Here is an example of a class extending AppenderTracker in Logback:

import ch.qos.logback.core.spi.AppenderAttachable;
import ch.qos.logback.core.spi.AppenderTracker;

public class MyAppenderTracker<E> extends AppenderTracker<E> {
    private AppenderAttachable<E> appenderAttachable;

    public MyAppenderTracker(AppenderAttachable<E> appenderAttachable) {
        this.appenderAttachable = appenderAttachable;
    }

    @Override
    public void add(String key, E appender) {
        // add logic
        super.add(key, appender);
    }

    @Override
    public void removeStaleComponents(long now) {
        // remove stale components logic
        super.removeStaleComponents(now);
    }

    @Override
    public E find(String key) {
        // find logic
        return super.find(key);
    }
}

This class is an example of a custom Appender Tracker in Logback, which is used to track appenders associated with a specific key. It extends AppenderTracker, a class that provides an efficient way to track appenders which are associated with a key. The AppenderAttachable is passed in the constructor and it is used to attach the appenders when they are added.

The add method is overridden to define the logic for adding an appender for a specific key. The removeStaleComponents method is overridden to define the logic for removing stale appenders. The find method is overridden to define the logic for finding an appender for a specific key.

It is important to note that this class is extending AppenderTracker which is a base class for tracking appenders. The AppenderTracker class provides a simple and efficient way to track appenders which are associated with a key.