How to Create a Custom Event in Java Using the Observer Pattern?

Create a Custom Event in Java: Leveraging the Observer Pattern

When developing Java applications, it's often necessary to implement communication between different objects in a reliable and flexible manner. Events and listeners provide a robust solution for this purpose.

Consider the following scenario:

When event "object 1 say 'hello'" happens, then object 2 responds to that event by saying "hello".

To create such an event-driven interaction, the Observer Pattern comes in handy. The Observer Pattern defines a one-to-many relationship between objects, where an "observable" (initiator) notifies all its registered "observers" (listeners or responders) when an "event" occurs.

To implement this pattern, follow these steps:

1. Define the Event: Create an interface that represents the event. In this case, it could be:

   interface HelloEvent {
    void someoneSaidHello();
   }

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2. Create the Observable: This class will manage the event subscriptions and notify observers when the event occurs. It can be named "Initiater":

   class Initiater {
    private List<HelloEventListener> listeners = new ArrayList<>();
    public void addListener(HelloEventListener toAdd) { listeners.add(toAdd); }
    public void sayHello() { System.out.println("Hello!!"); for (HelloEventListener hl: listeners) hl.someoneSaidHello(); }
   }

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3. Create the Observer: This class implements the event interface and defines the behavior that should be triggered when the event occurs. Here's a sample responder:

   class Responder implements HelloEventListener {
    @Override public void someoneSaidHello() { System.out.println("Hello there..."); }
   }

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4. Register Observers: Connect the observers to the observable using the "addListener" method:

   Initiater initiater = new Initiater();
   Responder responder = new Responder();
   initiater.addListener(responder);

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5. Trigger the Event: The observable triggers the event by calling a specific method (e.g., "sayHello"):

   initiater.sayHello(); // Output: "Hello!!!" "Hello there..."

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By following these steps, you'll have successfully created a custom event mechanism in Java using the Observer Pattern, enabling objects to communicate and react to specific events in a decoupled and extensible manner.

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