It is a question of the usability.
If you use zoom in / out buttons, you must think about the location of the buttons.
If you're using small buttons, it will be hard to tap a button or maybe you tap on the map itself and do something you won't to do.
It would be helpful to see your existing UI where you implement the map. If you already have a lot element, I wouldn't use buttons, because it will get hard to handle on smaller smartphones.
Especially when you develop for android devices, hold in mind that there are different screen sizes. Your app should be easy to handle on small screens and also on bigger screens. Here a quote of a Stackoverflow Answer about the minimum screen resolution:
According to the latest Compatibility Definition Document, the
definition for screen configurations require that:
Screens MUST be at least 2.5 inches in physical diagonal size Density
MUST be at least 100 dpi
The aspect ratio MUST be between 1.333 (4:3) and 1.779 (16:9)
The display technology used consists of square pixels
In practical terms, the lowest screen resolution currently being used
is QVGA (240x320). There are a couple of phones that use this
resolution including the HTC Tattoo and Wildfire. The smallest device
I'm aware of is the 2.55" QVGA Sony Ericsson X10 Mini.
And another question is, how important is the design of your app?
If you already have a UI around the map, you maybe could implement the buttons into the existing UI.
IMO, there are three possibilities:
Use the Buttons in your already existing UI
You implement the map in a self-made UI and implement the zoom buttons in it.
And please apologize my stunning paint-skills ;-)
Use the in-map zoom buttons by Google itself
The Google Map API provide Zoom In / Out Buttons by default, which just must be activated with
They look like the following screenshot:
Don't use any buttons
If you don't use any buttons, the screen looks cleaner. The most of the user know how to zoom wihtout buttons on google maps. If you want a clean screen or you already have many UI elements on the screen, don't implement the buttons.