How to Create Custom Listeners in Java for Event Handling

What are the steps to create custom listeners in Java?

public interface CustomListener {
    void onEvent(String eventDetails);
}

public class EventSource {
    private List<CustomListener> listeners = new ArrayList<>();

    public void addListener(CustomListener listener) {
        listeners.add(listener);
    }
    
    public void triggerEvent(String eventDetails) {
        for (CustomListener listener : listeners) {
            listener.onEvent(eventDetails);
        }
    }
}

In Java, custom listeners are interfaces that define methods to be invoked when specific events occur. Creating custom listeners allows for a cleaner and more modular way to handle events, adhering to the observer design pattern.

// Define the custom listener interface.
public interface MyClickListener {
    void onClick(String buttonId);
}

// Implementing the listener in a class.
public class MyButton {
    private MyClickListener listener;

    public void setMyClickListener(MyClickListener listener) {
        this.listener = listener;
    }

    public void click(String buttonId) {
        if (listener != null) {
            listener.onClick(buttonId);
        }
    }
}

Causes

• When an application needs to respond to specific user actions or system events.
• To decouple the event source from the event handler, promoting better code organization.

Solutions

• Define an interface for your custom listener.
• Implement the listener in a class that will handle the events.
• Set up methods to register and trigger these listeners.

Mistake: Not implementing the listener interface correctly.

Solution: Ensure the class implementing the listener provides implementations for all methods defined in the interface.

Mistake: Forgetting to register the listener before triggering events.

Solution: Always register your listener before the event is triggered, or implement checks to ensure listeners are present.