public/subjects/java/piscine/Observer/README.md

Observer

Instructions

Now let's implement the Observer Design Pattern

classDiagram
class ValuePublisher {   
    -List~NumericBaseObserver~ observers
    +updateState(int value)
    +subscribe(NumericBaseObserver observer)
    +unsubscribe(NumericBaseObserver observer)
}

class NumericBaseObserver {
    <<interface>>
    +updateState(int state)*
    +getEvents()* List~String~
}

class BinaryBaseObserver {
    -List~String~ events
    +updateState(int state)
    +getEvents() List<String>
}
class DecimalBaseObserver {
    -List~String~ events
    +updateState(int state)
    +getEvents() List<String>
}
class HexaBaseObserver {
    -List~String~ events
    +updateState(int state)
    +getEvents() List<String>
}

ValuePublisher o-- NumericBaseObserver
NumericBaseObserver <|.. BinaryBaseObserver
NumericBaseObserver <|.. DecimalBaseObserver
NumericBaseObserver <|.. HexaBaseObserver

Here is the matching class diagram. Create the matching classes in the matching files.

Here is the description :

• For ValuePublisher :
  • updateState calls the updateState method of all observers.
  • subscribe should add the observer to the list of observers.
  • unsubscribe should remove the observer from the list of observers.
• For BinaryBaseObserver :
  • updateState should add the binary String representation of its parameter to the list of events (e.g. for 13, should add "1101")
  • getEvents should return the list of events.
• For DecimalBaseObserver :
  • updateState should add the decimal String representation of its parameter to the list of events (e.g. for 13, should add "13")
  • getEvents should return the list of events.
• For HexaBaseObserver :
  • updateState should add the hexadecimal String representation of its parameter to the list of events (e.g. for 13, should add "d")
  • getEvents should return the list of events.

Usage

Here is a possible ExerciseRunner.java to test your function :

public class ExerciseRunner {

  public static void main(String[] args)  {
    ValuePublisher publisher = new ValuePublisher();

    BinaryBaseObserver binaryObserver = new BinaryBaseObserver();
    DecimalBaseObserver decimalObserver = new DecimalBaseObserver();
    HexaBaseObserver hexaObserver = new HexaBaseObserver();

    publisher.subscribe(binaryObserver);
    publisher.subscribe(decimalObserver);
    publisher.subscribe(hexaObserver);

    publisher.updateState(1254);
    publisher.updateState(45839);
    publisher.updateState(5984382);

    System.out.println("Binary " + binaryObserver.getEvents());
    System.out.println("Decimal " + decimalObserver.getEvents());
    System.out.println("Hexa " + hexaObserver.getEvents());
  }
}

and its output :

$ javac *.java -d build
$ java -cp build ExerciseRunner 
Binary [10011100110, 1011001100001111, 10110110101000001111110]
Decimal [1254, 45839, 5984382]
Hexa [4e6, b30f, 5b507e]
$

Notions

Class diagram