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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.

How to create large deployable programs with multiple languages?

Hi all this is my post on stackoverflow.
I am normally a lurker and find everything I need without posting but was drawing some blanks when trying to picture how this works. Maybe if someone can provide some insight I would greatly appreciate it.
So I understand the software development cycle and know a decent amount about code
(c++ and Java, and visual basic, I am a CIS major with a CS minor in my last year). I can write some programs in elipse using Java and I can even make some basic GUI components in eclipse using Java.
Now onto my 3 part question: I understand one could make a .jar from several .java files and distribute it, but while thinking about a common program like say yahoo messenger (just as an example) I came to the following three part question:
First, being could you write a program like yahoo messenger in just eclipse using Java or would you need other tools like other languages and other development environments, how does the project go from start to finish? (I think I am kind of lost here because I have only wrote command line and simple GUI applications).
Second, Say it was easier to create a feature in yahoo messenger using a different language, how can one write one program in more than one language? Say for example you wanted to write some code in python or c++ and your the majority of your code was in Java, ie your main method is java and you are compiling using JVM. (assuming you cant just stick python or c++ code in a java program) I did some Googling around and saw some things about linking the compiler and including native code to include other language code in a Java project. Links to other reading material is acceptable too if the explanation is too long.
Third- How does deployment work? Say I am done writing the code for my program and want to turn it into an .exe (for windows users) and stick it on my site for people to download. I know windows comes built in with an iexpress utility to create .exe's. Besides distributing a .jar how would one go about turning source into an exe? Thanks again for all your input and time. I am a beginner and trying to wrap my head around these concepts. The answers can be provided in a technical realm or just conceptual either is greatly appreciated.
-Mark

1) I most cases you can write your program in java without needing any other programming language. There are rather rare cases where you need to call a dll from java to interface with some proprietary program, for this you would need to use JNI and C or C++. A perhaps more common case for using multiple languages is for adding scriptability to your application. For example, my company offers a server/client application that is scriptable by users using Groovy, but the server and the rich client itself are written in Java only.
2) The integration of java and another programming language depends on the other language. Integrating Groovy is easy, and I think integrating Python (using JPython) or Ruby (using JRuby) is fairly easy. But it is an effort (not to mention the mental stress of programming in different languages) and I would not recommend doing that unless there's a specific requirement for this.
3) As always, there are several options. See how-can-i-convert-my-java-program-to-an-exe-file for creating a windows executable. Or you can create windows installer using e.g. NSIS. Or use Java Web Start.
If the intent behind this question is getting an idea how some big java rich-client (desktop) applications are written and deployed, I recommend the Eclipse RCP book. This book will walk you through the development and deployment of an XMPP/Jabber messenger client using the Eclipse RCP framework. Be aware though that there is no one true way of creating a big application and other java application frameworks do things differently.

Several IM clients are written in Java, though I would hazard a guess that the mainstream ones would be mostly written in C, C++ or (on the Mac) Objective-C.
Writing the one program in multiple languages has numerous challenges, and the nature of the challenges varies depending on the combination of languages you want to use. In many cases, you will probably not have much luck combining more than two languages. One set of impedance problems is bad enough, three is an almost guaranteed disaster.
You can avoid these problems by splitting a single application across multiple programs, each of which is written in a single language and communicates with the other programs via some kind of IPC mechanism.
Creating an "exe" is also a very language-specific concern. For instance, Java, C#, C/C++, and Python all have radically different deployment stories.

Say I am done writing the code for my program and want to turn it into an .exe (for windows users) and stick it on my site for people to download.
In that case, I'd say you were foolish. ;)
Java Web Start is a better option for deploying a Java based rich client app. from a web site. JWS works for any platform with Java.

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.

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I've just finished a relatively large project for the Android, and it's left a bitter taste in my mouth with the knowledge that it will never run on one of the most ubiquitous handsets this side of the solar system (the one by that fruity little club).
So, for my next project, I want to write it in a way that makes most of the components easily transportable between the iPhone and Android platforms. The way I'm thinking of doing this is by coding most of it in Objective-C, and then adding the platform-specific parts in more Objective-C and Java respectively. On the Android side, this will require using the the NDK.
My knowledge of C is good, but my knowledge of Objective-C is close to zero, and I have no desire to learn C++. How sane is the approach above, and is there a better one? Is there any way I can code in Java and still reach the un-hacked iPhone market? And how likely is it that the people I know (iPhone users) will have an Android phone by next year?

Step back and think about what in the end you will logically be able to share.
The UI models are fairly different, the components are different. In the end what you might be able to share is data object classes, possibly some algorithms. It's not even like you could realistically end up sharing network code as in the old days because you aren't directly using sockets, you are using HTTP libraries.
So will all of the effort you are putting into this really find a payoff in the end? It seems to me the end result will be a brittle mess that is hard to update, and is mediocre on both platforms instead of being great on either.
Why are you writing applications? To make life easier for you, or your users?

Others have said basically this, but I'd like to make it more explicit. Your best bet is probably to write:
Cross-platform data models & core logic, using:
bits of GNUstep (Obj-C), or
CF-Lite (C), or
Whatever you'd like, as long as it's cross-platform :P
iPhone-only interface code, using Cocoa Touch (Obj-C)
Android-only interface code, however they do it for the Android.
That's as close as you can get; any attempt to write cross-platform interface code will undoubtedly result in a mediocre app on both platforms. But making all the rest of your code portable and just wrapping a device-specific interface around it is done all the time and has been worked great for some iPhone developers.

Objective-C without Cocoa is not so useful and won't bring you much closer to haveing a working iPhone codebase. You'd probably be better off writing your core in C with Core Foundation and using either Java or Objective-C for the platform specific parts. Apple has open sourced a large chunk of Core Foundation as CF-Lite, and it's toll-free bridged with Cocoa on OS X (i.e. you can use many CF classes interchangeably with their Cocoa counterparts).

My guess, which has no experience to back it up, is that you probably could write Obj-C with Google's NDK somehow, given that GCC exists for ARM, is open source, has an Obj-C compiler and a basic Obj-C runtime (which if it doesn't already probably could be hacked up to work on a new architecture), etc.
That might also be a lot of work for questionable benefit.
And of course "Obj-C" (without the NS classes) means something very different than "Cocoa", which is what most people really mean when they say "Obj-C". You might be able to re-use some of GNUstep for some that, but... Honestly, I doubt it. Sounds again like a lot of work.
So, yes, I think it is possible. It's also a lot of work and I don't think it's worth it.
Given what you've said, if I were attempting this, I would be tempted to write as much of your core logic as possible in C, then wrap it with two separate GUIs for each platform.

Coming at this from a different angle... I know that you said you wanted to try and stick with Java, but if you know C# then you could go with the MonoTouch framework for the iPhone. Mono is essentially and open source implementation of the .Net stack. The Mono team is working on bringing Mono to the Android so you could basically write a shared C# library for your business logic and have different Views/Controllers per platform. This would all be in C# of course and it is a bit more expensive, but it does solve the problem of writing everything in different languages.
I believe it is called MonoTouch on the iPhone and MonoDroid on Android.

The Apportable SDK is an Objective-C approach to write once and deploy to both IOS and Android. It will cross-compile a running IOS Xcode project to an Android SDK.
See here for sample apps that run on both platforms in minutes after download.

I'm not sure about Android but with the iPhone you can essentially write straight C as long as you wrap it up in Objective-C classes.

The Objective-C runtime has not been ported to Android yet. It shouldn't be too much work, but still, without a working knowledge of the language I doubt you'll have an easy time porting it.
What you are trying to do is going to be hard for a generic application, but should be possible for games, if you choose to develop the game in plain C (which is supported by both the Android NDK and the iPhone). You'd have to write up some glue code to pass input events from the Obj-C and Java environments into your C code, but that shouldn't be much of a problem - Objective-C allows you to directly call into your C code and there are plenty of example projects which do exactly this for Android.

I haven't tried this myself or finished watching the talk yet, but there is a Google Tech Talk on Developing iPhone Applications using Java up on YouTube that looks pretty promising.

XMLVM is a project which is capable of translating (some) Android applications to the iPhone. For more infromation, visit http://xmlvm.org/android/

I realize this may be a tad late, but it seems the industry is going in the direction of web apps these days to achieve app portability. That is, embedding a web-browser in your "skeleton native app", and writing javascript, css and html for Android, iOS and the other major smartphone platforms.
There are tools that help you with this. You might want to check out PhoneGap and Sencha Touch, but there are many more. Note that this approach may not be ideal for real-time/animation-intensive apps.

Here is a talk from facebook's mobile # scale conference where two teams (dropbox and orchestra) used similar approaches. Dropbox used C++ to create libdropbox and Orchestra (mailbox) used Objective-c to create libmailbox.
Again, they wrote their front ends in the platform native language and used their cross platform libs for core logic and data.
Key benefits I took away: Mailbox went from ios to android in 5 weeks because it was just building UI code. Dropbox can beta test changes to core functionality that are in the shared library with Android beta deployments were it's easier to do massive deploys at scale for beta builds.

If you can wait until later this year (exact amount of time unknown), Adobe will have AIR for Android and a compiler to iPhone. Thus you can write an app in AIR for the Android and use most of the same code to compile to the iPhone.
http://labs.adobe.com/technologies/flashcs5/appsfor_iphone/
Even if you can't wait see: http://www.insideria.com/2008/12/actionscript-to-cocoa---protot.html where it explains the similarities between ActionScript and Cocoa.
Also check out: http://labs.adobe.com/technologies/air2/ for the AIR version capable of using the touch screen.
So you can soon write once and deploy to Android and iPhone using ActionScript 3.