Anyone with an Android gadget most likely knows about Google Play, as it's become the go-to source for Android apps. That said, I wanted to point out a pre-defined search on the Google Play website that allows users to view available apps for learning Japanese.
Japanese is one language for which there is no shortage of apps to discover, test and use. This is a good thing, but it can also be daunting. Apps with good reviews (and lots of them) are generally a good place to start.
Though there are many free Android apps for learning Japanese, some are more in-depth than others. This article cannot go into the specifics of each app, but Free Language will begin reviewing specific apps in due time.
For now, have a look at the available apps for Japanese and see if there's something in there that will work for you. Note that some are focused on teaching spoken Japanese, some on written Japanese, some on Japanese for travel and still others combining many aspects into a single app, including vocabulary, grammar, etc. Additionally, some of these apps are geared toward having fun while learning, almost like playing a game, while others take a more straightforward educational approach.
Be aware than while many of these Japanese Android Apps are free, some only provide you with a portion of the features and will require upgrading to a pay version.