Private methods in interfaces in Java 9

Java SE 9 is slowly moving towards the finishing line. One new feature is private methods on interfaces.

Private methods on interfaces in Java 9

In Java 7 and all earlier versions, interfaces were simple. They could only contain public abstract methods.

Java 8 changed this. From Java 8, you can have public static methods and public default methods.

public interface HolidayCalendar {
  // static method, to get the calendar by identifier
  public static HolidayCalendar of(String id) {
    return Util.holidayCalendar(id);
  }
  
  // abstract method, to find if the date is a holiday
  public abstract boolean isHoliday(LocalDate date);
  
  // default method, using isHoliday()
  public default boolean isBusinessDay(LocalDate date) {
    return !isHoliday(date);
  }
}

Note that I have chosen to use the full declaration, with "public" on all three methods even though it is not required. I have argued that this is best practice for Java SE 8, because it makes the code clearer (now there are three types of method) and prepares for a time when there will be non-public methods.

And that time is very soon, as Java 9 is adding private methods on interfaces.

public interface HolidayCalendar {
  // example of a private interface method
  private void validateDate(LocalDate date) {
    if (date.isBefore(LocalDate.of(1970, 1, 1))) {
   throw new IllegalArgumentException();
 }
  }
}

Thus, methods can be public or private (with the default being public if not specified). Private methods can be static or instance. In both cases, the private method is not inherited by sub-interfaces or implementations. The valid combinations of modifiers in Java 9 will be as follow:

• public static - supported
• public abstract - supported
• public default - supported
• private static - supported
• private abstract - compile error
• private default - compile error
• private - supported

Private methods on interfaces will be very useful in rounding out the functionality added in Java 8.