Event-Driven Programming

You can write code to process events such as a button click, mouse movement, and keystrokes.

Suppose you wish to write a GUI program that lets the user enter a loan amount, annual interest rate, and number of years and click the Calculate button to obtain the monthly payment and total payment, as shown in Figure below. How do you accomplish the task? You have to use event-driven programming to write the code to respond to the button-clicking event.

Before delving into event-driven programming, it is helpful to get a taste using a simple example. The example displays two buttons in a pane, as shown in Figure below.

(a) The program displays two buttons. (b) A message is displayed in the
console when a button is clicked.

To respond to a button click, you need to write the code to process the button-clicking action. The button is an event source object—where the action originates. You need to create an object capable of handling the action event on a button. This object is called an event handler, as shown in Figure below.

Not all objects can be handlers for an action event. To be a handler of an action event, two
requirements must be met:

1. The object must be an instance of the EventHandler interface. This interface defines the common behavior for all handlers. denotes that T is a generic type that is a subtype of Event.
2. The EventHandler object handler must be registered with the event source object using the method source.setOnAction(handler).

The EventHandler interface contains the handle(ActionEvent) method for processing the action event. Your handler class must override this method to respond to the event. Listing 15.1 gives the code that processes the ActionEvent on the two buttons. When you click the OK button, the message “OK button clicked” is displayed. When you click the Cancel button, the message “Cancel button clicked” is displayed, as shown in Figure above (a) and (b).

package application;
import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;

public class HandleEvent extends Application {
    @Override // Override the start method in the Application
    public void start(Stage primaryStage) {
        // Create a pane and set its properties
        HBox pane = new HBox(10);
        pane.setAlignment(Pos.CENTER);
        Button btOK = new Button("OK");
        Button btCancel = new Button("Cancel");
        OKHandlerClass handler1 = new OKHandlerClass();
        btOK.setOnAction(handler1);
        CancelHandlerClass handler2 = new CancelHandlerClass();
        btCancel.setOnAction(handler2);
        pane.getChildren().addAll(btOK, btCancel);

        // Create a scene and place it in the stage
        Scene scene = new Scene(pane);
        primaryStage.setTitle("HandleEvent"); // Set the stage title
        primaryStage.setScene(scene); // Place the scene in the stage
        primaryStage.show(); // Display the stage
    }

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

class OKHandlerClass implements EventHandler<ActionEvent>{
    @Override
    public void handle(ActionEvent e) {
        System.out.println("OK button clicked");
    }
}

class CancelHandlerClass implements EventHandler<ActionEvent>{
    @Override
    public void handle(ActionEvent e) {
        System.out.println("Cancel button clicked");
    }
}

Two handler classes are defined in lines 37–42. Each handler class implements EventHandler to process ActionEvent. The object handler1 is an instance of OKHandlerClass (line 19), which is registered with the button btOK (line 20). When the OK button is clicked, the handle(ActionEvent) method (line 39) in OKHandlerClass is invoked to process the event. The object handler2 is an instance of CancelHandlerClass (line 21), which is registered with the button btCancel in line 22. When the Cancel button is clicked, the handle(ActionEvent) method (line 46) in CancelHandlerClass is invoked to process the event.

You now have seen a glimpse of event-driven programming in JavaFX.