javac HelloWorld.java
HelloWorld.java:2: error: package javafx.application does not exist
import javafx.application.Application;
                         ^
HelloWorld.java:3: error: package javafx.event does not exist
import javafx.event.ActionEvent;
                   ^
HelloWorld.java:4: error: package javafx.event does not exist
import javafx.event.EventHandler;
                   ^
HelloWorld.java:5: error: package javafx.scene does not exist
import javafx.scene.Scene;
                   ^
HelloWorld.java:6: error: package javafx.scene.control does not exist
import javafx.scene.control.Button;
                           ^
HelloWorld.java:7: error: package javafx.scene.layout does not exist
import javafx.scene.layout.StackPane;
                          ^
HelloWorld.java:8: error: package javafx.stage does not exist
import javafx.stage.Stage;
                   ^
HelloWorld.java:10: error: cannot find symbol
public class HelloWorld extends Application {
                                ^
  symbol: class Application
HelloWorld.java:16: error: cannot find symbol
    public void start(Stage primaryStage) {
                      ^
  symbol:   class Stage
  location: class HelloWorld
HelloWorld.java:12: error: cannot find symbol
        launch(args);
        ^
  symbol:   method launch(String[])
  location: class HelloWorld
HelloWorld.java:15: error: method does not override or implement a method from a supertype
    @Override
    ^
HelloWorld.java:18: error: cannot find symbol
        Button btn = new Button();
        ^
  symbol:   class Button
  location: class HelloWorld
HelloWorld.java:18: error: cannot find symbol
        Button btn = new Button();
                         ^
  symbol:   class Button
  location: class HelloWorld
HelloWorld.java:20: error: cannot find symbol
        btn.setOnAction(new EventHandler<ActionEvent>() {
                            ^
  symbol:   class EventHandler
  location: class HelloWorld
HelloWorld.java:20: error: cannot find symbol
        btn.setOnAction(new EventHandler<ActionEvent>() {
                                         ^
  symbol:   class ActionEvent
  location: class HelloWorld
HelloWorld.java:23: error: cannot find symbol
            public void handle(ActionEvent event) {
                               ^
  symbol: class ActionEvent
HelloWorld.java:22: error: method does not override or implement a method from a supertype
            @Override
            ^
HelloWorld.java:28: error: cannot find symbol
        StackPane root = new StackPane();
        ^
  symbol:   class StackPane
  location: class HelloWorld
HelloWorld.java:28: error: cannot find symbol
        StackPane root = new StackPane();
                             ^
  symbol:   class StackPane
  location: class HelloWorld
HelloWorld.java:30: error: cannot find symbol
        primaryStage.setScene(new Scene(root, 300, 250));
                                  ^
  symbol:   class Scene
  location: class HelloWorld
20 errors