Google opens the door for Android developers writing in Java to port apps to the iPhone with their open-source j2ObjC (Java to Objective-C) tool.
Developers make apps and apps sell phones, so this move by Google should endear them even more strongly to Android developers. Their new J2ObjC software tool “converts Java classes to Objective-C classes that directly use the iOS Foundation Framework." (Googles announcement). The Register reminds developers the tool is not a cross-platform miracle.
The open-source command-line tool from Google will help Android developers port their code to the iPhone, but won't do all the work. But it will keep more of their code in a single base that can be used for web-based apps, as well as Android and iOS. UI functions still need to be in Objective-C.
This sort of presumes that Android developers would develop for that platform first. Why? Android users aren't known for buying apps. There's an enormous amount of piracy on that platform.
iOS users, on the other hand, buy a lot of apps. Developers know that they can make money on Apple's platforms, so it makes more sense financially to develop for that platform first then perhaps do Android versions later if they have the time and resources.
Developers interested in iOS should start with the iOS Dev Center: