Android Functions
ready to use

public static String getRandomString(int length) {
  final String s = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
  SecureRandom sr = new SecureRandom();
  StringBuilder sb = new StringBuilder(length);
  for (int i = 0; i < length; i++) {
    sb.append(s.charAt(sr.nextInt(s.length())));
  }
  return sb.toString();
}

public static void moveFile(File src, File dst) throws IOException {
  if (dst.exists()) {
    dst.delete();
  }
  InputStream input = new FileInputStream(src);
  OutputStream output = new FileOutputStream(dst);

  byte[] buffer = new byte[1024];
  int bytesRead;
  while ((bytesRead = input.read(buffer)) != -1) {
    output.write(buffer, 0, bytesRead);
  }
  src.delete();
}

public static void copyInputStreamToFile(InputStream src, File dst) throws IOException {
  OutputStream output = new FileOutputStream(dst);

  byte[] buffer = new byte[1024];
  int bytesRead;
  while ((bytesRead = src.read(buffer)) != -1) {
    output.write(buffer, 0, bytesRead);
  }
}

@NonNull
public static String readTextFileFromAsset(Context c, String path) {
  StringBuilder s = new StringBuilder();
  InputStream fIn = null;
  InputStreamReader isr = null;
  BufferedReader input = null;
  try {
    fIn = c.getResources().getAssets().open(path);
    isr = new InputStreamReader(fIn);
    input = new BufferedReader(isr);
    String line;
    while ((line = input.readLine()) != null) {
      s.append(line);
    }
  } catch (Exception e) {
    e.getMessage();
  } finally {
    try {
      if (isr != null)
        isr.close();
      if (fIn != null)
        fIn.close();
      if (input != null)
        input.close();
    } catch (Exception e2) {
      e2.getMessage();
    }
  }
  return s.toString();
}

public static void openDialer(Context c, String number)    {
  Intent i = new Intent(Intent.ACTION_DIAL);
  i.setData(Uri.parse("tel:" + number));
  c.startActivity(i);
}

public static void copyFile(File src, File dst) throws IOException {
  InputStream input = new FileInputStream(src);
  OutputStream output = new FileOutputStream(dst);

  byte[] buffer = new byte[1024];
  int bytesRead;
  while ((bytesRead = input.read(buffer)) != -1) {
    output.write(buffer, 0, bytesRead);
  }
}

public static float pixelTodp(Context c, float pixel) {
  float density = c.getResources().getDisplayMetrics().density;
  float dp = pixel / density;
  return dp;
}

public static float dpTopixel(Context c, float dp) {
  float density = c.getResources().getDisplayMetrics().density;
  float pixel = dp * density;
  return pixel;
}

public static Bitmap rotateBitmap(Bitmap src, float rotationDegree) {
        Matrix matrix = new Matrix();
        matrix.setRotate(rotationDegree);
        return Bitmap.createBitmap(src, 0, 0, src.getWidth(), src.getHeight(), matrix, true);
}

public static Bitmap flipBitmap(Bitmap src, int type/*0=horizontal, 1=vertical*/) {
    Matrix matrix = new Matrix();
    if(type == 0) {
        matrix.preScale(-1.0f, 1.0f);
    }
    else if(type == 1) {
        matrix.preScale(1.0f, -1.0f);
    }
    else    {
        return null;
    }
    return Bitmap.createBitmap(src, 0, 0, src.getWidth(), src.getHeight(), matrix, true);
}

public static boolean isValidEmail(CharSequence target) {
    if (TextUtils.isEmpty(target)) {
        return false;
    } else {
        return android.util.Patterns.EMAIL_ADDRESS.matcher(target).matches();
    }
}

public static boolean isNetworkAvailable(Context c) {
    ConnectivityManager connectivityManager = (ConnectivityManager) c.getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo();
    return activeNetworkInfo != null && activeNetworkInfo.isConnected();
}