Observer Pattern

Question 1

Subject

Answer 1

The object being observed; owns the state and notifies observers when it changes.

Question 2

Observer

Answer 2

Dependent objects that are interested in changes to the subject’s state.

Question 3

Concrete Subject

Answer 3

Implementation of the subject interface; includes state and logic for managing observers.

Question 4

Concrete Observer

Answer 4

Implements the observer interface and defines how it reacts to changes in the subject.

Question 5

notifyObservers()

Answer 5

Called by the subject to update all registered observers.

Question 6

update()

Answer 6

Called by the subject on each observer to notify them of a state change.

Question 7

registerObserver(o)

Answer 7

Adds an observer to the subject’s list.

Question 8

removeObserver(o)

Answer 8

Removes an observer from the subject’s list.

Question 9

Observer Pattern

Answer 9

A behavioural design pattern where a subject (publisher) maintains a list of observers (subscribers) and notifies them automatically of any state changes.

Question 10

Observer Pattern Structure

Answer 10

Classes:
- Subject Interface
- Concrete Subject
- Observer Interface
- Concrete Observer

Question 11

Subject Interface (Observer Pattern Classes)

Answer 11

• Methods: registerObserver(), removeObserver(), notifyObservers()
• Maintains a list of observers.

Question 12

Concrete Subject (Observer Pattern Classes)

Answer 12

• Stores actual state (e.g. weather data or clock time).
• Implements methods from Subject interface.

Question 13

Observer Interface (Observer Pattern Classes)

Answer 13

Declares update() method.

Question 14

Concrete Observer (Observer Pattern Classes)

Answer 14

• Has its own state.
• Implements update() to stay consistent with the subject.