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FAQ: version 0.5 NEW POSTERS READ FIRST (UPDATED 14 July 2005) (Read 31016 times)
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shendley
Moderator
Senior. Member
Posts: 346
J2ME drains the soul
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Welcome to J2me.org, the most helpfull j2me forum in the world. There is a group of very smart and professional posters on this board who share their knowledge with the rest of us. We dont ask much of new posters but there is one thing that really gets up our noses and thats when people just post blindly without first searching. To make that a little easier we have started collecting the most frequent problems and solutions in this thread.
We also have a few other tips for new posters:
- When you search for a problem, especially if your app is giving you an error message try searching for the name of the error.
- Give us a detailed description of your problem; start with what device or emulator you are using and tell us exactly what you know about what is not working. This includes stack traces, error messages, debug output, and where you think the code is getting to.
- Post some of your source code. Most problems are easier to debug if we have the code. But when I say some code I mean it if you post the 15 lines of your networking code you think is broken someone will take the time to look through it and find potential problems. If you post your entire 300 line midlet most of us will not even bother to look through it to find the one imortant section. (remember to remove the actual web addresses, passwords from your postings)
- Do not ask about how to pirate, rebrand or otherwise ripoff J2ME apps. This is how we make our living and you are asking us to tell you how to steal from us.
How do I add API X (bluetooth, wma, mmapi, Image.getRGB, ...) to a phone that does not have built in support for API X?
You cant, the only way to add that sort of functionality to J2ME is to upgrade the Virtual Machine on the phone which is something only the phone manufacturer can do.
I am using a third party library in my app and when I try to emulate it I gives me an error like: preverfication error: com/cool/library/ClassA. What am I doing wrong?
You are most likely putting the library jar in the "libs" directory when you should be putting it in the "src" directory. If that does not work try unzipping it into the source directory. (Some IDE's require this)
But its a library therefore it goes into the library directory right?
Nope, its confusing but the library directory is there to let you make the library that you are compiling your application against be the same as what is actually what is going to be on the phone. Your third party library is not going to be on the phone so you need to include it like any other code.
What language should I write my server in?
Whatever you feel most confortable in, as long as it can handle HTTP requests then it will work to talk to J2ME phones.
How do I talk to a FTP server, database, www.something.com:xxx?
You need to have a phone that handles socket connections and handle everything on that level.
How do I write a xxxx game?
By knowing how to write it, we are here to help each other program not design, code and debug apps for people.
Can I access the phone's xxxx? (memory, phone book, inbox, pictures...)?
Generally no, this is considered a security risk and most manufacturers don't allow it. However, a very small minority do, so check out their developer's site.
Where can I find xxxx's developer's site? Where can I find xxxx's SDKs?
Here is a list of the majority of the developer sites:
Nokia: http://forum.nokia.com
Motorola: http://www.motocoder.com
SonyErcisson: http://developer.sonyericsson.com
Samsung: http://developer.samsungmobile.com/eng/front_zone/bbs/bbs_main.jsp?p_menu_id=1500
Vodafone/Sharp: http://via.vodafone.com
Orange: http://www.developers.orange.com
How do I make a game? Do I need to learn Java first?
Yes. Learning Java in order to make games/apps on J2ME devices means understanding the workings of OOP, Threading (especially scheduling differences in embedded devices), Memory Management, Exceptions (checked, runtime, errors). MYTH: J2ME is not a lesser version of J2SE when it comes to the Java language. It is not like comparing VBA to VB. J2ME maybe a smaller subset of the J2SE API but the language and all its features are identical in both platforms.
I want to display a "Busy, please wait..." screen while i do my networking/any other lengthy process, but the waiting screen never appears, and after the process is over the result screen appears. What's wrong?
Most likely you are blocking the main thread that is in charge of refreshing the screen. This is the same thread that calls all the input methods (like commandAction() or keyPressed()), so if you perform a lengthy process in this thread you are blocking it and there is no way the screen will be updated. Perform the lengthy operation in a separate thread.
How do I connect to an Database? / Can I use SQL?
J2ME has no support for SQL or JDBC. If you want to connect to a database, you need to use the network (http, udp, https) to connect to a server which handles the database queries.
The next best thing on J2ME is to use the Record Store Management (RMS) system, which allows the storage of byte arrays.
What does this mean? "Unable to create MIDlet SimpleGame java.lang.ClassNotFoundException: SimpleGame"
99% of the time this error is due to a bad JAD entry. The line "MIDlet-1" needs to contain the full name of the MIDlet class as the last item. The full name of the class is allways the package name and the class name.
How do I capture keypresses on a Form/List/Alert?
You don't, atleast not in MIDP 1.0, the high level apis were written so that the programmer did not have to worry about programming a response to every keypress. If you have to have that level of control you will need to use a Canvas and do everything yourself including creating your own textboxes, lists, option buttons....
Ok, then how do I put a canvas on the form or use a highlevel widget on the canvas?
You cant see above.
What are the basic differences between developing in J2SE and J2ME?
* J2ME devices use a Kilobyte Virtual Machine (KVM) instead of a Java Virtual Machine (JVM) due to memory restrictions.
* Only the basic packages from the J2SE API are available in J2ME and the classes within those packages have some methods removed.
* J2ME specific packages and classes have been added to give control over the devices resources.
* Optional/Vendor specific APIs are available to allow finer control over a particular model or series' functionality.
* Thread scheduling on lesser devices is usually handled cooperatively meaning a thread may hog the CPU and prevent other threads from getting a chance to execute whereas in J2SE the thread scheduling is usually preemptive in nature (actually depends on OS!) meaning each thread gets a fair slice of the CPU's time. Setting the thread's priority in either environment does not guarantee that one thread will get more time than another!
* J2ME devices do not have a rich file system. They use a RecordStore to store simple binary streams by name. Typically small amounts of data such as high scores, progress, preference data are stored here.
{b]How do I do xxxxx? (Which isn't realated to or doesn't use J2me)[/b]
Simple, you use GOOGLE or find a forum that covers that topic. This is a J2ME forum!
-DinkyDino
Why doesnt my app switch between my two/three/fifty-seven canvases when I call myDisplay.setCurrent(nextCanvas)?
Well the technical answer is that you are most probably doing something like this:
Code:
myDisplay.setCurrent(canvas1);
someFunctionUsingCanvas1();
myDisplay.setCurrent(canvas2);
That doesnt work beacuse the function allways returns instantly and the display is actually updated when the KVM feels like it. Having said that, the real answer is dont use multiple canvases. They are memory hogs and having more than one in a Midlet is a very bad idea. Use a single canvas that just has differnt states and then paint the correct info depending on what state the app is in. Not only that but the state machine architecture is very simple and easy to expand as your app evolves.
-shendley
Where can I find out about XXX api to see if it is possible to do YYY with j2me ?
http://www.jcp.org/en/jsr/tech?listBy=1&listByType=platform
This is the location of the j2me related jsrs, you can see an individual technology and get hold of the specification for that JSR.
Where can I find out if my phone supports XXX JSR ?
Your favourite phone manufacturer's developers portal, check the list above for links.
Keep em' coming.
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« Last Edit: July 14, 2005, 12:55:30 pm by kbateman »
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JFGI just isnt getting my point across; how about TGF - Try-Google-First.
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shareme
J2me god
Posts: 1051
Java is the only drug
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How can I find out about whether a specific phone supports soemthing with closed APIs?
Most vendors that do support or extend MIDP1.0 have those issues lsited within the developer portals for j2me developers.
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Wotan
Senior. Member
Posts: 415
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Q : I've just developed a J2ME app in an emulator. Now I want to try it in a real phone. How can I transfer my app to a phone? Do I need to do it Over the Air (OTA) and pay a call charge each time?
A : Many phones support some kind of local connectivity through a USB cable, infra-red or Bluetooth, allowing you to transfer J2ME apps directly to your phone from your PC. Whether any particular phone does or not can be determined by consulting the phone's documentation on one of the manufacturer's web sites listed above.
The PC software required to allow you to do these local transfers will usually either be supplied with the mobile phone or can be downloaded from the manufacturer's web site. If a USB cable is required, it will sometimes be supplied with the mobile phone at purchase time but more usually will have to be bought separately, either from the manufacturer itself or from a mobile phone accessory supplier. The USB cables required vary from one phone model to another (ie. there is no "One size fits all" USB cable available) but typically work with a number of phones within the manufacturer's product range.
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Wotan
Senior. Member
Posts: 415
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Is it possible to access the SIM card from a java midlet in order to get the operator ID and the telephone number?
No.
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Claymore
Global Moderator
J2me god
Posts: 4978
Quantity != Quality
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Can I access the Pictures/SMS/Phones numbers that are already on my phone?
Generally, NO. If you can find one of the small percentage of phones that support the file:// protocol then you may be able to access these files.
However...
Pictures: You can take your own pictures using the phone's camera and MMAPI
SMS: You can write your own app to send and recieve SMS, but you must use a different port from the standard SMS port.
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Wotan
Senior. Member
Posts: 415
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Where can I find information about using Bluetooth in J2ME and some example code?
www.benhui.net
is a good place to look for Bluetooth info and examples and also has a forum where you can ask questions.
www.javabluetooth.com
is the website of the book "Bluetooth for Java" by Bruce Hopkins and Rajith Anthony. The source code examples from the book, which cover most aspects of the use of Bluetooth from Java, is available for free download from the site.
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« Last Edit: January 22, 2005, 07:47:02 am by PG »
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kbateman
Global Moderator
J2me god
Posts: 4362
Woohooo it works ...
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How can I automatically update my MIDlet from within the MIDlet ?
Since this seems to be asked quite a lot, here is the answer... using MIDP1 there isn't really a nice solution. However for MIDP2, this is how you do it ...
Use the platformRequest("http://<server-url>/<new jadfile>");
If the jad file is for the new version of your application it will update the existing application as if you had used a wap browser and gone to the jad file yourself... this allows you to have an UPDATE menu option within your MIDlet.
Here is the relevant bit from Sun about it:
MIDlet Life-Cycle Considerations
You must take the MIDlet life-cycle into account when you use platformRequest().
Some MIDP platforms are more restricted than others. For example, some don't support concurrent processing, so the MIDlet must exit before the platform can honor a service request. Recall that platformRequest() returns true in such cases, giving your MIDlet fair warning that it must exit.
If the platform request is to download and install an update of the currently loaded MIDlet suite, the currently running MIDlet is first terminated.
In both these cases, the MIDlet must be prepared to exit gracefully, first managing its state so as to resume appropriately later.
NOTE: apparently Motorola devices do NOT support JAD and JAR types through the platformRequest() method
Cheers
Kirk
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« Last Edit: July 08, 2005, 12:51:04 pm by kbateman »
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Kirk Bateman
Managing Director
Synaptic Technologies Limited (UK)
E-Mail: kbateman (at) synaptic-technologies (dot) com
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Claymore
Global Moderator
J2me god
Posts: 4978
Quantity != Quality
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Why when I set the GameCanvas to full screen does getHeight() not return the right value?
The main reason for this is that if you call setFulLScreenMode(true) within the constructor of a class - the actual update wont happen until after the constructor has finished. If you have the calls to getHeight() within the same constructor, or within another constructor in a sequence of class initialisations, then it will return the wrong value.
Here is one solution (assuming you use a thread for your game loop):
1) Move your call to setFullScreenMode(true); outside of any constructor - preferably move it to the startApp() method for your main MIDlet:
(e.g.)
public void startApp()
{
if(canvas== null)
{
canvas = new MyCanvas(this);
canvas.setFullScreenMode(true);
}
Display.getDisplay(this).setCurrent(canvas);
canvas.start(); // To start the Thread.
}
2) You then need to override the sizeChanged(int w, int h) method of GameCanvas:
(e.g)
public void sizeChanged(int _w, int _h)
{
slx = _w;
sly = _h;
screenUpdated = true;
}
You will notice here that I am using a boolean flag to identfy whether the screen has been updated or not...
3) In your thread's run() method, inside the while loop, add the following call:
while(!screenUpdated)
{
currentThread.yield();
}
This will yield() your current Thread and wait until the Canvas has been updated and hopefully you should now have the right values.
Note: Some devices and emulators may not call the sizeChanged() method - in those cases you may have to hard code the height value.
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nobody nowhere
Senior. Member
Posts: 495
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Memory Management In J2ME
Most java programmers are aware of the System.gc() and/or Runtime.gc()
method. However most programmers have only a partial grasp of the
effects of calling one of these methods. This document is about
dispelling some of the myths of garbage collection and about memory
management in general. The target audience ranges from newbie to
mid-level Java programmer.
What is garbage collection?
Garbage collection is a service that can be provided by the Java
Virtual Machine to reclaim memory that is deemed eligible for garbage
collection. All classes of Java (J2ME, J2SE, and J2EE) are capable of
providing this service.
Where is the Java equivalent of delete?
Unlike C/C++ which allows the programmer direct access to memory
addresses and makes him/her responsible for allocating and deallocating
memory, Java hides the true location of allocated objects and does not
offer a means for the programmer to directly deallocate them. In other
words the JVM/KVM is in charge of such actions.
What happens when I call System.gc()?
There is no definitive answer to this question. The JVM/KVM is NOT
required to even perform garbage collection services. That's right.
Read that statement again and embed it in your mind. Now, what good is
a JVM without garbage collection? I don't know. Perhaps none, but if
you wanted to implement your own JVM you could make it a spec-compliant
one without collecting any garbage at all.
It is very important to realize that calling this method is merely a
suggestion to the JVM to perform garbage collection. Even if it
supports garbage collection it may choose not to perform it. Also the
JVM may decide to perform garbage collection on its own without the
program calling System.gc() if it deems such an action necessary (such
as dwindling free memory).
Assuming garbage collection does occur, how does it work?
Again, no definitive answer. It may take a "mark and sweep" approach or
other. This is up to the maker of the JVM to decide. In most JVM implementations
however the garbage collection seems to occur in its own thread and as
such slows the application down somewhat.
This tutorial is pretty useless, what can you tell me that is
definitive?
This much is for sure, in order for an object to be eligible for garbage
collection, no live thread can have any way to access it. This can be
accomplished in a few different ways:
1. A thread dies, taking the sole reference of the object with it to the
grave.
2. All live threads nullify their references to it.
3. All references to an object are isolated (A points to B but A's
reference was nullified so no way to access B anymore).
How the JVM knows that an object has references to is unknown. It
might maintain counters or might have lookup tables or other... It is
enough that the JVM knows which objects are and are not eligible for
garbage collection.
How can I make it easy to ensure an object's eligibility for garbage
collection?
Simple, maintain as few as possible references to the object. A good
approach is to use the singleton pattern for having a "reference manager".
Since only it maintains single references to all objects only one
reference need be nullified.
Another approach that a member of these forums pointed out was to kill
the thread that has only its own local references to objects. Once the
thread is killed the references die with it, making the objects elegible
for garbage collection.
How can I prevent my object from being elegible for garbage
collection?
In the class' finalize method you can assign an outside reference to the
object before it is collected. ( ex: OutsideClass.referenceToMe = this; )
This will only work the first time the object is attempted to be
collected since the finalize method is only called once.
My device has XX kilobytes of memory but I cannot allocate all of it.
What is going on?
As mentioned earlier, references aren't the same as pointers. Memory
allocations can be made anywhere on the heap and we have no guarantee of
where they occur. As such a small object might be allocated right in
the center of the heap. A subsequent allocation of a large chunk may
not be able to fit before or after this smaller chunk. To avoid these
situations try not to allocate large chunks of memory.
When I print out an object I get something like Object@345563. Isn't
this the address of the object?
No, that is the object's reference ID. It helps the VM lookup the
actual object instance in the heap at some other location. Each JVM can
decide how to implement this ID. You can still use this ID as a means
to test whether two references refer to the same object ( a == b ).
How do I deal with OutOfMemoryErrors?
In theory you don't or at least shouldn't. Errors, as opposed to
Exceptions are not meant to be caught by your application. The fact
that this error occurs is a sign that you are trying to allocate more
memory than is available as a single chunk. This usually occurs when
you attempt to load a large image after loading many smaller ones. The
smaller images used up half the heap and the large one can't be provided
a large enough chunk in which to fit. To overcome this, call System.gc() more
often (even if its performance is not guaranteed) and avoid loading such
large resources into memory. This way you shouldn't need to catch the
error.
Is there any way to determine the real addresses of objects on the
heap?
No, at least not any offical way. Imagine the havoc that would be
caused if there were. You could easily prevent the JVM from doing its
job properly if you were able to manually delete an object by yourself
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Claymore
Global Moderator
J2me god
Posts: 4978
Quantity != Quality
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Ok, I've finished what Kirk had kindly started - thanks Kirk! 
All of the above FAQ stuf is on the Wiki now, so, probably best to post future FAQ stuff there. It could do with some proper formatting - like restricting the width of the entries. I've no idea how to do that though.
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