Exception in Application constructor while making a constructor

Question

When I am running the below mentioned code it is working

import javafx.application.Application;
public class Client {
public static void main(String[] args){
    Test t2 = new Test();
    Application.launch(t2.getClass(),args);
    }
}

where the test class is

package com.temp.com.serverclient;

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

public class Test extends Application {
@Override
public void start(Stage primaryStage) {
    primaryStage.setTitle("No Main");
    StackPane root = new StackPane();
    root.getChildren().add(new Label("It worked!"));
    primaryStage.setScene(new Scene(root, 300, 120));
    primaryStage.show();
    }
}

But if I am trying to add constructor,it is getting Exception in Application constructor,Error. The code is

package com.temp.com.serverclient;

import javafx.application.Application;

public class Client {
public static void main(String[] args){

    Test t1 = new Test("Pass this String to Constructor");
    Application.launch(t1.getClass(),args);
    }
}

Test class

package com.temp.com.serverclient;

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

public class Test extends Application {
String str;
public Test(String str) {
    this.str = str;
}
@Override
public void start(Stage primaryStage) {
    primaryStage.setTitle("No Main");
    StackPane root = new StackPane();
    root.getChildren().add(new Label("It worked!"));
    primaryStage.setScene(new Scene(root, 300, 120));
    primaryStage.show();
    }
 }

How Can I sovle the problem? I need to pass the String to collect information from previous class.

fabian · Accepted Answer · 2018-04-29 11:51:48Z

Application.launch always uses a public parameterless constructor to create a instance of the application class to launch. (It does not provide any benefit to create a instance in the main method BTW. Simply pass the class without creating a instance, i.e. Application.launch(Test.class, args);.)

In fact you can only pass String parameters to the new instance of your application class without using static members and it's done via the args parameter of Application.launch:

public class Client {
    public static void main(String[] args) {
        Application.launch(Test.class, "Pass this String to Constructor");
    }
}

public class Test extends Application {

    String str;

    @Override
    public init() {
        this.str = getParameters().getRaw().get(0);
    }

    @Override
    public void start(Stage primaryStage) {
        primaryStage.setTitle("No Main");
        StackPane root = new StackPane();
        root.getChildren().add(new Label("It worked!"));
        primaryStage.setScene(new Scene(root, 300, 120));
        primaryStage.show();
    }
}

Note that accessing the parameters property is also possible for the start method.

JavaFX 9 introduced a new possiblility: using Platform.startup but you need to handle the lifecycle of the application class yourself:

Application app = new Test("Pass this String to Constructor");
app.init();
Platform.startup(() -> {
    Stage stage = new Stage();
    try {
        app.start(stage);
    } catch (Exception ex) {
        throw new IllegalStateException(ex);
    }
});

This does not properly call the Application.stop method though. Also the parameters are not assigned.

vatbub · Accepted Answer · 2018-04-29 10:27:40Z

0

AFAIK, Application.launch creates a new instance of Test. Therefore, you need another way to get the value to the instance, e. g. using a static getter method in your Client class which is called from Test

answered Apr 29, 2018 at 10:27

vatbub

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