Encapsulation

COMP122 Lectures 2021-03-02

Public and Private Content

Attributes and methods can be public or private:

• private members of a class cannot be accessed from outside the object.
• public can be accessed from outside.
  • This can be done via the dot notation:

    int t = alarmClock.time;
    alarmClock.setAlarm(t+5);
  

  The public members form the object’s interface. This consists of the messages it can receive and the services it can offer.

Information Hiding

It is good practice to declare attributes private to avoid accidental outside interference or messy dependencies.

This principle is called information hiding or encapsulation.

Setting Private Attributes via Methods

You can introduce public methods whose sole job it is to get or set the value of private attributes.

• Methods that modify attributes are called mutators.
• Methods that retrieve the value of an attribute are called accessors.

It is convention to call them getX or setX for private attributes X.

Alarm Clock Example

classDiagram
class AlarmClock{
	-time int
	-alarm int
	+getTime()
    +setAlarm(int)
}

- means private and + means public.

AlarmClock a  new AlarmClock();
int t = a.getTime();
a.setAlarm(t + 5);