Basic Event Handling | Java Subject Material

Step 1 of 13

5.1 Event-Driven Programming

Learning objective

Contrast sequential console flow with event-driven GUI flow.

Programming flow
Console program	Event-driven JavaFX program
Runs statements in a planned sequence	Starts the interface and waits
Often blocks while reading input	Receives events from controls, keys, and the mouse
The program asks for the next value	The user chooses the next action

After stage.show(), the JavaFX event loop continues running. When the user clicks, types, or changes a selection, JavaFX creates an event and dispatches it to a matching handler.

Check your understanding: What determines which handler runs in an event-driven application?

The event that occurs and the handler registered for its source or target.

Step 2 of 13

5.2 What is an Event?

Learning objective

Define an event and identify common JavaFX event categories.

Common events

ActionEvent for command controls
KeyEvent for keyboard input
MouseEvent for mouse actions
InputMethodEvent for composed text input
Selection and value changes exposed through observable properties

Check your understanding: What event commonly represents a Button activation?

ActionEvent.

Step 3 of 13

5.3 Event Source

Learning objective

Identify the object where an event originates.

Read an event source

submit.setOnAction(event -> {
    Button source = (Button) event.getSource();
    result.setText(source.getText() + " clicked");
});

Check your understanding: In a button-click example, what is normally the event source?

The Button that was activated.

Step 4 of 13

5.4 Event Object

Learning objective

Use event-object information inside a handler.

Inspect event details

scene.setOnKeyPressed(event ->
        status.setText("Key: " + event.getCode()));

root.setOnMouseClicked(event ->
        status.setText("Mouse: " + event.getX()
                + ", " + event.getY()));

Check your understanding: Which MouseEvent methods return the pointer position relative to the node?

getX() and getY().

Step 5 of 13

5.5 Event Handler

Learning objective

Describe the code executed in response to an event.

Register a handler

button.setOnAction(event -> {
    result.setText("Operation completed");
});

A useful handler usually

Reads the event or current control values
Validates input
Updates application state
Updates the interface
Reports errors clearly

Check your understanding: Does registering a handler execute it immediately?

No. It runs when the matching event is dispatched.

Step 6 of 13

5.6 EventHandler<ActionEvent>

Learning objective

Implement the EventHandler interface explicitly and compare it with a lambda.

Anonymous EventHandler object

button.setOnAction(new EventHandler<ActionEvent>() {
    @Override
    public void handle(ActionEvent event) {
        result.setText("Clicked");
    }
});

Equivalent lambda expression

button.setOnAction(event -> result.setText("Clicked"));

EventHandlerDemo.java

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class EventHandlerDemo extends Application {
    @Override
    public void start(Stage stage) {
        Button button = new Button("Click Me");
        Label result = new Label("Waiting");
        button.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent event) {
                result.setText("Handled by EventHandler");
            }
        });
        stage.setScene(new Scene(new VBox(12, button, result), 340, 180));
        stage.setTitle("EventHandler Demo");
        stage.show();
    }
    public static void main(String[] args) { launch(args); }
}

Expected result

Before clicking: Waiting
After clicking: Handled by EventHandler

Check your understanding: Which method is implemented by EventHandler<ActionEvent>?

handle(ActionEvent event).

Step 7 of 13

5.7 Button Click Event

Learning objective

Handle a command button and validate numeric input.

Addition button event

addButton.setOnAction(event -> {
    try {
        double first = Double.parseDouble(firstField.getText());
        double second = Double.parseDouble(secondField.getText());
        result.setText("Sum: " + (first + second));
    } catch (NumberFormatException ex) {
        result.setText("Enter valid numbers");
    }
});

AdditionEventDemo.java

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

public class AdditionEventDemo extends Application {
    @Override
    public void start(Stage stage) {
        TextField first = new TextField();
        TextField second = new TextField();
        Button add = new Button("Add");
        Label result = new Label();
        add.setOnAction(e -> {
            try {
                double a = Double.parseDouble(first.getText());
                double b = Double.parseDouble(second.getText());
                result.setText("Sum: " + (a + b));
            } catch (NumberFormatException ex) {
                result.setText("Enter valid numbers");
            }
        });
        stage.setScene(new Scene(new VBox(10, first, second, add, result), 340, 240));
        stage.setTitle("Addition Event Demo");
        stage.show();
    }
    public static void main(String[] args) { launch(args); }
}

Expected result

For inputs 15 and 25:
Sum: 40.0

For invalid input:
Enter valid numbers

Check your understanding: Which exception should be handled when converting invalid text to a number?

NumberFormatException.

Step 8 of 13

5.8 TextField Events

Learning objective

Respond when the user submits text or when the text value changes.

Submit with Enter

nameField.setOnAction(event ->
        greeting.setText("Hello " + nameField.getText()));

Observe every text change

nameField.textProperty().addListener(
        (observable, oldValue, newValue) ->
                count.setText("Characters: " + newValue.length()));

Check your understanding: How can a handler run when the user presses Enter in a TextField?

Register it with setOnAction().

Step 9 of 13

5.9 Checkbox Events

Learning objective

Respond to independent selected-state changes.

Update selected subjects

EventHandler<ActionEvent> updateSubjects = event -> {
    String selected = "";
    if (javaBox.isSelected()) selected += "Java ";
    if (pythonBox.isSelected()) selected += "Python ";
    result.setText("Subjects: " + selected.trim());
};

javaBox.setOnAction(updateSubjects);
pythonBox.setOnAction(updateSubjects);

Check your understanding: Can several CheckBox handlers report selected choices at the same time?

Yes, because CheckBoxes are independent.

Step 10 of 13

5.10 RadioButton Events

Learning objective

Respond to a mutually exclusive selection and safely read the selected toggle.

Report the selected option

EventHandler<ActionEvent> showGender = event -> {
    RadioButton selected =
            (RadioButton) genderGroup.getSelectedToggle();
    result.setText(selected == null
            ? "Select gender"
            : "Gender: " + selected.getText());
};

male.setOnAction(showGender);
female.setOnAction(showGender);

Check your understanding: Why must getSelectedToggle() be checked for null?

A group may have no selected RadioButton.

Step 11 of 13

5.11 ComboBox Event

Learning objective

Respond when a new drop-down item is chosen.

Display the selected branch

branchBox.setOnAction(event -> {
    String branch = branchBox.getValue();
    result.setText(branch == null
            ? "Select a branch"
            : "Branch: " + branch);
});

Check your understanding: What must be handled before using the selected value?

The possibility that getValue() is null.

Step 12 of 13

5.12 Keyboard Events

Learning objective

Handle pressed, released, and typed keyboard input.

Show the pressed key

scene.setOnKeyPressed(event -> {
    status.setText("Pressed: " + event.getCode());

    if (event.getCode() == KeyCode.ESCAPE) {
        stage.close();
    }
});

Keyboard handler methods
Method	Meaning
setOnKeyPressed	Physical key goes down
setOnKeyReleased	Physical key goes up
setOnKeyTyped	A character is produced

KeyboardEventDemo.java

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.input.KeyCode;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class KeyboardEventDemo extends Application {
    @Override
    public void start(Stage stage) {
        Label status = new Label("Press a key");
        Scene scene = new Scene(new StackPane(status), 360, 180);
        scene.setOnKeyPressed(e -> {
            status.setText("Pressed: " + e.getCode());
            if (e.getCode() == KeyCode.ESCAPE) stage.close();
        });
        stage.setScene(scene);
        stage.setTitle("Keyboard Event Demo");
        stage.show();
    }
    public static void main(String[] args) { launch(args); }
}

Expected result

Pressing A displays:
Pressed: A

Pressing Escape closes the window.

Check your understanding: Why might a Scene key handler receive nothing while the window is open?

The intended node may not have focus, or another control may consume the event.

Step 13 of 13

5.13 Mouse Events

Learning objective

Handle mouse actions and combine multiple event types in one application.

IntegratedEventDemo.java

import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.input.KeyCode;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class IntegratedEventDemo extends Application {
    @Override
    public void start(Stage stage) {
        TextField name = new TextField();
        name.setPromptText("Enter name and press Enter");

        CheckBox emailUpdates = new CheckBox("Email updates");
        ComboBox<String> branch = new ComboBox<>();
        branch.getItems().addAll("CSE", "IT", "EC");
        branch.setPromptText("Select branch");

        Label status = new Label("Interact with the controls");
        Button submit = new Button("Submit");

        name.setOnAction(e ->
                status.setText("Hello " + name.getText()));
        emailUpdates.setOnAction(e ->
                status.setText("Email updates: "
                        + emailUpdates.isSelected()));
        branch.setOnAction(e ->
                status.setText("Branch: " + branch.getValue()));
        submit.setOnAction(e ->
                status.setText("Submitted for " + name.getText()));

        VBox root = new VBox(12,
                name, emailUpdates, branch, submit, status);
        root.setPadding(new Insets(20));
        root.setOnMouseClicked(e ->
                status.setText("Mouse: " + (int)e.getX()
                        + ", " + (int)e.getY()));

        Scene scene = new Scene(root, 430, 300);
        scene.setOnKeyPressed(e -> {
            if (e.getCode() == KeyCode.ESCAPE) stage.close();
        });

        stage.setScene(scene);
        stage.setTitle("Integrated Event Demo");
        stage.show();
    }

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

Expected result

The status label changes according to the latest event.

Examples:
Hello Vatsal
Email updates: true
Branch: IT
Submitted for Vatsal
Mouse: 120, 210

Pressing Escape closes the window.

Common mouse handlers
Handler	Responds to
setOnMouseClicked	Click or double-click
setOnMousePressed	Mouse button goes down
setOnMouseReleased	Mouse button goes up
setOnMouseEntered	Pointer enters a node
setOnMouseExited	Pointer leaves a node
setOnMouseMoved	Pointer moves without dragging
setOnMouseDragged	Pointer moves while a button is held

MouseEventDemo.java

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

public class MouseEventDemo extends Application {
    @Override
    public void start(Stage stage) {
        Label status = new Label("Click anywhere");
        StackPane root = new StackPane(status);
        root.setOnMouseClicked(e -> status.setText(
                "Mouse: " + (int)e.getX() + ", " + (int)e.getY()
                + " | Clicks: " + e.getClickCount()));
        stage.setScene(new Scene(root, 420, 240));
        stage.setTitle("Mouse Event Demo");
        stage.show();
    }
    public static void main(String[] args) { launch(args); }
}

Expected result

Example single click:
Mouse: 120, 90 | Clicks: 1

Example double-click:
Mouse: 120, 90 | Clicks: 2

Check your understanding: Which method reports a double-click count?

MouseEvent.getClickCount(), which returns 2 for a double-click.