Use DatePicker spinners in API 24

gistfile1.java

/**
 * Workaround for this bug: https://code.google.com/p/android/issues/detail?id=222208
 * In Android 7.0 Nougat, spinner mode for the DatePicker in DatePickerDialog is
 * incorrectly displayed as calendar, even when the theme specifies otherwise.
 *
 * Modified slightly from the equivalent fix for TimePicker from @jeffdgr8: 
 *     https://gist.github.com/jeffdgr8/6bc5f990bf0c13a7334ce385d482af9f
 */
private void fixSpinner(Context context, int year, int month, int dayOfMonth) {
    // The spinner vs not distinction probably started in lollipop but applying this
    // for versions < nougat leads to a crash trying to get DatePickerSpinnerDelegate
    if (Build.VERSION.SDK_INT == Build.VERSION_CODES.N) {
        try {
            // Get the theme's android:datePickerMode
            final int MODE_SPINNER = 1;
            Class<?> styleableClass = Class.forName("com.android.internal.R$styleable");
            Field datePickerStyleableField = styleableClass.getField("DatePicker");
            int[] datePickerStyleable = (int[]) datePickerStyleableField.get(null);
            final TypedArray a = context.obtainStyledAttributes(null, datePickerStyleable,
                    android.R.attr.datePickerStyle, 0);
            Field datePickerModeStyleableField = styleableClass.getField("DatePicker_datePickerMode");
            int datePickerModeStyleable = datePickerModeStyleableField.getInt(null);
            final int mode = a.getInt(datePickerModeStyleable, MODE_SPINNER);
            a.recycle();

            if (mode == MODE_SPINNER) {
                DatePicker datePicker = (DatePicker) findField(DatePickerDialog.class,
                        DatePicker.class, "mDatePicker").get(this);
                Class<?> delegateClass = Class.forName("android.widget.DatePicker$DatePickerDelegate");
                Field delegateField = findField(DatePicker.class, delegateClass, "mDelegate");
                Object delegate = delegateField.get(datePicker);

                Class<?> spinnerDelegateClass = Class.forName("android.widget.DatePickerSpinnerDelegate");

                // In 7.0 Nougat for some reason the datePickerMode is ignored and the
                // delegate is DatePickerCalendarDelegate
                if (delegate.getClass() != spinnerDelegateClass) {
                    delegateField.set(datePicker, null); // throw out the DatePickerCalendarDelegate!
                    datePicker.removeAllViews(); // remove the DatePickerCalendarDelegate views

                    Constructor spinnerDelegateConstructor = spinnerDelegateClass
                            .getDeclaredConstructor(DatePicker.class, Context.class,
                                    AttributeSet.class, int.class, int.class);
                    spinnerDelegateConstructor.setAccessible(true);

                    // Instantiate a DatePickerSpinnerDelegate
                    delegate = spinnerDelegateConstructor.newInstance(datePicker, context,
                            null, android.R.attr.datePickerStyle, 0);

                    // set the DatePicker.mDelegate to the spinner delegate
                    delegateField.set(datePicker, delegate);

                    // Set up the DatePicker again, with the DatePickerSpinnerDelegate
                    datePicker.updateDate(year, month, dayOfMonth);
                }
            }
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
}

private Field findField(Class objectClass, Class fieldClass, String expectedName) {
    try {
        Field field = objectClass.getDeclaredField(expectedName);
        field.setAccessible(true);
        return field;
    } catch (NoSuchFieldException e) {
        Timber.i(e); // ignore
    }

    // search for it if it wasn't found under the expected ivar name
    for (Field searchField : objectClass.getDeclaredFields()) {
        if (searchField.getType() == fieldClass) {
            searchField.setAccessible(true);
            return searchField;
        }
    }
    return null;
}