How can I get the distance between two point by latlng?

LatLng startLatLng = new LatLng(startLatitude, startLongitude);
LatLng endLatLng = new LatLng(endLatitude, endLongitude)
double distance = SphericalUtil.computeDistanceBetween(startLatLng, endLatLng);

also add

compile 'com.google.android.gms:play-services-maps:15.0.1'
compile 'com.google.maps.android:android-maps-utils:0.5+'

to your gradle dependencies

Distance in meters

There is an api provided by android itself to get distance between two points. Use:

Location.distanceBetween(
    startLatitude,
    startLongitude,
    endLatitude,
    endLongitude,
    results);

which returns distance in meters.

Try below code.

public float distance (float lat_a, float lng_a, float lat_b, float lng_b ) 
{
    double earthRadius = 3958.75;
    double latDiff = Math.toRadians(lat_b-lat_a);
    double lngDiff = Math.toRadians(lng_b-lng_a);
    double a = Math.sin(latDiff /2) * Math.sin(latDiff /2) +
    Math.cos(Math.toRadians(lat_a)) * Math.cos(Math.toRadians(lat_b)) *
    Math.sin(lngDiff /2) * Math.sin(lngDiff /2);
    double c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));
    double distance = earthRadius * c;

    int meterConversion = 1609;

    return new Float(distance * meterConversion).floatValue();
}

If you didn't understood, please check this link, same code available here.