books that talks about web services, they mainly explain what webservices are, what is xml..etc... and of course, they did talk about the differenct type of messaging format available. (JAX-RPC..etc..)
4. In the java world - how do you use webservices? Is it the same way that i am familiar with? Like, that we have a front end gui, that it will call the web services, pass it a few param, and get the xml result back, and parse it and display / store.. it? Or that is not the correct way to consume the web services in java world?
5. In .net - as long as i have the .net framework sdk, and a IIS - i am good to go. In java - what do i need in order to deploy a web services?? I already have tomcat / jdk / wsdp / IDE - do i have enough to create a simple ws? if so, can you show me a basic tutorial??
based on some reading, i understand that the ws on the service side, needs an SEI implementing java.rmi.remote, and we need another class implementing this SEI. But I would really want to see more example, or downloads, where i can just download / view the whole thing all at once... I googled "java web service", "jax-rpc example"..etc.. most of the return links are from either java.sun.com, or ibm, or some other vendor's web site, explaining the architecture... I really want to look at the code, to get a better understand, what is the relationship between each of these softwares (tomcat, apache http server, axis...)
Any suggestions??
Thanks,
T

I think I can answer your Q1
Apache is very good at serving simple HTML pages
Tomcat is capable of serving active content
So the two complement each other
Or that's how I understand it.
Steaker

Interesting question in Java!!!!!!!!

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