Android map v2 zoom to show all the markers

You should use the CameraUpdate class to do (probably) all programmatic map movements.

To do this, first calculate the bounds of all the markers like so:

LatLngBounds.Builder builder = new LatLngBounds.Builder();
for (Marker marker : markers) {
    builder.include(marker.getPosition());
}
LatLngBounds bounds = builder.build();

Then obtain a movement description object by using the factory: CameraUpdateFactory:

int padding = 0; // offset from edges of the map in pixels
CameraUpdate cu = CameraUpdateFactory.newLatLngBounds(bounds, padding);

Finally move the map:

googleMap.moveCamera(cu);

Or if you want an animation:

googleMap.animateCamera(cu);

That's all :)

Clarification 1

Almost all movement methods require the Map object to have passed the layout process. You can wait for this to happen using the addOnGlobalLayoutListener construct. Details can be found in comments to this answer and remaining answers. You can also find a complete code for setting map extent using addOnGlobalLayoutListener here.

Clarification 2

One comment notes that using this method for only one marker results in map zoom set to a "bizarre" zoom level (which I believe to be maximum zoom level available for given location). I think this is expected because:

1. The LatLngBounds bounds instance will have northeast property equal to southwest, meaning that the portion of area of the earth covered by this bounds is exactly zero. (This is logical since a single marker has no area.)
2. By passing bounds to CameraUpdateFactory.newLatLngBounds you essentially request a calculation of such a zoom level that bounds (having zero area) will cover the whole map view.
3. You can actually perform this calculation on a piece of paper. The theoretical zoom level that is the answer is +∞ (positive infinity). In practice the Map object doesn't support this value so it is clamped to a more reasonable maximum level allowed for given location.

Another way to put it: how can Map object know what zoom level should it choose for a single location? Maybe the optimal value should be 20 (if it represents a specific address). Or maybe 11 (if it represents a town). Or maybe 6 (if it represents a country). API isn't that smart and the decision is up to you.

So, you should simply check if markers has only one location and if so, use one of:

• CameraUpdate cu = CameraUpdateFactory.newLatLng(marker.getPosition()) - go to marker position, leave current zoom level intact.
• CameraUpdate cu = CameraUpdateFactory.newLatLngZoom(marker.getPosition(), 12F) - go to marker position, set zoom level to arbitrarily chosen value 12.

Google Map V2

The following solution works for Android Marshmallow 6 (API 23, API 24, API 25, API 26, API 27, API 28). It also works in Xamarin.

LatLngBounds.Builder builder = new LatLngBounds.Builder();

//the include method will calculate the min and max bound.
builder.include(marker1.getPosition());
builder.include(marker2.getPosition());
builder.include(marker3.getPosition());
builder.include(marker4.getPosition());

LatLngBounds bounds = builder.build();

int width = getResources().getDisplayMetrics().widthPixels;
int height = getResources().getDisplayMetrics().heightPixels;
int padding = (int) (width * 0.10); // offset from edges of the map 10% of screen

CameraUpdate cu = CameraUpdateFactory.newLatLngBounds(bounds, width, height, padding);

mMap.animateCamera(cu);

I couldnt use the onGlobalLayoutlistener, so here is another solution to prevent the "Map size can't be 0. Most likely, layout has not yet occured for the map view. Either wait until layout has occurred or use newLatLngBounds(LatLngBounds, int, int, int) which allows you to specify the map's dimensions." error:

mMap.setOnMapLoadedCallback(new GoogleMap.OnMapLoadedCallback() { 
@Override 
public void onMapLoaded() { 
    mMap.moveCamera(CameraUpdateFactory.newLatLngBounds(builder.build(), 15));
 } 
});

So

I needed to use addOnGlobalLayoutListener to get the appropriate sample

for example, your Google Map is inside RelativeLayout:

RelativeLayout mapLayout = (RelativeLayout)findViewById(R.id.map_layout);
mapLayout.getViewTreeObserver().addOnGlobalLayoutListener(new OnGlobalLayoutListener() {
        @Override
        public void onGlobalLayout() {
            //and write code, which you can see in answer above
        }
    });