I'm trying to create a program, which should draw a rectangle on a canvas based on two left mouse clicks and clears the canvas with one right click.
The rectangle should be created in a way, where the first mouse click simulates one corner of the rectangle, and the next mouse click simulates the diagonal corner of the rectangle compared to the first mouse click.
I'm stuck on how to store the coordinates for the first mouse click, and then put the second mouse click to good use, as the rectangle per definition is created based on only 1 set of coordinates, which is the upper-left corner of the rectangle.
Right now, all my code does is that it draws fixed sized rectangles (50x25) which obviously isn't what I want. This is just to see if the clearing part works.
This is what I've got so far:
package application;
import javafx.application.Application;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.input.MouseButton;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.StackPane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;
public class DrawRectangle extends Application {
public static void main(String[] args) {
launch(args);
}
#Override
public void start(Stage primaryStage) {
primaryStage.setTitle("Title");
Canvas canv = new Canvas(500, 400);
GraphicsContext gc = canv.getGraphicsContext2D();
EventHandler<MouseEvent> event = new EventHandler<MouseEvent>() {
#Override
public void handle(MouseEvent event) {
if (event.getButton() == MouseButton.PRIMARY) {
double x = event.getX();
double y = event.getY();
gc.setFill(Color.BLUE);
gc.fillRect(x, y, 50, 25);
}
}
};
canv.addEventHandler(MouseEvent.MOUSE_CLICKED, event);
EventHandler<MouseEvent> event2 = new EventHandler<MouseEvent>() {
#Override
public void handle(MouseEvent event2) {
if (event2.getButton() == MouseButton.SECONDARY) {
gc.clearRect(0, 0, 500, 400);
}
}
};
canv.addEventHandler(MouseEvent.MOUSE_CLICKED, event2);
StackPane root = new StackPane();
root.getChildren().add(canv);
primaryStage.setScene(new Scene(root, 500, 400));
primaryStage.show();
}
}
I don't know if it matters which corners of the rectangle the mouse clicks should simulate, maybe someone got any ideas?
Best regards and happy new year
Just save the first click point in a member variable, then use it and reset it when the user makes a second click point.
Note: the sample code below uses a switch expression from Java 13+. If switch expressions are not available in your Java version, convert it to a standard switch statement or if/else clauses.
import javafx.application.Application;
import javafx.geometry.*;
import javafx.scene.Scene;
import javafx.scene.canvas.*;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.StackPane;
import javafx.scene.paint.Color;
import javafx.stage.Stage;
public class DrawRectangle extends Application {
private static final int W = 500;
private static final int H = 400;
private Point2D savedPoint = null;
#Override
public void start(Stage stage) {
Canvas canvas = new Canvas(W, H);
GraphicsContext gc = canvas.getGraphicsContext2D();
canvas.setOnMouseClicked(event -> {
switch (event.getButton()) {
case PRIMARY -> handlePrimaryClick(gc, event);
case SECONDARY -> gc.clearRect(0, 0, W, H);
}
});
stage.setResizable(false);
stage.setScene(new Scene(new StackPane(canvas), W, H));
stage.show();
}
private void handlePrimaryClick(GraphicsContext gc, MouseEvent event) {
Point2D clickPoint = new Point2D(event.getX(), event.getY());
if (savedPoint == null) {
savedPoint = clickPoint;
} else {
Rectangle2D rectangle2D = getRect(savedPoint, clickPoint);
gc.setFill(Color.BLUE);
gc.fillRect(
rectangle2D.getMinX(),
rectangle2D.getMinY(),
rectangle2D.getWidth(),
rectangle2D.getHeight()
);
savedPoint = null;
}
}
private Rectangle2D getRect(Point2D p1, Point2D p2) {
return new Rectangle2D(
Math.min(p1.getX(), p2.getX()),
Math.min(p1.getY(), p2.getY()),
Math.abs(p1.getX() - p2.getX()),
Math.abs(p1.getY() - p2.getY())
);
}
public static void main(String[] args) {
launch(args);
}
}
Related
I am working on a javafx program to create three buttons which are "circle", "ellipse" and "reverse". The circle button is the first thing shown when the program is run, the reverse button is supposed to correspond with the movement of the rectangle around the circle. I am having trouble getting the reverse button to work, I set autoReverse to true but it isn't doing anything. My second issue with the code is when the ellipse button is clicked the class MyEllipse isn't showing the ellipse shape like it is supposed to and it's not removing the circle animation from the pane. I tried to create a new pane in the EventHandler for the buttonellipse but I am assuming that isn't the correct way to do it. Any help on these two issues would be greatly appreciated.
import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.layout.Pane;
import javafx.animation.PathTransition;
import javafx.scene.paint.Color;
import javafx.animation.Timeline;
import javafx.scene.shape.Circle;
import javafx.util.Duration;
import javafx.scene.shape.Rectangle;
import javafx.scene.shape.Ellipse;
import javafx.scene.control.Button;
import javafx.event.ActionEvent;
import javafx.scene.layout.HBox;
import javafx.event.EventHandler;
import javafx.stage.Stage;
public class exam3b extends Application {
#Override
public void start(Stage primaryStage) {
Rectangle rectangle = new Rectangle (0, 0, 25, 50);
rectangle.setFill(Color.ORANGE);
Circle circle = new Circle(115, 90, 45);
circle.setFill(Color.WHITE);
circle.setStroke(Color.BLACK);
PathTransition pt = new PathTransition();
pt.setDuration(Duration.millis(4000));
pt.setPath(circle);
pt.setNode(rectangle);
pt.setOrientation(
PathTransition.OrientationType.ORTHOGONAL_TO_TANGENT);
pt.setCycleCount(Timeline.INDEFINITE);
pt.setAutoReverse(false);
pt.play();
circle.setOnMousePressed(e -> pt.pause());
circle.setOnMouseReleased(e -> pt.play());
HBox panel = new HBox(10);
panel.setAlignment(Pos.BOTTOM_CENTER);
Button button = new Button("Circle");
Button buttonellipse = new Button("Ellipse");
Button reverse = new Button("Reverse");
panel.getChildren().addAll(button,buttonellipse,reverse);
button.setOnAction(new EventHandler<ActionEvent>()
{
#Override
public void handle(ActionEvent e)
{
reverse.setVisible(true);
}
});
reverse.setOnAction(new EventHandler<ActionEvent>() //supposed to make rectangle move in reverse direction
{
#Override
public void handle(ActionEvent e)
{
pt.setAutoReverse(true);
}
});
buttonellipse.setOnAction(new EventHandler<ActionEvent>() //button to make ellipse appear from class MyEllipse, reverse button is supposed to disappear
{
#Override
public void handle(ActionEvent e)
{
reverse.setVisible(false);
Pane ellipse = new Pane();
ellipse.getChildren().add(new MyEllipse());
}
});
Pane pane = new Pane();
pane.getChildren().addAll(panel,circle,rectangle);
Scene scene = new Scene(pane, 350, 250);
primaryStage.setTitle("exam3b");
primaryStage.setScene(scene);
primaryStage.show();
}
public static void main(String[] args) {
launch(args);
}
}
class MyEllipse extends Pane
{
private void paint() {
getChildren().clear();
for (int i = 0; i < 16; i++) {
Ellipse e1 = new Ellipse(getWidth() / 2, getHeight() / 2,
getWidth() / 2 - 50, getHeight() / 2 - 50);
e1.setStroke(Color.color(Math.random(), Math.random(),
Math.random()));
e1.setFill(Color.WHITE);
e1.setRotate(i * 180 / 16);
getChildren().add(e1);
}
}
#Override
public void setWidth(double width) {
super.setWidth(width);
paint();
}
#Override
public void setHeight(double height) {
super.setHeight(height);
paint();
}
}
You had an issue with how you were using MyEllipse, in that you were never adding it to your root pane nor were you settign the width/height (which leads to your paint method being called). I renamed your root pane to 'root'. Created an instance of 'MyEllipse', and in the 'Ellipse' button i add the instance of 'MyEllipse' to the root pane.
import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.layout.Pane;
import javafx.animation.PathTransition;
import javafx.scene.paint.Color;
import javafx.animation.Timeline;
import javafx.scene.shape.Circle;
import javafx.util.Duration;
import javafx.scene.shape.Rectangle;
import javafx.scene.shape.Ellipse;
import javafx.scene.control.Button;
import javafx.event.ActionEvent;
import javafx.scene.layout.HBox;
import javafx.event.EventHandler;
import javafx.stage.Stage;
public class exam3b extends Application {
#Override
public void start(Stage primaryStage) {
Rectangle rectangle = new Rectangle(0, 0, 25, 50);
rectangle.setFill(Color.ORANGE);
Circle circle = new Circle(115, 90, 45);
circle.setFill(Color.WHITE);
circle.setStroke(Color.BLACK);
PathTransition pt = new PathTransition();
pt.setDuration(Duration.millis(4000));
pt.setPath(circle);
pt.setNode(rectangle);
pt.setOrientation(
PathTransition.OrientationType.ORTHOGONAL_TO_TANGENT);
pt.setCycleCount(Timeline.INDEFINITE);
pt.setAutoReverse(true);
pt.play();
circle.setOnMousePressed(e -> pt.pause());
circle.setOnMouseReleased(e -> pt.play());
HBox panel = new HBox(10);
panel.setAlignment(Pos.BOTTOM_CENTER);
Button button = new Button("Circle");
Button buttonellipse = new Button("Ellipse");
Button reverse = new Button("Reverse");
panel.getChildren().addAll(button, buttonellipse, reverse);
button.setOnAction(new EventHandler<ActionEvent>() {
#Override
public void handle(ActionEvent e) {
reverse.setVisible(true);
}
});
reverse.setOnAction(new EventHandler<ActionEvent>() //supposed to make rectangle move in reverse direction
{
#Override
public void handle(ActionEvent e) {
pt.setAutoReverse(true);
}
});
Pane root = new Pane();
MyEllipse myEllipse = new MyEllipse();
myEllipse.setWidth(200);
myEllipse.setHeight(400);
buttonellipse.setOnAction(new EventHandler<ActionEvent>() //button to make ellipse appear from class MyEllipse, reverse button is supposed to disappear
{
#Override
public void handle(ActionEvent e) {
// reverse.setVisible(false);
if (root.getChildren().contains(myEllipse)) {
root.getChildren().remove(myEllipse);
} else {
System.out.println("adding ellipse");
root.getChildren().add(myEllipse);
}
}
});
root.getChildren().addAll(panel, circle, rectangle);
Scene scene = new Scene(root, 350, 250);
primaryStage.setTitle("exam3b");
primaryStage.setScene(scene);
primaryStage.show();
}
public static void main(String[] args) {
launch(args);
}
}
class MyEllipse extends Pane {
public MyEllipse() {
}
private void paint() {
getChildren().clear();
for (int i = 0; i < 16; i++) {
Ellipse e1 = new Ellipse(getWidth() / 2, getHeight() / 2,
getWidth() / 2 - 50, getHeight() / 2 - 50);
e1.setStroke(Color.color(Math.random(), Math.random(),
Math.random()));
e1.setFill(Color.WHITE);
e1.setStrokeWidth(1);
e1.setRotate(i * 180 / 16);
getChildren().add(e1);
}
}
#Override
public void setWidth(double width) {
super.setWidth(width);
paint();
}
#Override
public void setHeight(double height) {
super.setHeight(height);
paint();
}
}
When running the following code, produces just a blank window as in no visible circles are being drawn. It is expected to produce randomly placed black circles on the window.
Here is my code:
package circles;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.VBox;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.stage.Stage;
import java.util.Random;
import javafx.scene.paint.Color;
public class Circles extends Application {
private Canvas canvas = new Canvas(400, 400);
#Override
public void start(Stage primaryStage) {
VBox root = new VBox();
root.getChildren().add(canvas);
Scene scene = new Scene(root);
primaryStage.setTitle("Draw Circles");
primaryStage.setScene(scene);
primaryStage.show();
}
private void drawCircles() {
// radius of the circles
final int RADIUS = 20;
Random rand = new Random();
GraphicsContext gc = canvas.getGraphicsContext2D();
gc.clearRect(0, 0, canvas.getWidth(), canvas.getHeight());
gc.setFill(Color.WHITESMOKE);
for (int i =0; i <50;i++) {
int x = rand.nextInt(400);
int y = rand.nextInt(400);
gc.fillOval(x, y, RADIUS, RADIUS);
}
}
public static void main(String[] args) {
Circles x = new Circles();
x.drawCircles();
launch(args);
}
}
The Application.launch(...) method creates an instance of the application class, and (after some other work) calls start() on it.
In your code, you first create an instance of your application class Circles, call drawCircles() on it, and then you call Application.launch(...). This causes a second instance of Circles to be created, and start() to be invoked on that second instance.
Consequently, drawCircles() is not called on the same object as start(), and so the circles are drawn on a canvas that is never displayed.
Simply move the call to drawCircles() to the start() method:
package circles;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.VBox;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.stage.Stage;
import java.util.Random;
import javafx.scene.paint.Color;
public class Circles extends Application {
private Canvas canvas = new Canvas(400, 400);
#Override
public void start(Stage primaryStage) {
VBox root = new VBox();
root.getChildren().add(canvas);
drawCircles();
Scene scene = new Scene(root);
primaryStage.setTitle("Draw Circles");
primaryStage.setScene(scene);
primaryStage.show();
}
private void drawCircles() {
// radius of the circles
final int RADIUS = 20;
Random rand = new Random();
GraphicsContext gc = canvas.getGraphicsContext2D();
gc.clearRect(0, 0, canvas.getWidth(), canvas.getHeight());
gc.setFill(Color.WHITESMOKE);
for (int i =0; i <50;i++) {
int x = rand.nextInt(400);
int y = rand.nextInt(400);
gc.fillOval(x, y, RADIUS, RADIUS);
}
}
public static void main(String[] args) {
launch(args);
}
}
I just started working with JavaFX and have a question. In my project i want to use a rotating rectangle. But the rectangle only rotates about its center and i want it to rotate around its upper left corner.
Like in this picture (from here):
Here some code like in my project:
import javafx.animation.AnimationTimer;
import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.input.KeyCode;
import javafx.scene.paint.Color;
import javafx.stage.Stage;
import javafx.scene.shape.Rectangle;
import java.awt.*;
import java.awt.event.KeyEvent;
import java.util.*;
public class Main extends Application {
#Override
public void start(Stage stage) throws Exception {
Group root = new Group();
Scene scene = new Scene(root, 500, 500);
//create rectangle
Rectangle rect = new Rectangle(10, 10, 200, 15);
rect.setTranslateX(250);
rect.setTranslateY(250);
rect.setFill(Color.BLACK);
root.getChildren().add(rect);
AnimationTimer timer = new AnimationTimer() {
#Override
public void handle(long now) {
stage.getScene().setOnKeyPressed(e -> {
if (e.getCode() == KeyCode.LEFT) {
rect.setRotate(rect.getRotate()-5); //<-- rotate rectangle here
} else if (e.getCode() == KeyCode.RIGHT){
rect.setRotate(rect.getRotate()+5); //<-- rotate rectangle here
}
});
}
};
timer.start();
stage.setScene(scene);
stage.show();
}
public static void main(String[] args) {
launch(args);
}
}
In this case the rectangle rotates if the arrow keys are pressed.
You need to use Transform and Rotate specifically to rotate a node around a custom pivot.
//Create a Rotate Object
Rotate rotate = new Rotate();
rotate.setPivotX(node.getX()); //Pivot X Top-Left corner
rotate.setPivotY(node.getY()); //Pivot Y
rotate.setAngle(angle); //Angle degrees
//Add the transform to the node
node.getTransforms().add(rotate);
In your code
final Rotate rotate = new Rotate();
rect.getTransforms().add(rotate);
AnimationTimer timer = new AnimationTimer() {
#Override
public void handle(long now) {
stage.getScene().setOnKeyPressed(e -> {
rotate.setPivotX(rect.getX());
rotate.setPivotY(rect.getY());
if (e.getCode() == KeyCode.LEFT) {
rotate.setAngle(rotate.getAngle() - 5);
} else if (e.getCode() == KeyCode.RIGHT){
rotate.setAngle(rotate.getAngle() + 5);
}
});
}
};
I have a program where 2 circles are dragged in a pane. There is also a line connecting them and the distance displayed above it. My problem lies when I drag the circles at a slow pace with the mouse they move fine, but when I move it more quickly the circles stop.
here is where the circle drag is calculated
pane.setOnMouseDragged(e -> {
if (circle1.contains(e.getX(), e.getY())) {
pane.getChildren().clear();
circle1.setCenterX(e.getX());
circle1.setCenterY(e.getY());
pane.getChildren().addAll(getLine(circle1, circle2), circle1,
circle2, getText(circle1, circle2));
}
else if (circle2.contains(e.getX(), e.getY())) {
pane.getChildren().clear();
circle2.setCenterX(e.getX());
circle2.setCenterY(e.getY());
pane.getChildren().addAll(getLine(circle1, circle2), circle1,
circle2, getText(circle1, circle2));
}
});
I think what is happening is that when the mouse moves quickly, the distance moved between processing two consecutive events takes it outside the bounds of the circle, so the if condition becomes false. You probably need to register the mouse handlers on the circles themselves, instead of the pane. (As an aside, why clear and rebuild the pane, instead of just update the line?)
Here's an example using these techniques:
import javafx.application.Application;
import javafx.beans.property.ObjectProperty;
import javafx.beans.property.SimpleObjectProperty;
import javafx.geometry.Point2D;
import javafx.scene.Scene;
import javafx.scene.layout.Pane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.scene.shape.Line;
import javafx.stage.Stage;
public class DraggingCircles extends Application {
#Override
public void start(Stage primaryStage) {
Pane pane = new Pane();
Circle circle1 = createDraggingCircle(50, 50, 25, Color.BLUE);
Circle circle2 = createDraggingCircle(350, 350, 25, Color.RED);
Line line = new Line();
line.startXProperty().bind(circle1.centerXProperty());
line.startYProperty().bind(circle1.centerYProperty());
line.endXProperty().bind(circle2.centerXProperty());
line.endYProperty().bind(circle2.centerYProperty());
pane.getChildren().addAll(circle1, circle2, line);
Scene scene = new Scene(pane, 400, 400);
primaryStage.setScene(scene);
primaryStage.show();
}
private Circle createDraggingCircle(double x, double y, double radius, Color fill) {
Circle circle = new Circle(x, y, radius, fill);
ObjectProperty<Point2D> mouseLocation = new SimpleObjectProperty<>();
circle.setOnMousePressed(e -> {
mouseLocation.set(new Point2D(e.getX(), e.getY()));
});
circle.setOnMouseDragged(e -> {
double deltaX = e.getX() - mouseLocation.get().getX();
double deltaY = e.getY() - mouseLocation.get().getY();
circle.setCenterX(circle.getCenterX() + deltaX);
circle.setCenterY(circle.getCenterY() + deltaY);
mouseLocation.set(new Point2D(e.getX(), e.getY()));
});
return circle ;
}
public static void main(String[] args) {
launch(args);
}
}
I am trying to learn javafx. I did most of the code but I am having trouble with the start method.
What I wanted to do was add spots to the screen by clicking on it. And if I press either 1 or 0 future spots that will be added will change to some different color. Therefore, I know that I must use setOnMouseClicked
and setOnKeyPressed methods but there isn't much on the internet on it.
import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
public class Spots extends Application {
public static final int SIZE = 500;
public static final int SPOT_RADIUS = 20;
private LinkedList<Spot> spotList;
private Color color;
public static void main(String...args) {
launch(args);
}
public void start(Stage stage) {
stage.setTitle("Spots");
dotList = new SinglyLinkedList<>();
Group root = new Group();
Scene scene = new Scene(root, 500, 500, Color.BLACK);
Spot r;
// ...
stage.show();
}
private class Spot extends Circle {
public Spot(double xPos, double yPos) {
super(xPos, yPos, SPOT_RADIUS);
setFill(color);
}
public boolean contains(double xPos, double yPos) {
double dx = xPos - getCenterX();
double dy = yPos - getCenterY();
double distance = Math.sqrt(Math.pow(dx, 2) + Math.pow(dy, 2));
return distance <= SPOT_RADIUS;
}
}
}
The reason the circle is not accepting is that it is not focused. For nodes to respond to key events they should be focusTraversable. You can do that by
calling setFocusTraversable(true) on the node. I edited your start() method and here is the code I ended up with.
public void start(Stage primaryStage) throws Exception {
Pane pane = new Pane();
final Scene scene = new Scene(pane, 500, 500);
final Circle circle = new Circle(250, 250, 20);
circle.setFill(Color.WHITE);
circle.setStroke(Color.BLACK);
pane.getChildren().add(circle);
circle.setFocusTraversable(true);
circle.setOnKeyPressed(new EventHandler<KeyEvent>() {
#Override
public void handle(KeyEvent e) {
if ((e.getCode() == KeyCode.UP) && (circle.getCenterY() >= 5)) {
circle.setCenterY(circle.getCenterY() - 5);
}
else if ((e.getCode() == KeyCode.DOWN && (circle.getCenterY() <= scene.getHeight() - 5))) {
circle.setCenterY(circle.getCenterY() + 5);
}
else if ((e.getCode() == KeyCode.RIGHT) && (circle.getCenterX() <= scene.getWidth() - 5)) {
circle.setCenterX(circle.getCenterX() + 5);
}
else if ((e.getCode() == KeyCode.LEFT && (circle.getCenterX() >= 5))) {
circle.setCenterX(circle.getCenterX()-5);
}
}
});
//creates new spots by clicking anywhere on the pane
pane.setOnMouseClicked(new EventHandler<MouseEvent>() {
public void handle(MouseEvent event) {
double newX = event.getX(); //getting the x-coordinate of the clicked area
double newY = event.getY(); //getting the y-coordinate of the clicked area
Circle newSpot = new Circle(newX, newY,20);
newSpot.setFill(Color.WHITE);
newSpot.setStroke(Color.BLACK);
pane.getChildren().add(newSpot);
}
});
primaryStage.setTitle("Move the circle");
primaryStage.setScene(scene);
primaryStage.show();
}
Also take look at the answers for the following links:
Handling keyboard events
Cannot listen to KeyEvent in
JavaFX
Solution Approach
You can monitor the scene for key typed events and switch the color mode based on that. You can place an mouse event handler on your scene root pane and add a circle (of the appropriate color for the prevailing color mode) to the scene when the user clicks anywhere in the pane.
Sample Code
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.Pane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.stage.Stage;
// Java 8+ code.
public class Spots extends Application {
private static final int SIZE = 500;
private static final int SPOT_RADIUS = 20;
private Color color = Color.BLUE;
public void start(Stage stage) {
Pane root = new Pane();
root.setOnMouseClicked(event ->
root.getChildren().add(
new Spot(
event.getX(),
event.getY(),
color
)
)
);
Scene scene = new Scene(root, SIZE, SIZE, Color.BLACK);
scene.setOnKeyTyped(event -> {
switch (event.getCharacter()) {
case "0":
color = Color.BLUE;
break;
case "1":
color = Color.RED;
break;
}
});
stage.setScene(scene);
stage.show();
}
private class Spot extends Circle {
public Spot(double xPos, double yPos, Color color) {
super(xPos, yPos, SPOT_RADIUS);
setFill(color);
}
}
public static void main(String... args) {
launch(args);
}
}
Further Info
For detailed information on event handling in JavaFX, see the Oracle JavaFX event tutorial.
Generally, you'd use setOnAction as shown in the Oracle tutorials.
Example:
btn.setOnAction(new EventHandler<ActionEvent>() {
public void handle(ActionEvent event) {
System.out.println("Hello World");
}
});
If the particular Node you're trying to use does not have a clickHandler method, try doing something like this (on Group for example):
group.addEventHandler(MouseEvent.MOUSE_CLICKED, new EventHandler<MouseEvent>() {
#Override
public void handle(MouseEvent event) {
System.out.println("Hello!");
}
});