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Can iOS Apps be developed using Java?

I'm a web developer and am thinking of digging into mobile app development. The languages I know so far are PHP, Python, SQL, Js, HTML and CSS, all of them pretty well now. Currently I'm starting to dig into Java, as it's the language to be used for native Android App Development, as far as I've learned. Interestingly, I'm coming across posts (also in this forum) saying that Java can also be used to develop iOS - compatible Apps, while other sources comment that Apple is completely against cross-devs, and that you should develop iOS apps using Swift when it comes to the native choice and optimal UX. So I was wondering about the answer in this forum; can you develop iOS apps using Java or not?

Post is a few months old now, but I figure someone with a similar question will probably find this with web search, so this reply is for them. (Disclaimer: I wrote Java professionally for about five years, and I grew up when Java was supposed to be the future.)
If the question is, "Can you write Java code and run it on iOS?" then the answer is, "Technically yes, but you almost certainly don't want to."
It requires jumping through all kinds of hoops and sleight-of-hand to make it work. The project linked in a previous answer, Codename One, is a pretty heavyweight framework and the kind of thing you'd only want to use if you were heavily committed to a Java codebase already.
There are advantages to knowing Java in 2022, but cross-platform mobile development isn't one of them.
Some reasons why you might want to learn Java:
Developing on Android. Android now supports/recommends Kotlin for new projects, but Java was the sole first-class language on the platform for 10+ years, so it's advantageous to be able to read/write/reuse Java code and libraries.
Using long-standing big-data tools (spark, hadoop, neo4j, etc.). While this domain has increasingly moved to Python over the past 5-7 years, there's still a lot of production Java code that someone needs to maintain. [Edit 3/31/22: See also the Apache http server, whose name never sat right with me because as far as I know, the people who wrote it aren't Apache.]
Stylistic similarity to C-family languages. Java can be a more intuitive introduction to C-style languages than jumping right into, say C++. These days, though, I would recommend C# over Java since it's used in more domains.
Continuity with some JS dialects. React code in particular is full of Object Oriented(tm) design conventions that are very strongly influenced by Java practices, and those design choices in turn have influenced changes in the JS spec. So being familiar with Java conventions can help make sense of that stuff. It can also help you identify which of those practices are bad ideas so you can avoid them in JS. [See footnote 1.]
As far as reasons why you won't see widespread Java usage outside of those legacy domains, I'd say there are a lot of good reasons people have moved away from Java since 2006 or so:
It's incredibly verbose, and requires a lot of esoteric knowledge about implementation details to write code that runs efficiently. This is the kind of problem that languages like Scala and Kotlin have tried to solve, while retaining compatibility with the JVM platform for ease of migration.
The JVM platform itself introduces substantial overhead, both in resource usage and integration complexity.
Changes in approach and policy since Sun's acquisition by Oracle. This is related to licensing issues (see Oracle v Google), as well as complexity/feature creep, which has given rise to compatibility issues across codebases.
Emergence of viable alternative languages in the 2000's and 2010's.
Widespread adoption of virtualization technology has yielded alternate ways to solve the compatibility issues that Java was designed to address.
It's ironic, since the motto of Java used to be "write once, run everywhere", but that's the way things go.
Footnote 1. Actually, this is kind of a trick question, since most commonly accepted Java practices are bad ideas. This can be demonstrated by the fact that in 1995, object-oriented Java applications would run in a typical web browser, whereas 27 years later Java is considered too heavyweight for that purpose.
Postscript, 3/31/22: This is my first and likely only Stack Overflow post. If you want to read more like this, check out my github: https://github.com/ubuvoid

Yes you can.
One tool used for this is Codename One.
https://www.codenameone.com/
Also for more info on this subject, feel free to consult this thread:
How can one develop iPhone apps in Java?

Yes, this is possible. https://github.com/Anuken/Mindustry is an open-source RTS game written in Java. It has an iOS port that is compiled via an open-source fork of RoboVM.

IOS works on objective C and Android works in Java and Kotlin. I don't know how much some conversion tool can help you to achieve all the functionality. So in my opinion answer is No.
Hybrid Application development can help you. You have one source code that will run in both platform. You have to develop the app again.

First: Java is used in more Domains than C#. Needed information because C# has no way to get over with ONE UI to Desktop, Mobile and Web.
By the way, the recommended Solution is CodenameOne or - if you want to implement your App with JavaFX - use Gluonmobile which is using the GraalVM, an ahead of time compiler for IOS.
In both Cases you have access to Hardware of the Device for Android and IOS because there is an abstraction which helps you with implementations for both Platforms - Android And IOS. Write in this Case one App and compile for both worlds.

Multiplatform app development with individual guis

I am currently planning the development of a multiplatform app, and I am not sure wich way to do it fits my requirements best, since all possibilties I could think of failed to satisfy me.
First I want to write an app for android, which should feel like a normal typical android app. So I want to use the standard actionbar and android design look and feel in my other gui elements. After finishing the android app I am planning on developing an ios app, which should have a different design, so i am going to redevelop the gui for this anyway.
But I don´t want to rewrite the other code wich represents the intelligence of the app, independent from the gui. I came up with the following possibilities:
1. Java GUI With native library
Here I abstract all the code of my app in a C++ library (since as far as I know ios supports the usage of c++ libraries too) and develop the gui android typically in java. The library would than have a function to start and would inform the gui about every change via callback functions.
Pro
I can reuse all the code that would be the same on both platforms. I just would implement the gui seperately
The design of the android gui is straightforward as I want it to be. It looks like typical android because it is typical android.
Cons
I dislike the usage of JNI very much. Especially the signature and names of the callback functions (calling java functions from c++) are not checked at compile time and require a lot of manual work. If I rename a function and forget to rework the native part I only notice this mistake at runtime.
2. Build the GUI on the native side
Here I had difficulties finding out what is possible, especially for 2.2
2.1 Use Qt
I have only a few first step experiences with Qt in general but as far as I understood i would have the following pros and cons:
Pro
Reuse most parts of the code for ios and Android. I would than redesign the gui for each platform to make them feel natural. I can´t evaluate how much qt may even assists me at doing that
Cons
I have to copy the android gui by using other qt widgets. This is more effort and I don´t know if one can replicate the android gui elements (like the actionbar) so that the user wouldn´t notice it.
2.2 Using the android framework from the native side
I dont know if this is possible at all, I wasn´t able to find this information. Can I use the class "NativeActivity" and use the android framework to build the gui and use e.g. the actionbar? If this is possible somehow it would have the pros from 1. and maybe wouldn´t have it cons?
Do you have any feedback to my ideas or maybe even new approaches I didn´t think of? How do other multiplatform apps like WhatsApp solve this problem? Do they have redundant code for each platform?
Thank you,
Tobi

I would say that it depends very much on your application requirements. By my opinion, a better solution is to develop a separate application for each platform using recommended SDK's for that platform, and implement in native C++ only the time consuming data processing algorithms.
Application runtime on mobile platforms is not so straightforward as on desktop platforms. You should take into account background and foreground processing, specific application life cycling, accessing system resources such as network, file system, etc. And all these issues do differ on iOS and Android.
Regarding possibilities that you listed.
Qt/QML is ok only in case two requirements are met:
1.1 Your application is a foregroud application without any background operations.
1.2 You purchase a commercial Qt license because only commercial Qt can be submitted to Apple iTunes app store (even GPL apps are under
question).
Using NDK Native Activities on Android with cross-platform C/C++ backend. Android NDK API offers much less API then Android Java SDK, so a lot of things you will have to implement or wrap manually. It is a hard road.
Mixing Java code and C/C++ using JNI gives you more of Android SDK API. But you should remember that an Android activity's life cycle is not somewhat that you're used to deal with when developing on C/C++.
Approach that we are using
We've been developing an application with a huge amount of cross platform functionality that should work on Windows/Linux/Mac OSx/Android/iOS. We're using the approach as follows.
Cross-platform core is written in pure C++.
We have adaptors to GUI interface for each platform.
On desktop we use Qt as it reaches all desktop platforms with minor adapting to each platform.
On iOS the GUI is built using iOS SDK with Objective-C and C++ core is linked as a Framework. Still, we had to patch our core in some way for iOS background requirements and so.
On Android we wrap our C++ core in a background process and build all the GUI using only Android Java SDK. Foreground GUI activities interact with the core via local sockets, so we don't need to bother with activities life cycling in our C++ core. But the adaptor is a litle bit complicated.
Nevertheless, both mobile platforms often require workarounds and adaptations in C++ core which add a number of #ifdef'ed branches in code for each platform.

What should i select for my mobile applciation development? Native way or cordova or react,Node.js like things

I want to develop application in android and iOS both.
But i am confuse between native development and development using react and node.js etc.
Please help me to choose (If possible specify reason).
P.S Here cost is not an issue.

If you need high performance and attractive UI and don't want to be blocked by some functionalities that may not br provided by cross platform, I recommend you to use native platform. But if your app doesn't need to be very attractive and doesn't need too much processing, then you can use cross-platform. Actually for many features you need to add plugins if you are building cross platform app and adding number of plugins makes your application slower.

OK, well there isn't much data to go on, but there are several options if you want to write cross platform apps.
There is:
Sencha Touch - HTML and JavaScript
PhoneGap - HTML, JavaScript and CSS
QT - C++ and QML
Appmethod - C++ and Object Pascal
Xamarin - C#
Visual Studio - looks like you can do C# or C++, or Unity for games.
and probably a lot more...
Why would you choose one of these instead of writing in the native language for the platform?
Well, the pros seem to outweigh the cons. If you are targeting several platforms, I would choose one of the options above, simply because you only have to write everything once. If you did it native for each platform, you would write everything twice or maybe three or even four times. Using one of the above tools will be much easier and save you time, and they might be just as seamless and workable as native development.
To simply answer your question, If you would like to be able to write your app once, and be able to manage all future updates by managing only one code base, do cross platform using one of the above tools. The only downside is that you might have to learn a new language (but with a place like Stack Overflow, learning new languages is pretty awesome). It will save you time in the long run.

Android Cross-Platforming

I want to know more about Cross-Platform. I'm currently working on my thesis and decided to make an Android Application but not all target users uses Android device. So, We've come up with an idea to Cross-Platform the android application.
I have read the same topic but it didn't get to me too well. iOS / Android cross platform development. I want to know if there is another way than using the frameworks discussed in the link.
I want know what is the best/shortest way to cross platform a Android Application?

Cross Platform tools are in my opinion not a good idea at all. Android is Java, and iOS is Objective C. Android can use NDK to use a C/C++ library so theoretically if you did most of your code in C/C+ it could be used on both sides. I think two natives is more in order. Study NDK to see what could run on both devices. Lets say you built your app as a C++ Library except for UI interactions. Then in theory the library should work on both sides. C/C++ a lot easier on iOS than android however.
Also it really depends on the dependencies your app will have. Lets say you want to use a library but its only available in Java or C++ how does this effect your decision.
You might also consider designing your app so that it just captures data on the devices and the actual processing of the data takes place in a web service.

Another way is use Mobile Web HTML5 Framework like Phonegapp, Sencha touch and more.
Although web technologies don’t perform as well as native, but it is a useful thing that you can try.
See Choosing Mobile Web HTML5 Framework and http://operationproject.com/2011/adventures-in-html5-part-one/

Android application are apk files that are basically zip file that contains dex/odex files and all other resources that you may wish to add.But I think you can use any coding language to develop as long as you have IDE that converts your code to apk and dex .And most important of all your programming language must support corresponding api or must have something similar to cross compile.The reason Google used Java for Android was they felt it is convenient to do it(read it during case Google v/s Oracle)
Personally I feel cross-platform is good since it increases the developers base and understanding of the System (here Android).And I think the same analogy would go with other Systems as well.

Sharing code between iOS & Android projects

I've been hired to develop a mobile framework for a webservice created by my employer. Ideally management would like to have some reusable components that can be shared across mobile platforms (initially iOS & Android, probably Windows Phone 7 at some later point).
I've been wondering how feasible this is. One of the requirements is a native interface, so we would use Cocoa Touch on iOS & whatever (Java-based) toolset is used to create a native UI for Android. The application will interact with webservices, mainly the ones that we've been creating internally. The webservices have been developed in .NET.
As far as reusable components go, I guess we could use some C++ code to make webservice calls and perhaps even more of the backend of an application, yet I wonder if this would be a good approach. Apple's Foundation Framework has some excellent capabilities build-in to access webservices, not to mention other open-source libraries, e.g. ASIHttpRequest, SBJSON, etc... I guess the same would be true for Android (though I have no real Android experience for now). Also, when looking at projects done by companies like Google, Twitter & Facebook, each of these companies offers native libraries built for the major mobile OS platforms. If the big companies take this approach, it seems logical for us to follow suit.
Perhaps we should focus more on a general architecture that we should offer across platforms instead of an implementation that can be shared.
Would anyone advise us to make use of C/C++ to develop such a framework (shared library) for the mobile site of our webservices? If so, why?

It really depends on what level of customized code you want to have. I worked at a game company previously, and we made two games for both iPhone and Android, and at least 90% of the codebase was shared between the two platforms in c++ code. Sometimes it is significantly easier to implement elements with specialized third-party libraries, like for Facebook and the like, but maintaining that code means continually doing it for both platforms. That was one of the reasons why we even implemented our own UI objects in c++ for our games. Because even though the initial setup would have been easier to do with Interface Builder and Android's XMLs, the maintenance and tweaking necessary ended up being significantly less because we went with a shared codebase.
In short, I would highly recommend writing any shared customized code in C/C++, and things that are significantly easier in their native codebases (java for android, or obj-c for iphone) and you don't expect to change much, to keep separate.

Depending on which WS protocol you use it may be more or less hard to do, but anyway I see little advantage in doing that in C/C++ for a mobile platform.
If you were using SOAP, I'd never consider C/C++, even for a desktop app. I already used SOAP libraries for C++ and they are a lot harder to use than their Java/.net counterparts, and the way they are implemented (mapping SOAP objects to C structs) is very prone to crashes if the format changes. Not to mention that you have to recompile your client when the WSDL changes.
As I understood in your case you plan to use REST. I never found a good REST library for C/C++, but recently I did a desktop project in which I implemented a C++ REST client using simply WinHTTP (in Windows) and libCURL (in Linux). Of course, they provide just the HTTP part, so I had to add cppdom for XML parsing. If you use JSON, there are many good libs like jsoncpp, libjson.
I'd say to you that even in a desktop environment it was harder to do than would be in .net or java, and was only done this way because it was part of a larger application already written in C/C++.
Anyway, you'll have more work and not much advantage since all those modern mobile platforms provide rich libraries that do the same thing, and probably the user of your API will develop in the platform's main language, so you'll have the double extra work of implementing the WS access code AND the binding code. As I assume all (or at least most) of your logic is in your server, not the client, there's not much common code between the platforms to justify using C/C++.