Advanced Java Programming (BSc CSIT, CSC409): the questions likely to come

JavaFX ChoiceBox Application

ChoiceBox is a JavaFX control that shows a drop-down list of items for single selection. We listen to its selection model with a ChangeListener (or a binding) and update a Label whenever the chosen color changes.

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.ChoiceBox;
import javafx.scene.control.Label;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class ColorChoiceApp extends Application {
    @Override
    public void start(Stage stage) {
        ChoiceBox<String> choiceBox = new ChoiceBox<>();
        choiceBox.getItems().addAll("Red", "Green", "Blue", "Yellow");
        choiceBox.setValue("Red");

        Label label = new Label("Selected color: Red");

        // Update label whenever the selected color changes
        choiceBox.getSelectionModel().selectedItemProperty().addListener(
            (obs, oldVal, newVal) -> label.setText("Selected color: " + newVal));

        VBox root = new VBox(10, choiceBox, label);
        Scene scene = new Scene(root, 300, 150);
        stage.setTitle("ChoiceBox Demo");
        stage.setScene(scene);
        stage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}

When the user picks a color from the ChoiceBox, the listener fires and the Label text changes to reflect the selection.

Steps for Writing CORBA Programs

CORBA (Common Object Request Broker Architecture) is a standard from the OMG that lets objects written in different languages on different machines communicate through an ORB (Object Request Broker) using IDL (Interface Definition Language).

Steps:

1. Define the interface in IDL — declare the remote operations in a language-neutral .idl file.
2. Compile the IDL with the IDL-to-Java compiler (idlj) to generate the stub (client side), skeleton (server side), and helper/holder classes.
3. Implement the servant — write a Java class that extends the generated skeleton (...POA) and implements the interface operations.
4. Write the server — create the ORB, get the POA, instantiate the servant, register/bind its object reference with the Naming Service, and wait for requests.
5. Write the client — initialize the ORB, look up the object reference from the Naming Service, narrow it to the interface type, and invoke remote methods.
6. Run — start orbd (the naming/activation daemon), then the server, then the client.

Example IDL — Hello.idl

module HelloApp {
    interface Hello {
        string sayHello();
    };
};

Servant

import HelloApp.*;
class HelloImpl extends HelloPOA {
    public String sayHello() { return "Hello from CORBA server"; }
}

Server (skeleton)

ORB orb = ORB.init(args, null);
POA rootpoa = POAHelper.narrow(orb.resolve_initial_references("RootPOA"));
rootpoa.the_POAManager().activate();
HelloImpl servant = new HelloImpl();
org.omg.CORBA.Object ref = rootpoa.servant_to_reference(servant);
Hello href = HelloHelper.narrow(ref);
// bind 'href' in the naming service, then orb.run();

Client (skeleton)

ORB orb = ORB.init(args, null);
// resolve "Hello" from naming service into 'helloRef'
Hello helloRef = HelloHelper.narrow(ncRef.resolve_str("Hello"));
System.out.println(helloRef.sayHello());

Thus CORBA achieves language- and platform-independent distributed object communication via IDL and the ORB.