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cross-platform apps with c#Is

This is a question isn't asking whether is't possible (because I've already checked that) but as too way it's generally frowned upon.
My background in programming is C and C# and I've been more interested in app development since the announcement of Windows 10. I have also been interested in Android development for some but don't have the time to really dwell into Java. I do know however that C# can be used to write applications for Windows and Android so sticking wit C# seems all the more compelling.
Looking at the new Visual Studio RC, I notice that they are saying that you'll be able to run native android apps using C#. However, of this forum it's apparent that people don't like the idea of using another language other than Java to do so. Would there even be such a performance drop when many android devices have quad-core SoC's with 2-3GB RAM? I'm really intrigued in using C# for cross-platform development the Windows platform is interesting me even more.
As I should state, I'm already aware of Xamarin cross-platform development. But this question is specifically on the performance difference and the fairly mute enthusiasm by many android devs when suggesting so.

C# and Java are pretty close langues so that is why the conversion from C# to Java is easier. Based on your code and the approaches you follow, the performance of course is to change, however, as mentioned since C# and Java are syntactically very close it will just be converted to the Java code and compiled using Process Virtual Machine. The only difference would how Java and C# takes same code block, and compiles it. I think you will just need to research that point, and then you should be safe to go.

You will need Xamarin if you want to use C# to develop Android apps. Xamarin claims "native performance".

Can Google's j2objc process and convert binary JAR files?

I'm looking to get started with mobile development (specifically iOS), and while I'm usually pretty good at learning languages, Objective-C confounds me at almost every turn. So, looking at j2objc (because I already know Java), I think it might fit the bill in terms of what I'm looking for.
My question, though, is pretty simple: can the converter properly handle a situation where my source code references, say, Apache Commons code in a binary-only (bytecode) JAR file?

No, j2objc requires source code (it uses the front-end of Eclipse's compiler). The RoboVM project works from class files, though.

J2objc is good for writing business logic only once. If you plan to write an app for Android, iOS and GWT, use it. But you will always need to write native code for UI, network and other specific platform stuff (Camera, Geoloc., etc.). If you plan to become a mobile development expert, my advice is to start learning Objective-c right now (I was just like you 2 years ago. I invested time to learn Objective-c and it's the only way to become a cross-platform mobile geek :-). I started with this book : Programming in Objective-C 2.0 (Stephen G. Kochan). Good luck !

No: j2objc requires source code. However j2objc allows transpiling source code that references .jar files. However the jar files with .class files wont be transpiled

I am coming from Java background as well and learned Objective-C by myself (and taught it as well).
If you really want to develop mobile apps for iOS the best thing is to invest in learning Objective-C. J2ObjC is not sufficient to create iOS apps since it does not provide UI support and it is not stable yet (between alpha and beta quality)
J2ObjC does not provide any sort of platform-independent UI toolkit, nor are there any plans to do so in the future.
Objective-C looks strange in the beginning, especially for people with Java or C/C++ background, but once learn it; you will start to appreciate the language.
This is an excellent course to get you started and apple provides excellent resources to learn iOS development and Objective-C.
I strongly doubt that any language translation tool ,e.g. J2ObjC, will be sufficient to build good iOS apps.

Translating/transposing an Iphone app to Android

My team and I are within days of finishing our first iphone app. However, we went to program the same app onto android. None of us have any experience with android. I've been reading up on some tutorials for android beginners.
Since I know there are many knowledgable and experienced android/iphone programers on stackoverflow, i was wondering if there are any tips/resources you guys could shoot my way.
thanks!

For the best experience (for your users), you really ought to learn how to develop on Android using the standard Android tool chain. Applications shouldn't just be 'translated' from platform to platform, they really should be redesigned.
There are differences between platforms that really require rethinking of how things should work, both at the user level and at the code level. Simply translating between platforms results in a poor experience for the result.

I don't know how your project is structured out or what technology you use. Anyway, for me, I implemented all the logics and coding in C++ other than the interface itself. Then, I used JNI for the C++ code on Android. At the end of the day, it's a matter of building Java GUI programming on Android.
Alternatively, you could use some portable framework like Titanium and PhoneGap. But they do need some learning curves and you might experience weird behaviors.

Are there any huge differences between objective-c and Java, or iPhone and Android?

Edit: My bad, I meant objective-c, not c#. Some reason I got it into my head it was c# the iphone used. So the answers for c# were great, thanks, but theyre a bit irrelevant, sorry about that.
I've had a look but can't find anything that answers this, though a few have shortened the question by answering parts of it. Between a small group, we were planning on doing some work on iPhone and Android, the 2 seperate for the most part but helping each other out, and with some guys doing graphics work split between them.
But we were thinking about the possibilities of moving things between the two, not necessarily apps, maybe just useful classes or something. Looking at objective-c and Java, they seem to have about the same features that the biggest obstacle would be system interface stuff, so we were wondering whether, if we created an abstraction over these on each system so they could be given the same input (which unless I'm wrong wouldn't put too much strain on the system?), would there be any problems in writing something to convert between objective-c and Java, worse than the locations of methods in the sdks? Or are there key features or something in one language that the other doesn't have which we've missed that would mean the only way to do it would be rewriting from scratch.

I wouldn't waste time trying to find commonality between iOS and Android.
Cross-platform is almost always a waste of time and resources unless the cross-platform capability is central to the apps functioning. That is especially true for platforms such as iPhone and Android which have custom OS and work very tightly with the hardware.
Cross-platform development environments add rather than reduce complexity long term. Yeah, it sounds neat but usually you get 90% what you want easily and then you hit a roadblock that destroys all the savings you made and then starts putting you in the hole. There are simply to many compromises and square pegs jammed into round holes.
Unless your app could in theory work from a generic web page, cross-platform is not for you.
In the specific case of Objective-C and Java, although Java is descended from Objective-C they have no modern interoperability. You can't use code from one on the other.
You should spend the time to learn each platform's specific API. There are no shortcuts.

Tools like XMLVM, Codename One and iSpectrum tell us that you can actually cross-compile Java code (from an Android app or creating one from scratch) to Objective-C code that is running on an iPhone without having to install any virtual machine on the Apple side of things.
I suggest watching Developing iPhone Applications using Java
Unfortunately Apple's license
agreement for the iPhone SDK prohibits
the porting of the Java virtual
machine to the iPhone. In this
presentation we introduce an Open
Source Java-to-Objective-C
cross-compiler as well as a Java-based
implementation of the Cocoa library.
With the help of these tools, iPhone
applications can be written in pure
Java. Using the Java version of Cocoa,
it is possible to run a Java-based
iPhone application as a Java
desktop/applet application that can be
cross-compiled to run natively on the
iPhone. The talk will discuss the
challenges of the Java-to-Objective-C
cross-compiler as well as the
Java-based version of Cocoa. Details
are available at http://www.xmlvm.org/
and for more insight a more recent talk about Cross-Compiling Android applications to the iPhone from Frebruary 2010 is available here, which is documented at http://www.xmlvm.org/android/ :
Android is an Open Source platform for mobile devices. Initiated by Google, Android has received much attention. Android applications are developed using Java, although a special compiler converts class files to a proprietary, register-based virtual machine that is used on Android devices to execute applications. Android defines its own API for writing mobile applications. With the help of XMLVM it is possible to cross-compile Java-based Android applications to native iPhone applications.
The Android application is written in
Java and makes use of an Android
specific API. XMLVM offers a
compatibility library, written in
Java, that offers the same API as
Android, but only makes use of the
Java-based API for Cocoa Touch. During
the cross-compilation process, both
the application and the Android
compatibility library are
cross-compiled from Java to
Objective-C and linked with the Cocoa
Touch compatibility library to yield a
native iPhone application.
From the 2008 talk about building Java applications for the iPhone http://www.xmlvm.org/iphone/ :
Apple's iPhone has generated huge
interest amongst users and developers
alike. Like MacOS X, the iPhone
development environment is based on
Objective-C as the development
language and Cocoa for the GUI
library. The iPhone SDK license
agreement does not permit the
development of a virtual machine.
Using XMLVM, we circumvent this
problem by cross-compiling Java to the
iPhone. Just like a Java application
can be cross-compiled to AJAX, XMLVM
can be used to cross-compile a Java
application to Objective-C. The
cross-compilation is also accomplished
by mimicking a stack-based machine in
Objective-C.

If you focus on abstracting away things like system interactions, and stick to the common subset of the languages, you can probably build classes that would require little or no modification to move from one language to the other.
C# does have a better implementation of generics than Java, in that C#'s generics retain strong typing at run-time.
C# also has LINQ, which as far as I know has no equivalent (yet) in Java. LINQ provides a SQL-like query capability, built into the language and fully supported by the .NET frameowrk, that lets you query object collections, and does so in a very functional-language style. LINQ can be extended by query providers to work with other data formats, such as XML and SQL, once the query provider provides an object model that translates to the other format. LINQ also makes parallelization very easy for multi-core work. It's a huge extension to language capability.

This topic is often filled with the dogma of technology jihadis, so I shall try and steer clear of that in my answer.
In my experience, I would largely agree with people saying that trying to share code between the two platforms would be difficult. However, there are some important exceptions:
I would consider cross-platform development where your business logic:
is non-trivial;
should be standardized across platforms; and
has well defined interactions with the outside world (e.g. network stack or UI).
(bonus) is written already.
Apple is the rate determining step here, given the now infamous 3.3.1 restrictions on source code languages. You can write code in Javascript according to the bundled Webkit, C, C++ and Objective C.
If you don't want to install extra languages on a 'droid, you can use the Java that compiles down to dalvik code, the Javascript as per the slightly different build of webkit or something from the NDK. Then you'll be looking at C/C++. You could cross-compile Objective-C, but I haven't had any experience of that.
The clean separation of business logic from UI and networking is important, as you'll be looking to write adapters for the networking layer, something else for the UI.
I would not attempt to write cross platform UI code in C/C++, and would either write something that used HTML/CSS/JS, or more likely write something completely custom to take advantage of the different UI metaphors on each platforms - e.g. there is no analogue to the notification bar on the iPhone. Animation on the iPhone is orders of magnitude simpler to implement than on Android.
If you don't need the UI to be massively integrated with the OS, then a webview and some HTML5 may be sufficient. Titanium is a good option here, and my colleagues in the know tell me it is better (i.e. compiles, not interpreted) than PhoneGap. Again I don't know.
Going the other way in complexity, Open GLES is available on both platforms.
It should also be noted that SVG is not available on the current crop of Android OSes.

If you actually want to do cross phone development, as long as it is not something really computationally expensive, I would probably go with PhoneGap.
PhoneGap is an open source development framework for building cross-platform mobile apps. Build apps in HTML and JavaScript and still take advantage of core features in iPhone/iTouch, iPad, Google Android, Palm, Symbian and Blackberry SDKs.
As far as I can tell, it's pretty much the only cross platform toolkit that is allowed in the iPhone app store.
If you're dead set on native development, I think you'll find that unless you're making a game and targeting c, it will be a lot of work to port. The frameworks are very different and the way of building UI is very different. That said, to answer you're direct question of objective-c vs. java, it's not that bad. It is different enough, though, that an abstraction tool would not be that easy - not like Java vs C#. Objective-c does not have garbage collection (on the iPhone anyway), for example, and the syntax is very different. Objective-c is more loosely typed in the way that it does method calls (messages in objective-c). That said, they aren't wildly different in terms of programming paradigms. They are both object oriented imperative languages. They both have classes and methods, public and private. If you wanted to port the code by hand, it wouldn't be the end of the world, I just think you might wind up spending a lot more time trying to build an abstraction layer.

If you're thinking about doing Android development my best advice is to go for java. There is no benefit at all of writing a complete abstraction (assuming you can) layer to convert from C# to Java. As for the IPhone development dont doubt about using Objective-C for the same purpose.

You can't develop for the iPhone in the same language as for Android. For the iPhone, you can only program in Objective C, it's developed by Apple. You can't use it for android, and the only way to do iPhone development is with that language (it's in the user agreement).
As for C# vs. Java, the whole principle is different. Java is one language for every platform, .Net is one platform for every language. They are not compatible. You can use Java on Android, but I'm not sure if you can use C# (.Net) as well.
Anyway, you'll have to build two different apps for these two mobile operating systems.

Develop app to run natively or go cross platform? [closed]

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What's your opinion on whether one should write an application for a specific platform or use a cross platform approach and reach more potential clients?
It would be a GUI based application and I'm thinking of using Java.
Most of the applications that I've used built on Java don't run as smooth as the native applications on the respective platforms.
The two platforms I'm thinking of are windows and macs.
If I was creating the app just for my own use, and I had to choose one system, I would choose to focus on the mac.
I would like to hear your thoughts.

I would use Qt and yes, definitely target Cross Platform...
The major reason isn't in fact what you think it is (being cross platform by itself) but actually that by targeting multiple platforms your code by some freaking weird magical reasons tends to end up with far better code quality. I've seen this happen countless times in my own projects. Every time I add a new platform the quality of the code increases and the code becomes more clean...
Also every serious app should have as few "dependencies" as possible, being cross platform means you have less dependencies... ;)
Then as a "bonus" you get to your app to run on multiple platforms...

Requirements, requirements, requirements.
In all seriousness, "it depends" is the right answer.
Truly cross platform is hard work; Working where you/your users are familiar is worth something. Solving the problem in an effective manner is really the highest priority.

Cross platform development, in general, is a bit overrated as an artform. Java is the leading technology here, but even in this case, most Java programmers don't really go cross platform. There are a few spectacular examples of tools like Vuze and LimeWire and JRipper which work pretty well just about anywhere, but these are exceptions, statistically speaking. Most Java apps, especially of the corporate variety, are coded with one platform in mind, and they never really move around.
If you want to go Mac and Windows, consider spending time in the Limewire and Vuze codebase. Both of these apps work extremely well on both platforms.
You will definetly need two builds. One build probably won't do.

I suggest to develop it first for the platform you feel comfortable with, and where you can make use of all platform gimmicks; in this case, this would be OS X.
Later, if your app is successful for the Mac, look at porting options. It's IMO not worth to put effort into porting until it is certain that there would be significant interest in the app in the first place.

In this day and age, is there really a good reason to write native code?
Yes. The languages and libraries available do not allow excellent programming to be done well. At best they allow one to write code that works ok across platforms, but never excel.
Pick one and focus on it. Focus on that audience (you'll find the audiences are different in their tastes and desires anyway).
Once you've got an app people are clamoring for on the other side, and just before someone makes a competitor, evaluate the cost/benefit to porting your finely tuned and well designed product over.
If you try to 'serve two masters' you'll make design decisions based on what's easiest for both platforms, and that's not a good design process.
-Adam

If your app uses an MVC design, you can write portable model classes using C++, and native controller and view classes for each platform you want to support. That gives you the best of both worlds - portability where that's practical, and a native interface for which your users will thank you.

I agree with your observation about Java applications versus native applications, although I'm sure that an experienced Java programmer could probably make it work.
The bottom line is that the programming APIs for both Mac and Windows are the best tools to use for building interfaces on their respective systems. If you want your app to run smoothly on a particular system, you should use that system's API.
If it is absolutely essential to have a cross-platform implementation, consider writing all of the core functionality (non user-interface portion) for your program in a library that can be compiled for both systems (c++ is an obvious choice), and then build a nice user interface for each target system that can makes calls to the common library.

IMHO, this discussion should be answer by the customer needs.
Most of the time IT provide solution, the way the solution is build is only a detail for the customer.
However making the code reusable could be useful in case your first choice of development environment was wrong.
many other criteria may come into the thinking.
for instance if your software is for health care you may want to support for 10 years, then you have to select an OS supported for more than 10 years (forget windows and mac). etc.

Depends on how much time you wish to spend on the application. Fastest way is Java, otherwise you can build your model with C++ and do separate GUI code using MFC and Cocoa.

Why don't you consider Web Application.
You can build rich UI Web site running in browser which runs everywhere and all platforms,
If you are familiar with Java, you can try GWT for the 'Weblication'
Check out the demo page of GWT

Make it a web app ?
Otherwise Java i guess. It depends of many things, type of your app, target etc.

I have had good luck with cross platform Java using SWT. This will give you a nice native-look on any OS, and there aren't a whole lot of problems out there that require any JNI code. With any luck, you should be able to distribute the exact same codebase to both targets, and it should just work.
Having said that, you should evaluate whether or not you really need the cross platform functionality. At the very least, it will require you to test the GUI layer of your app on each OS (testing layouts, presentation, etc). With a large app, this becomes cumbersome very quickly.
In either case, choosing a cross-platform language from the start will give you flexibility in the future. If you were to write native code from the start, it would be non-trivial to port the code to another platform. It would likely be accompanied by several bugs, and would also require a port of all the test code to the new platform. You would run into synchronization issues with any new features, bug fixes, etc. With Java, you can write JUnit tests which will run in both OS's, and any bugs or new features will be propogated to both versions automatically.
It really does depend on your specific application. If you are anticipating a user base on both OS's, then that should dictate your language choice.

It is possible to get a nice looking Java app to run on Windows, Linux and Java - look at SBCommander for example - and note that you can run it and look at the UI even if you don't have the hardware that it's supposed to control.
On the other hand, one thing I've found very hard to do is to write a cross-platform app that looks and behaves like a native app on each platform; there are just too many different assumptions about how an application should behave.

It really depends on who your users will be. Don't go cross-platform just because you can. There needs to be a real need regardless of the tool. And testing is going to eat up quite a bit of time. You'll still need to thoroughly test on both platforms.
I use REALbasic to create a wide variety of cross-platform desktop applications from a single code base and it has worked well for me and my clients.

Quite often I find PC developers attempt to capitalize on the Mac market (some do successfully). In turn, they development the application in the described above "cross-platform" architecture (Java, or some Qt framework, etc...) The result is usually, a hideous looking application with poor OS integration support and the community backlash commences in the reviews.
My advice, port it. Do a good job, understand your market before dumping your cross-platform product in your carpet bomb marketing approach.