midpoint between two latitude and longitude

You need to convert to radians. Change it to the following:

public static void midPoint(double lat1,double lon1,double lat2,double lon2){

    double dLon = Math.toRadians(lon2 - lon1);

    //convert to radians
    lat1 = Math.toRadians(lat1);
    lat2 = Math.toRadians(lat2);
    lon1 = Math.toRadians(lon1);

    double Bx = Math.cos(lat2) * Math.cos(dLon);
    double By = Math.cos(lat2) * Math.sin(dLon);
    double lat3 = Math.atan2(Math.sin(lat1) + Math.sin(lat2), Math.sqrt((Math.cos(lat1) + Bx) * (Math.cos(lat1) + Bx) + By * By));
    double lon3 = lon1 + Math.atan2(By, Math.cos(lat1) + Bx);

    //print out in degrees
    System.out.println(Math.toDegrees(lat3) + " " + Math.toDegrees(lon3));
}

Even easier with Android Google Maps Utilities:

LatLngBounds bounds = new LatLngBounds(start, dest);
bounds.getCenter();

Update: Better use the builder (for why see Bad Losers Answer):

LatLngBounds.builder().include(start).include(dest).build().getCenter();