Develop Reference

How to choose the platform for my Mobile App and how to evolve it into multiple platform?

I'm finishing my Masters Degree in Computer Science and i just had this idea for a Mobile App to solve a problem i've been having for the last year or so. I have good knowledge of Java and i know that it's a good language for Android programming, but the thing is that this App would be to target both Android and iOS users.
Considering this, how do mobile developers go about this problem? Is it possible to make the application viable for both platforms or usually it works as a separate application with the same funcionalities?
For either cases, what tools/programming languages would you recommend for me to develop my idea? I'm on my 4th of 5 years but we still had no contact with mobile development.
Any other sugestions you think i might need to take into consideration when starting to develop an app are welcome!

Java can't be used for cross-platform development, as of my knowledge, but here are some great choices out there for cross-platform development. Here are some:
Flutter - developed by Google (my personal favourite)
Ionic
React Native
Xamarin
The question is does a cross platform SDK suite your needs. If you need to interact a lot with native APIs then it certainly isn't for you and you'll have to build two separate apps.
Another downside to using cross-platform tools is usually app size (for Flutter around 3-5MB larger for a simple app).
Here is my take on why I prefer flutter:
It's easy to build nice material UIs, since Google includes many material widgets
There are a lot of plugins available at pub.dev
It has close to native performance, because in opposition to the other SDKs the dart code is compiled into native code.
Hope I helped! Good luck in the strange world of app development!

Compile Android app to iOS

Is it possible to create a new iOS app with exsisting sources?
I am working with xamarin in Visual-Studio. I've got a project, an app that runs on Android devices. But can I make a new project for an iOS app with those existing sources? I have a Macbook that I can use to do this. I also have a PC with windows 10 on it. So the hardware/software requierments shouldn't be a problem.
Getting to the point..
Is it possible to transform the project from an Android app to an iOS app without writing any code extra? And how can I accomplish this, any tutorials that I can watch/read?
The code language is C#. The programs that I am using are Visual studio and the xamarin extention.
I've read something about Platform Abstraction and Divergent Implementation on the xamarin ducomentation page, but this couldn't fully answer my question.

Short answer: NO.
Long answer:
There are a quite a few cross platform developing tools out there. Xamarin, like the one you are using, or Cordova. They simplifies the development by "A LOT" but you can't just convert one app to another without writing 0 code. Some of the libraries in one platform do not exist in the other. So, the long answer is that cross platform shortens the development cycle as you do not need to start from scratch on other platform, that does not mean it can completely convert.
P.S. Youtube is your friend, but the difficulty of the transformation solely depends on the scope of your project, as they vary a lot.

I second the other answers.
Furthermore, I'd like to propose to have a look at Flutter.
Flutter is based on Dart language . The framework and your application get compiled from Dart to the Android and the iOS target platform.
Flutter is a product of Google and likely to get the base development environment for Fuchsia, a new OS from Google.
My own experiments with Xaramin ended up kind of frustrating. The development was slow, several times problems with Xamarin itself or plugins made me stop the effort.
At the other side, I created an app with Flutter quite fast. While Dart and Futures might seem at first quite different compared to other concepts/languages, after some time things turn out to be very natural and well designed.
While Flutter originally was based on Material design, Google added an iOS theme using CupertinoWidget.
A simply great feature is the enormous quick development turnaround time: The app under development very ofter adopts within just a second your source-code changes. Flutter/Dart hot reloads changed classes often instantly.
Android Studio is a great and free IDE for Flutter development. Plugins enhance to IDE with Flutter development tasks.

If you have android application written in Java you can use Migeran Analyzer to understand how much % of the code could be reused between android and iOS: https://analyzer.migeran.com/
Once you have the % you can use Intel Multi-OS Engine (free product) to create cross-platform application in Java: https://software.intel.com/en-us/multi-os-engine. It will have module with cross-platform shared code, android specific code module and iOS specific code module

Is developing an application for Android using Lua only possible?

So I read this article on Mobile Orchard and, as I'm planning on having my first steps in Android development, it got me thinking: is it possible to develop for Android in Lua only? I Googled the topic but found no definitive answer. I know Java is the "base style" for Android but for the coding newbie which I am (elementary Python, basic C++), Lua seems somehow clearer and more friendly. Then here goes my question: can one develop an Android app using Lua only? If so, could you please show me some directions to move from here?

I've been developing a mobile application in Lua to control robots remotely from iOS and Android phones. The app is in pure Lua and runs on top of Mosync platform using MobileLua. There is still some plumbing required, but it's not too complex.
I also have a debugger and a Lua IDE (ZeroBraneStudio) that support debugging of Lua scripts running remotely (including MobileLua scripts running on Android or iOS devices). I provide a demo app you can use as a client to run simple scripts and check remote debugging in action.
[Update] If you are interested in more game oriented options, there are several available: (1) already mentioned Corona SDK, (2) Moai, and (3) Gideros Mobile. There is also Marmalade Quick and Cocos2d-x Lua bindings (for example, quick-cocos2d-x). All these toolkits/frameworks allow to develop in Lua.
Here is a detailed comparison of Moai vs. Corona.

I know this is an older question, but just to correct some misinformation (in the comments) for people who come later, Corona SDK (as of spring 2013) has a Starter Edition which is free -- no splash screen or anything. The main difference between it and the "Pro" version is you can't do in-app purchases in the Starter Edition. But you can use it to create apps for sale, etc. No expiration date on it or anything.
And in answer to the original question, learning Java may allow you to write Android apps, but if you then also want to target iOS you don't have any great options. By choosing something that's cross-platform from the beginning you can target Android or iOS or both. Lua (in any of the flavors already mentioned) is a great base language to learn for mobile development.

The question is what kind of app do you want to build and what skills do you have. If you are already familiar with JavaScript there is Phonegap/Cordova. If you are building games there are a number of frameworks for 2D and 3D which use Lua. Some are free, others have licensing fees but are worth it if it provides a solution to your problem. All have pros and cons. For example I'm not sure if I would want to build an RSS reader in a game framework. Java is just another language, I would recommend just learning Java and the Android framework to start and knowing what is there before adding another application specific framework layer on top.

Corona SDK allows you to write iOS and Android apps in Lua. Its very easy to learn, but its not free.
Edit:
I saw you posted that you found Corona just after I sent my post...

You can try http://scormpool.com/luastudio
Develop and run code on your device including iOS, Android, Mac, Win.
Exporter can help you to make standalone app on listed above platforms.

This may not be useful to the OP, however for other people interested in using lua to write their apps, I'd share my thoughts. So to answer the question:
Yes, you can use only lua to write apps for android using LuaJava from the kepler project (though I don't believe its being maintained anymore). You can create and manipulate Java objects seemlessly, and interact with the Android APIs through lua. You can even create UI elements dynamically using just lua. However, a small amount of java bootstrap code is needed to start the lua VM from your app. The AndroLua project is a great place to start for Android. The creator has done a lot of work making lua work with Android and packaging it all into an app. From experience I can say it works great! However, I would in no way recommend this to a beginner, as it requires a lot of knowledge about java and lua to make things work, meaning more work not less.
For AndroLua, just clone the repo and use that as a base for your own app. Last time I used it, I just imported it into android-studio, and after fixing a few problems with my own setup, had it compiling and running on my device.

If only pure Lua then the answer is no.. unless you want to use a lot of libraries
If you want to make little Android app/games then use a game engine, learning is a step by step process, don't dive straight into the hard part.

iOS and Android development on Windows

I'm facing a dilemma: I have a solid ASP.NET development experience in C# and looking to start mobile development. I have done research and know that there are some hacks for getting a development environment for iOS on windows, and recently after Apple lifted ban on 3rd party dev apps, Adobe Flash CS5 can be used to package iOS apps. I do have some prior experience with Flash and AS2.
So my question is: should i use Flash CS5 for iOS and MonoDroid for Android or should i just get Mac and learn Objective C for iOS and learn Java for Android? What would be the most practical solution?
Thank you in advance
EDIT: In the end, after trying out both Java Development in Eclipse against Android SDK and Mono for Android, I decided it's best to write Android apps in native environment. Eclipse FTW! As for iOS, I think it will also be best to use native Mac tools and Objective C.

While there are ways to get around using Objective-C for iOS development, I'm going to recommend that you learn it anyway. You need a Mac to deploy anyway, you might as well use it to develop as well.
Objective-C is bloody bizarre when you first start using it, with its strange square brackets and odd ways of doing things. However, once you've spent some time writing code in it, a change happens in your brainmeats and you suddenly realize that this is really an elegant way to work. Xcode is also quite an excellent IDE.
You can start with the iPhone Applications Programming lessons on iTunesU from Stanford. It's free, and covers what you'll need to know pretty throughly.
Worst case, you've learned a new language, and how Apple thinks iPhone apps should be developed. Even if you ultimately use something else, that understanding will help you out long term.

Take a look at monotouch & MonoDroid (Coming Soon), it still requires a mac. But you can open your solutions in VS2010 and use resharper, and just use the Mac for deploying and building. Take a look at this Here to open monodevelop solutions in Visual Studio.
http://monotouch.net

For Android, I would recommend learning Java. I just started working with Android in Java on my Windows machine after developing some ASP.NET sites. I'm familiar with Java, but the syntax is so similar to C# that it's very easy to understand and pick up. So far it is more difficult figuring out and remembering the details of the architecture of mobile applications as opposed to the language. It also helps that there are a number of good books and online resources for converting from C# to Java and vice versa.
Update: I found a good post over on programmers.stackexchange.com that talks about C# vs. Java for Android development with good discussion on the pros and cons of switching: https://softwareengineering.stackexchange.com/questions/33076/as-a-c-developer-would-you-learn-java-to-develop-for-android-or-use-monodroid-i

#Dimitri I was almost your identical skill set when iOS 2.0 was released. I decided to try and learn Objective-C and now its my language of choice. Its a bit different to start with but most of the changes are simple to learn. Whether you are programming for iOS using Objective-C or something like Monotouch either way you have to learn the frameworks. If you use Monotouch you will never be able to be on the bleeding edge of the iOS enhancements. Using Objective-C you will be able to use the latest beta APIs and ensure your apps work right. You will need a Mac to deploy to the App store so just bite the bullet and get a Mac. Get a MacBook Air 13"

I went the route of Monotouch for iOS and Java for the Android. Unfortunately, Monotouch still requires a Mac. I am currently participating in the MonoDroid beta, which looks pretty promising too. It's all a matter of taste. I should also note that, for the most part, HTML/Javascript is a nice way to get portable apps in some circumstances. I would expect HTML5 apps to become even more viable as tooling improves.

I choose to use the Mono, MonoTouch and Mono for Android technology stack from Novell. When coupled with a micro framework like MonoCross (open source in Google code) it helps me write applications for both iPhone and Android devices while leveraging a shared code base for much of it. I would highly suggest the same route, it has allowed me to implement numerous solutions for small businesses and enterprise customers alike.
I didn't have the ASP .NET experience like you, but I was a Pocket PC/Windows CE developer so C# and the .NET Framework were also my strongest tools when I started on journey towards Mono many moons ago.

You can do iOS,Android and Windows using Xamarin.
Android and Windows can be done easily but doing iOS on Windows was tough so far like networked mac is required for debug and development but things have changed now.
Microsoft has introduced Xamarin Live Player using which you can debug,develop and deploy the app completely in windows without the help of any Apple system but to sign and release the app to appstore you will need Apple System though

Android development toolkits

Was wondering if anyone knew of some third party android development toolkits.
I have found Basic4Android.
Was wondering what other options are available.Language does not matter.
Thanks

Qt for Android (Alpha version) got launched last week.
Motorola has a custom dev environment built on top of eclipse (MOTODEV). It is not Motorola specific and you can use it for regular development. I have been playing around with it for a week and it is much more convenient than the standard Android Eclipse plugin.
EDIT: Forgot to mention there is AppInventor (currently in Beta) which is a drag and drop development environment from Google. It is a useful tool if you want to get a feel for development or you are developing just for your phone. It does not generate Java code (as yet) and there some restrictions on it like only single screen apps are supported. It has got a healthy community and lot of college students (in the U.S) seem to be using it.

MonoDroid is also another development stack for Android using C# and .NET API's. You can use Microsoft Visual Studio IDE to develop applications using MonoDroid.