public static Validator<CharSequence> number(@NonNull final Context context) {
        return new NumberValidator(context, R.string.default_error_message);
    }
public static Validator < CharSequence > number ( @ NonNull final Context context ) { return new NumberValidator ( context , R . string . default_error_message ) ; }
Creates and returns a validator, which allows to validate texts to ensure, that they only
contain numbers. Empty texts are also accepted.

@param context
The context, which should be used to retrieve the error message, as an instance of
the class {@link Context}. The context may not be null
@return The validator, which has been created, as an instance of the type {@link Validator}