The Cross-Platform is, also, a good way: you develop your application using the Javascript language and deploy it on the other different platforms (Android, BlackBerry, etc...), there are many frameworks to do it (Titanium, PhoneGab.....) !
Android phones are the most popular, so u need Java Language but if u are an HTML 5 developper you can deploy and publish you work on different platforms like blackberry, windows phone,ios...