<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"></uses-permission>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"></uses-permission>

import android.Manifest;
import android.app.Activity;
import android.content.pm.PackageManager;
import android.support.v4.app.ActivityCompat;

public class PermisionUtils {

 // Storage Permissions
 private static final int REQUEST_EXTERNAL_STORAGE = 1;
 private static String[] PERMISSIONS_STORAGE = {
  Manifest.permission.READ_EXTERNAL_STORAGE,
  Manifest.permission.WRITE_EXTERNAL_STORAGE};

 /**
 * Checks if the app has permission to write to device storage
 * If the app does not has permission then the user will be prompted to
 * grant permissions
 *
 * @param activity
 */
 public static void verifyStoragePermissions(Activity activity) {
 // Check if we have write permission
 int permission = ActivityCompat.checkSelfPermission(activity,
  Manifest.permission.WRITE_EXTERNAL_STORAGE);

 if (permission != PackageManager.PERMISSION_GRANTED) {
  // We don't have permission so prompt the user
  ActivityCompat.requestPermissions(activity, PERMISSIONS_STORAGE,
   REQUEST_EXTERNAL_STORAGE);
 }
 }
}

verifyStoragePermissions(this);