The class is named InventoryTrackers (with an s), the interface name is InventoryTracker (without the s, found statically in the class.) 
  
How do you use it, you must create a new thread and initialize it. You must also end the thread once the script is done. Dont for get to implement InventoryTracker. 
  
Example: 
public class ScriptName extends Script implements InventoryTracker	{
     private InventoryTrackers invTracker; //create a field
}
Next create a new thread, and initialize the tracker within the onStart 
Override
public void onStart() throws InterruptedException {
	this.invTracker = new InventoryTrackers(this, this);
	new Thread(invTracker).start();
}
Now within your script you should have the two method overrided. Keep the condition so it doesn't affect your script once its paused or not logged in. 
	@Override
	public void handleAdded(Item item) {
		if (!bot.getScriptExecutor().isPaused() && client.isLoggedIn())	{  }
	}
	@Override
	public void handleRemoved(Item item) {
		if (!bot.getScriptExecutor().isPaused() && client.isLoggedIn())	{  }
	}		
	
Kill the thread once the script is over. 
@Override
public void onExit() throws InterruptedException {	invTracker.stop();	}
if you have any question feel free to ask.  if im missing something let me know.