Jessid Leon Velez Gutierrez

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since Jan 19, 2011
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Recent posts by Jessid Leon Velez Gutierrez

Hello. I am using JFreeChart to plot some data I receive via serial port. Data I receive is the measure of temperature taken twice a day. Perhaps the client is going to receive data recorded during one month. I have plotted succesfully this data, but my question is this: is it possible to make JFreeChart to map the date (with hour and minute) with its corresponding measured temperature? Automatically it shows some dates in X axis, but does not correspond to the days I have measured temperature. I dont know if I am being clear. In short: show dates and temperatures in X and Y axis...

Thank you very, very much

Rob Spoor wrote:Java is not released under GPL; see

However, even if it were, only derivatives of the JDK, JRE or Oracle code would have to be released under the GPL. Any code you write using the tools is not a derivative as you could also use a different compiler and JVM to run them. Therefore, you are free to choose any license for the Java code you write yourself (provided it's not the derivative of some other code, you'd need to check its license for that).

Thanks Rob for answering. Just one last question about this topic:

If I decide to write an app using the SE of java and sell it, will I ever have to pay something to Oracle?

"Note that in either case (SCSL or JRL), if you decide to use your project internally for productive use or distribute your product to others, you must sign a commercial agreement and meet the Java compatibility requirements. Contact your local Sun office to obtain an agreement. "
7 years ago
Hello. I dont know if this is the correct place to ask this. Any way, I will ask:
I understand (do I?) that java has been released by sun under GPL license. My question is this:
If java is under GPL license, all programs I write HAVE to be under the same license? If so, Am i obligated to release the source code of my application?

Thanks a lot!
7 years ago
Hello friends. I wanted to share my happiness with you.... I have just presented the 310-083 exam and score 85%.... that is more than I was expecting....

...but but should I do next? some advice? I am thinking to study Ajax and perhaps struts or hibernate, but one thing at a time..... any suggestion will be welcome...

Goog bye!
8 years ago
Hello Pedro. thanks a lot for your answers. I have gotten the idea.... I hope to do it well in the exam

Pedro Kowalski wrote:Hello Jessid!

Firstly - did you try to run the example on your own? I guess so, as you spotted the compilation error, and it's true - if these are the exact files the question mentioned about - than a compilation error will occur.

Secondly - this kind of question is hardly possible to be present at the exam, but you should be very suspicious all the time. Also note that on the real exam (SCWCD 6) I personally bumped into (at least) two questions which source code won't even compile (!) and it was obvious that there was a mistake in the question (missing DD element, missing parenthesis etc.) because there was no 'compilation error' option. So be extra careful in such cases!

I would advice you to firstly assume that you are wrong and the question is prepared correctly. If you are 150% sure that the code is wrong because there is no correct answer, choose the one that best suits you (i.e. assume what the question is supposed to ask you about, what the author is trying to test you for, etc.). Just remember that this is really the last resort just to avoid receiving 0 points for not giving any answer.


Hello friends. I have a doubt about a question in, specifically number 6 in the "JSP standard actions quiz".

Here it is the question:

Given a jsp page called index.jsp with the content

And a menu.jsp file available to index.jsp containing

(I don't know why in the forum I see twice the word greeting in menu.jsp, one in red an one in blue before the "=" symbol. Just ignore this error)

What will happen when you attempt to compile and run index.jsp?
The correct answer is that it will compile and a runtime error will occur...

But in fact, I think this will not even compile, due to the variable called greeting, is not defined in menu.jsp ...
Well, I can see that they explicitly ask about index.jsp, but seems to me a little ambiguous...

1. am I wrong?
2. could this kind of questions be on the real exam?
3.If so, will I have to take literally what the question asks about?

I know this is kind of stupid question...anyway you have much more experience than I have...

Thanks a lot

Ulf Dittmer wrote:The problem would be detected at runtime of the web app, but at compile time of the JSP (since that's where the code containing the no-arg constructor is used).

mmm.... all right Ulf. I think I get it now. Thanks a lot!
Hello. I am studying for SCWCD exam and in one of the questions I found, said that a JavaBean will generate a compile time error just because it does not have non-arg constructor. Explicitly the author of the exam says:

"Since MyBean does not have a no-arg constructor, the container cannot instantiate it and hence a Compilation error
would occur."

MyBean is the class with a no arg-constructor. I think this is wrong, because it would generate a run-time error, when the container tries to instantiate the bean. Am I wrong?

thank you!

The exam i am talking about is Practice Test I, question number 19 from whizlabs.
Hello. I think there is a mistake in line 17. The () are incorrect syntax when defining a class... Am I wrong?
8 years ago
Hello. It would be easier to help you if you include some code and/or your web.xml file. Perhaps this exception is thrown because you havent configured your servlet in your web.xml or better yet, you havent implemented the doXXX method in your servlet. If you say you are having troubles when clicking a link in your welcome page, it means you are doing request using the HTTP GET method, so you should implement the doGet method.

I hope this is of some help.
8 years ago
this Bean is missing the no arg constructor, i think...

8 years ago
Hello Prasanth S Pillai. I am not a Java EE expert, but I think your approach for validating if the user has privileges for accessing some resources in your app should not rely on some "hidden" parameters. Perhaps you should use sessions or the security implemented by the container.
8 years ago

Bear Bibeault wrote:There are a number of errors with this code:

  • Firstly, using scriptlets in a JSP is a poor practice that should have been eradicated years ago. Bad idea.

  • Secondly, the code assues that the variable exception is not null. This is an invalid assumption. Hence, when it is null, a null pointer exception is raised.

  • I get it now. Thanks a lot!
    8 years ago
    Hello. I understand that there are better ways to solve what you are trying to do. Nevertheless here is a very simple idea (Please, if I am wrong, someone correct me):

    in the login jsp you could use a little code like this to kill your session. If you do it, you assure that the user is not in session when the browser is closed, but when the user is trying to re-login:

    <% request.getSession().invalidate(); %>
    8 years ago

    Stefan Evans wrote:Indeed. If you check out the setProperty tag you will find out that
    "If a parameter has an empty or null value, the corresponding bean property is not set. "

    An empty string counts as "empty" for purposes of this tag, and so the setter is not called.

    mmm now i understand. I did not know this.... thanks a lot for your answer
    8 years ago