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Is there any way to use Swift for iOS as well as android development?

When having a finished iOS app, coded with Swift via Xcode, is there any way to use the code and in general the project file and kind of transfer it into Java, C or C++ code which that can be used to create an android app?
Learning an additional programming language, such as Java or C, "just" so I can publish two these two stores isn't hopefully the only solution to this kind of problem.
I´d appreciate any of your past experiences and ideas related to my problem.

You could have wrote it initially in React Native for example, or using Flutter or Xamarin or whatever framework you wanted to write once and then run on both iOS and Android.
If you want an app with GUI and not a simple HelloWorld, i don't think so, as far as I am aware there is no way to do it.
There is a possibility to COMPILE a Swift code on Android, of that I am sure, but to run an app out of the box without using some additional frameworks - nope.

Mobile Apps Java and HTML

Does anyone know a framework to develop a mobile application for Android with Java and HTML?
I mean develop UI with HTML (to reuse it with an other OS) and native functions with Java.
For the moment I am trying to develop multiplatform apps with Qt but it's quite complex to use native functions from Java, that's why I'm looking for an easier way to do it.

I don't think you can find this kind of framework.
If you meet some problems to use native functions with Qt I recommend you to watch BogDan Vatra videos and pdf
https://www.qtdeveloperdays.com/sites/default/files/BogdanVatra_Extending_Qt_Android_Apps_with_JNI.pdf
Besides, you should look at the QtAndroidNamespace class and runOnAndroidThread function.
Edit : You can find the videos in the Tutorials part of QtCreator

I am not sure about the Java but using javascript,we have alot of Frameworks like Titanium, Sencha Touch and PhoneGap etc.In this you can simply use the html,js,and css3 bundle to creates the IOS Android and as well as web.

I would strongly advise against reusing the UI on different mobile operating systems - it will always look as if it doesn't belong on one of those systems. Creating an Android UI isn't that complicated, you should try it.
That said, you could have just one big WebView on your native UI, displaying your HTML UI. You might need a web server in your app, serving the UI, though - I'm not sure.

Exporting my android app to iphone?

Ive made an app on Android and I want to be able to sell the app for iphone and android. I dont want to have to learn objective C. Is there any possible way I can do this?

You have a lot of options but none of them are very nice. I know of no direct converter.
To reuse the majority of your code checkout RoboVM
http://www.robovm.com/
It is used by libgdx to allow for using Java to do cross platform development.
Pretty much all your other options as far as I have seen would involve using a proprietary language or recreating your app as HTML5.
In the end you will have to write some iOS code. At least now/soon you can use Swift instead of Objective-C =)

How to make Android App from existing JAva Code?

I have a multiplayer Chess program and I wanted to know if there is an easy way to make an app for it. If I worth with Android SDKs on Eclipse, do I have to code the whole thing again or I can just use the Java code for making the app?

It depends on what libraries you're using in your game and their support in Android.
If you use Java for the existing implementation, you'll hopefully, have to write the UI bits, and the erst of the logic should simply work.
If this is a desktop app you're trying to port, you may have performance issues (given you're running on a mobile device now), so there would be some changes necessary.
If you're using any graphics libraries, check their support on Android. Some libraries have a reputation of having issues on Android.

I solved it myself. You just need to insert your JS files or you could use the Url of your uploaded app.

Can I write an Android app in Java and convert it later?

I've got a lot of experience in Java but none developing mobile apps. I'd like to write an application using Java/Swing and then convert it for use on an Android phone.
Is this feasible or do I really need to develop from the ground up for the Android platform?
I don't own an Android phone as I can't afford one at the moment, and the Android emulator is so slow that I find myself wasting a lot of time sitting around waiting.

Alot of the code from Java is the same in Android but I would say don't. Android uses its own UI system and it would be hard to transfer from swing. Also when you get deeper into the Layouts the Android system has the application would be completely screwed up. Not to mention the whole Intent, Receiver, Content Provider aspects would not transfer properly. Then you need to think about memory. Simple answer in my opinion... no.

With Intent/Activities, in many ways Android applications need to be designed like web apps with pages connected by links. It's quite a different paradigm, so until you know it well I would develop straight onto Android.

If you use the control-model-view paradigm so that your application logic is separated from your display logic, then you should be able to port your application logic from ordinary Java to Android with few or no changes. As for the display logic, that will have to be rewritten using the Android SDK.

If you consider, J2ME may not support fullest functionality of swings. So, it breaks on the first step. Secondly, Android native development method uses XML for its presentation layer and java for Application and Business layers. If you can write the extending methods for android tags, then you may achieve your target at the basic level.
Rather I suggest use the phonegap and develop everything in HTML5, DOJO and CSS3. This would work on any mobile, any platform.