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Why can't I run Main.java on Eclipse "Errors exist in required project(s)"

Here is error when I press run:
Down is Main.java and I don't know what this is. Yesterday I used this file and nothing happened.
package application;
import javafx.application.Application;
import javafx.stage.Stage;
import javafx.stage.StageStyle;
import javafx.scene.Scene;
import javafx.scene.image.Image;
import javafx.scene.layout.BorderPane;
import javafx.fxml.FXMLLoader;
public class Main extends Application {
#Override
public void start(Stage primaryStage) {
try {
BorderPane root = (BorderPane)FXMLLoader.load(getClass().getResource("LogIn.fxml"));
Scene scene = new Scene(root, 700,512);
scene.getStylesheets().add(getClass().getResource("application.css").toExternalForm());
primaryStage.initStyle(StageStyle.DECORATED);
primaryStage.setScene(scene);
primaryStage.show();
primaryStage.setTitle("Log In");
Image icon = new Image(getClass().getResourceAsStream("sherr.jpeg"));
primaryStage.getIcons().add(icon);
primaryStage.setResizable(false);
} catch(Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
launch(args);
}
}
Here is what displayed on Console

Java FX 'cannot find symbol'

I'm new to java and java fx, I am trying to create a simple window but I keep getting the error
java: cannot find symbol
symbol: class Group
location: class sample.Main
This is my code:
package sample;
import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
public class Main extends Application {
public static void main(String[] args) {
launch(args);
}
#Override
public void start(Stage primaryStage) throws Exception{
Group root = new Group();
Scene scene = new Scene(root);
primaryStage.setScene(scene);
primaryStage.show();
}
}

You aren't importing javafx.scene.Group. Add
import javafx.scene.Group;

JavaFX; Eclipse points error at main method

I was trying to learn JavaFX and used the code on this video: https://www.youtube.com/watch?v=FLkOX4Eez6o.
I have Google it but could not find a solution.
Then Eclipse IDE shows this error:
import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
public class Testes extends Application{
Button botao;
public static void main(String[] args) {
launch(args);
}
#Override
public void start(Stage primaryStage) throws Exception {
primaryStage.setTitle("Controle de processos");
botao = new Button();
botao.setText("+");
StackPane layout = new StackPane();
layout.getChildren().add(botao);
Scene cena = new Scene(layout, 300, 250);
primaryStage.setScene(cena);
primaryStage.show();
}
}

Swap JavaFX scene when clicking a button

I am trying to create a program to teach people about GNU/Linux and the command line, I have my main.java
package sample;
import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;
public class Main extends Application {
Stage window;
public static void main(String[] args) {
launch(args);
}
#Override
public void start(Stage primaryStage) throws Exception {
window = primaryStage;
Parent root = FXMLLoader.load(getClass().getResource("login.fxml"));
primaryStage.setTitle("Learnix");
primaryStage.setScene(new Scene(root, 800, 500));
primaryStage.show();
}
}
And the controller to go with it.
package sample;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import java.io.IOException;
public class loginController {
public Button loginBtn;
public void loginBtnClick() throws IOException {
System.out.println("You are logged in");
}
}
I have tried things such as:
FXMLLoader.load(getClass().getResource("lessons.fxml"));
But I can't figure out how to get it to swap scenes. I have seen many tutorials on YouTube and it Stack Overflow but many of them have all of the JavaFX on the main.java and not in separate files as I am using scenebuilder.
Thank you.

You can either call Stage.setScene() to change the whole scene or just substitute a root to the new one by Scene.setRoot():
Parent newRoot = FXMLLoader.load(getClass().getResource("lessons.fxml"));
primaryStage.getScene().setRoot(newRoot);

Open PDF file with associated program in JavaFX

I want my java application such that if user chooses to click on a button the PDF opens using the default PDF reader that is installed in the computer.
The PDF which i want to be opened is present in same package "application".
The code which I am using is
package application;
import java.io.File;
import javafx.application.Application;
import javafx.application.HostServices;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.FileChooser;
import javafx.stage.Stage;
public class Main extends Application {
#Override
public void start(final Stage primaryStage) {
Button btn = new Button();
btn.setText("Load PDF");
btn.setOnAction(new EventHandler<ActionEvent>() {
#Override
public void handle(ActionEvent event) {
File pdfFile = new File("computer_graphics_tutorial.pdf");
getHostServices().showDocument(pdfFile.toURI().toString());
}
});
StackPane root = new StackPane();
root.getChildren().add(btn);
Scene scene = new Scene(root, 300, 250);
primaryStage.setScene(scene);
primaryStage.show();
}
public static void main(String[] args) {
launch(args);
}
}

If the PDF file is in the same package as the caller file (as you state), then
getHostServices().showDocument(getClass()
.getResource("computer_graphics_tutorial.pdf").toString());
should solve the problem.
The getResource method can be used really flexibly to locate files. Here is a small description how to use it: JavaFX resource handling: Load HTML files in WebView.