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Updating ZooKeeperList automatically​

The ZooKeeperList object needs some way of being notified whenever a change to the collection of zookeeper objects held by the administrator changes, in order that it can update its model and thus be reflected in the list of items displayed. This can be accomplished by defining suitable event and listener objects, as follows:

Create a sub-package under virtualzoo.core called event, within which you will define the class ZooKeeperEvent and the interface ZooKeeperListener.

Define a ZooKeeperEvent class in package virtualzoo.core.event:

• The class extends EventObject, which is a Java supplied class intended for sub-classing by different event types;
• The constructor requires a ZooKeeper object to be passed in and which is in turn passed to the superclass constructor (which accepts any Object);
• The EventObject class defines a getSource() method so client objects can determine the source of the event. Because getSource() returns it as an Object a cast is required to get it back into its ZooKeeper type which you know it has to be. To save client objects the inconvenience of having to do this cast you have defined a getZooKeeper() method that performs the cast here.

Define a ZooKeeperListener interface in package virtualzoo.core.event.

• The interface extends EventListener, which is a Java supplied interface intended for sub-classing by different event listeners
• Separate methods are specified for when a ZooKeeper is created, changed or removed. Note the argument for each is ZooKeeperEvent

Because ZooAdministrator stores the collection of ZooKeeper objects it is a natural place to maintain references to objects that want to know about new, changed or removed zoo keepers (i.e., ZooKeeperListeners) and to notify them whenever these events occur.

In ZooAdministrator define an import for virtualzoo.core.event and add the following instance variable to store a collection of listeners:

Instantiate the collection inside the constructor:

Now define two new public methods so that client objects can register themselves as listeners:

• The above methods simply delegate to the add() and remove() methods of the collection

Define a private helper method that notifies all registered listeners whenever a new zookeeper is added:

• The above method creates a ZooKeeperEvent and then iterates over each registered listener notifying them of the fact that a new zookeeper has been created

Modify the createZooKeeper() method to invoke fireZooKeeperCreated() once the ZooKeeper object has been added to the collection:

You can now define methods fireZooKeeperChanged() and fireZooKeeperRemoved(), each very similar to fireZooKeeperAdded():

Modify changeZooKeeper() and removeZooKeeper() to invoke the respective "fire" methods:

The ZooKeeperList class can now be modified to implement the ZooKeeperListener interface (note you need to import virtualzoo.core.event):

If you click the glyph that appears to the left of the class declaration statement above, you can click the option to implement all abstract methods. The following code will appear:

NetBeans generated the statements to throw an UnsupportedOperationException merely so that the code will compile. It is up to you to replace these lines with the actual code that should occur in response to the events. In all three cases you simply want the data in the list model (held in the ZooKeeperListModel inner class) to be reloaded. To do this you can invoke the loadModel() method from within the event listener methods, so make the following modifications:

You now need to wire up the ZooKeeperList object which is referenced in ZooKeeperPanel so that it listens to events. In ZooKeeperPanel define a new instance variable to reference the ZooAdministrator object (you need to import import virtualzoo.core):

Now change the constructor to obtain the ZooAdministrator object add call its addZooKeeperListener() method:

You can now run the application, enter and save one (or several) zookeepers, and you should find the list automatically updates itself:

It would be better to clear the editor form after adding a new zookeeper ready for adding a new one, so add a call to clearZooKeeper() in the saveButtonActionPerformed() method of ZooKeeperEditor:

Verify that the form gets cleared after successfully adding a new zookeeper.

Next lesson: 16.7 Modifying existing zookeeper details