When it comes to mobile devices, Android and iOS are the current dominant platforms.  There are other ones, but the ones that IMO have a bright future are the following trio:

– Tizen : Open Source Linux based platform, sponsored mainly by Samsung. Already running in Samsung smart watches.

– Firefox OS : Open Source Linux based platform, sponsored mainly by the Mozilla Foundation. Already running in many low end smart phones sold the world over. I have the latest ZTE one.

– Ubuntu Mobile : Open Source Linux based platform, sponsored mainly by Canonical. Promising, I have tested it on my old Nexus 4 and Nexus 7 devices.

Overall I see Firefox OS as the best mass market alternative because the low resources it needs and the focus on applications coded in HTML5 and JavaScript.
IMO, in the long run for mobile devices,  Java (or Java like) is a no-go, and .NET and objective C are not any better either.

On occasion, I still miss the simplicity of my long gone Palm Pilot III, and the elegance of my Palm Pilot V.
The Palm Treo 650 was my first Smart Phone, and probably the most usable and well integrated that I have ever used.

Now, a “Palm Treo 650”-like smart phone with Firefox OS, that will be my dream phone!