由null root引起的Application start方法中的异常

Question

我的代码给了我这个错误。现在，根据我对读取堆栈跟踪的有限知识，错误是由这行代码引起的：

primaryStage.setScene(new Scene(vb, 400, 300));`

错误是＆＃34; Root不能为空＆＃34;。由于我是初始化的，因此根目录为vb，我不明白它是如何为空的。任何帮助将不胜感激。

  import javafx.application.Application;
    import javafx.beans.value.ChangeListener;
    import javafx.beans.value.ObservableValue;
    import javafx.scene.Scene;
    import javafx.scene.control.Button;
    import javafx.scene.control.ComboBox;
    import javafx.scene.control.Label;
    import javafx.scene.control.ProgressBar;
    import javafx.scene.control.RadioButton;
    import javafx.scene.control.Slider;
    import javafx.scene.control.TextField;
    import javafx.scene.layout.VBox;
    import javafx.stage.Stage;
    import javafx.beans.value.ObservableValue;
    import javafx.scene.control.Toggle;
    import javafx.scene.control.ToggleGroup;
    import javafx.scene.layout.GridPane;
    import javafx.scene.layout.HBox;
    import javafx.event.ActionEvent;
    import javafx.event.EventHandler;
    import java.text.DecimalFormat;
// class definition
public class Projectile extends Application {

// init method
public void init() {

    // Projectile Type - ComboBox
    VBox vb = new VBox();
    projectile_type_combobox = new ComboBox<String>();
    projectile_type_label = new Label("Projectile Type");
    projectile_type_combobox.getItems().addAll("Human", "Piano");

    // Inital Speed ToggleGroup
    initial_speed_toggleGroup = new ToggleGroup();
    label_display = new Label("Radio Button 1 selected");
    initial_speed_slow = new RadioButton("Slow");
    initial_speed_medium = new RadioButton("Medium");
    initial_speed_fast = new RadioButton("Fast");

    initial_speed_slow.setToggleGroup(tg_radiobutton_group);
    initial_speed_slow.setToggleGroup(tg_radiobutton_group);
    initial_speed_fast.setToggleGroup(tg_radiobutton_group);

    // use the .setUserData command of the radio button to store speeds
    initial_speed_slow.setUserData("10");
    initial_speed_medium.setUserData("50");
    initial_speed_fast.setUserData("70");

    // Mass
    mass_textField = new TextField();
    mass_label = new Label("Mass(kgs)");

    // Angle
    angle_textField = new TextField();
    angle_label = new Label("Angle(Degrees)");

    // Speed
    intitial_speed_textField = new TextField();
    initial_speed_label = new Label("Initial Speed");

    // Range
    range_textField = new TextField();
    range_label = new Label();

    // Height
    height_textField = new TextField();
    height_label = new Label();

    // Time
    time_label = new Label();
    time_textField = new TextField();

    // Button
    fire_button = new Button("Fire");
    erase_button = new Button("Erase");

    // Prevent the following TextFields from being editable: angle,intial
    // speed range, height, time
    // setEditable(false);

    // Layout controls as per the diagram, feel free to improve the UI.
    // How many rows and columns do you want - work this out on paper first
    // My version has 7 rows, you can look at the JavaFX API to see how to
    // get controls to span more than one column

    gp = new GridPane();
    gp.addRow(0, projectile_type_label, projectile_type_combobox);
    gp.addRow(1, mass_textField, mass_label);
    gp.addRow(2, angle_label, angle_textField);
    gp.addRow(3, initial_speed_label, intitial_speed_textField);
    gp.addRow(4, range_label, range_textField);
    gp.addRow(5, height_label, height_textField);
    gp.addRow(6, time_label, time_textField);
    gp.addRow(7, fire_button, erase_button);

    // Method call (not declaration!) to initialize the controls based on
    // the projectile type.
    initalizeControlValues();



// Listener for angle Slider to set angle TextTield and the angle
        // variable
        angle_slider.valueProperty().addListener(new ChangeListener<Number>() {
            public void changed(final ObservableValue<? extends Number> observable, final Number oldValue,
                    final Number newValue) {
                angle = (double) newValue;
                angle_textField.setText("" + newValue);

            }

    });

    // Listener for inital_speed ToggleGroup to set initital_speed TextField
    this.initial_speed_toggleGroup.selectedToggleProperty().addListener(new ChangeListener<Toggle>() {
        public void changed(ObservableValue<? extends Toggle> ov, Toggle toggle, Toggle new_toggle) {

            intitial_speed_textField.setText("" + new_toggle);

        }
    });

    vb.getChildren().addAll(projectile_type_combobox, projectile_type_label, label_display, initial_speed_slow,
            initial_speed_medium, initial_speed_fast, mass_textField, mass_label, angle_textField, angle_label,
            intitial_speed_textField, initial_speed_label, range_textField, range_label, height_textField,
            height_label, time_label, gp, time_textField, fire_button, erase_button);





    // Listener to call the fire() method when the fire button is pressed

    fire_button.setOnAction(new EventHandler<ActionEvent>() {

        @Override
        public void handle(ActionEvent event) {
            fire();
        }
    });

    // Listener to initialize control values if the projectile type is
    // changed

    // Listener to initialize control values if the erase button is pressed
    // erase_button.setOnAction(new EventHandler<ActionEvent>() {

    // @Override
    // public void handle(ActionEvent event) {
    // erase();
    // }
    // });
}

// Overridden start method
@Override
public void start(Stage primaryStage) {

    primaryStage.setTitle("Projectile");

    primaryStage.setScene(new Scene(vb, 400, 300));
    primaryStage.show();

}

// Overridden stop method add functionality to this if you wish.
public void stop() {

}

// Entry point to our program
public static void main(String[] args) {
    launch(args);

}

// Method to harvest values from controls, perform calculation and display
// the results
private void fire() {
    // capture the values from the text fields outputting number errors
    // where relevant

    // don't forget to convert your angle input to radians for use with
    // Math.sin()

    // calculate the range of the projectile
    range = ((initial_speed * initial_speed) / gravitational_accelleration) * Math.sin(2 * angle);

    // calculate the flight time of the projectile
    time = ((2 * initial_speed) * Math.sin(angle)) / gravitational_accelleration;

    // calculate the max height of the projectile
    height = ((initial_speed * initial_speed) * Math.sin(angle)) / 2 * gravitational_accelleration;

    // display the results in the relevant TextFields

}

private void erase() {

    if (projectile_type_combobox.getValue() == "Human") {

        mass = 80;

        // Set slider scale 0 to 90, set slider value to 45 and ticks to
        // units
        angle_slider = new Slider(0, 45, 90);
        angle_slider.setShowTickMarks(true);
        angle_slider.setShowTickLabels(true);
        angle_slider.setMajorTickUnit(15);
        angle_slider.setBlockIncrement(0.5);

        // initalize the intital speed to fast

        this.initial_speed_fast.setSelected(true);
        this.intitial_speed_textField.setText((String) this.initial_speed_fast.getUserData());

    }

    else {
        // inital the mass to 400kg
        mass = 400;

        // Set slider scale 0 to 40, set slider value to 20 and ticks to 10
        // units
        angle_slider = new Slider(0, 20, 40);
        angle_slider.setShowTickMarks(true);
        angle_slider.setShowTickLabels(true);
        angle_slider.setMajorTickUnit(10);
        angle_slider.setBlockIncrement(0.5);
        // initalize the intial speed to slow
        this.initial_speed_slow.setSelected(true);
        this.intitial_speed_textField.setText((String) this.initial_speed_slow.getUserData());
    }

}

// Method to initalize the controls based on the selection of the projectile
// type
private void initalizeControlValues() {
    if (projectile_type_combobox.getValue() == "Human") {

        mass = 80;

        // Set slider scale 0 to 90, set slider value to 45 and ticks to
        // units
        angle_slider = new Slider(0, 45, 90);
        angle_slider.setShowTickMarks(true);
        angle_slider.setShowTickLabels(true);
        angle_slider.setMajorTickUnit(15);
        angle_slider.setBlockIncrement(0.5);

        // initalize the intital speed to fast

        this.initial_speed_fast.setSelected(true);
        this.intitial_speed_textField.setText((String) this.initial_speed_fast.getUserData());

    }

    else {
        // inital the mass to 400kg
        mass = 400;

        // Set slider scale 0 to 40, set slider value to 20 and ticks to 10
        // units
        angle_slider = new Slider(0, 20, 40);
        angle_slider.setShowTickMarks(true);
        angle_slider.setShowTickLabels(true);
        angle_slider.setMajorTickUnit(10);
        angle_slider.setBlockIncrement(0.5);
        // initalize the intial speed to slow
        this.initial_speed_slow.setSelected(true);
        this.intitial_speed_textField.setText((String) this.initial_speed_slow.getUserData());
    }
    // display ticks etc

    // clear the results fields and variables

    // The following variables SHOULD be initialized where appropriate as
    // declaring and
    // initializing separately is very verbose.

}

// Textfield

// Slider
private Label sli_display;
private Slider sli_slider;

// Radio Buttons
private RadioButton rb1, rb2, rb3;
private ToggleGroup tg_radiobutton_group;
private Label label_display;

private VBox vb;

// Layout
private GridPane gp;

// Projectile Type
private Label projectile_type_label;
private ComboBox<String> projectile_type_combobox;

// Mass
private Label mass_label;
private TextField mass_textField;
private double mass;

// Angle
private Label angle_label;
private Slider angle_slider;
private TextField angle_textField;
private double angle;
// Formating the values in the duration box
DecimalFormat df;

// Initial Speed
private Label initial_speed_label;
private ToggleGroup initial_speed_toggleGroup;
private RadioButton initial_speed_slow;
private RadioButton initial_speed_medium;
private RadioButton initial_speed_fast;
private TextField intitial_speed_textField;
private double initial_speed;

// Range
private Label range_label;
private TextField range_textField;
private double range;

// Height
private Label height_label;
private TextField height_textField;
private double height;

// Time
private Label time_label;
private TextField time_textField;
private double time;

// Gravity
private static final double gravitational_accelleration = 9.81; // m/s/s

// Calculate
private Button fire_button;
private Button erase_button;
}



Exception in Application start method
java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:389)
    at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:328)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.launcher.LauncherHelper$FXHelper.main(Unknown Source)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:917)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$155(LauncherImpl.java:182)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException: Root cannot be null
    at javafx.scene.Scene.<init>(Scene.java:336)
    at javafx.scene.Scene.<init>(Scene.java:223)
    at Projectile.start(Projectile.java:171)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$162(LauncherImpl.java:863)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$175(PlatformImpl.java:326)
    at com.sun.javafx.application.PlatformImpl.lambda$null$173(PlatformImpl.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$174(PlatformImpl.java:294)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
    ... 1 more
Exception running application Projectile

Answer 1

您没有初始化名为vb的字段：您在init()方法（也称为vb中声明了一个局部变量，并对其进行了初始化。

替换

VBox vb = new VBox();

与

vb = new VBox();