尝试从textField检索值并将其设置为标签的文本时，得到NPE

Question

StudenDetails类

import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.Node;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.TextField;
import javafx.stage.Stage;

import java.io.IOException;

public class StudentDetails {
    @FXML
    private TextField firstNameField;
    @FXML
    private TextField lastNameField;
    @FXML
    private Button okButton;

    private String firstName;



    @FXML
    private void handleOkButtonOnAction(ActionEvent event) throws IOException {
        if (firstNameField.getText().trim().isEmpty() || lastNameField.getText().trim().isEmpty()){
            firstNameField.setStyle("-fx-border-color: red");
            lastNameField.setStyle("-fx-border-color: red");
        }else {
                Parent qualification = FXMLLoader.load(getClass().getResource("qualification.fxml"));
                Scene qualificationScene = new Scene(qualification, 1050, 600);
                Stage qualificationStage = (Stage) ((Node) event.getSource()).getScene().getWindow();
                qualificationStage.setResizable(false);
                qualificationStage.setScene(qualificationScene);
                qualificationStage.show();

                firstNameField.setStyle("-fx-border-color: darkgray");
                lastNameField.setStyle("-fx-border-color: darkgray");
                firstName = firstNameField.getText().trim();
                System.out.println(firstNameField.getText().trim() + " h");
            }
    }

public String getFirstName() {
        System.out.println(firstName + "get");
        return firstName;
    }

}

StudentDetails.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.Label?>
<?import javafx.scene.control.TextField?>
<?import javafx.scene.layout.GridPane?>

<?import javafx.scene.control.Button?>
<GridPane fx:controller="sample.StudentDetails" id="studentGridPane"
          xmlns:fx="http://javafx.com/fxml" alignment="center" hgap="10" vgap="10" stylesheets="@styleCareer.css">

    <Label id="studentHead" text="Please enter your details as appropriate" alignment="CENTER" GridPane.rowIndex="0" GridPane.columnSpan="2"/>

    <Label text="First Name" GridPane.columnIndex="0" GridPane.rowIndex="1"/>
    <TextField fx:id="firstNameField" promptText="Enter your first name" GridPane.columnIndex="1" GridPane.rowIndex="1" />

    <Label text="Last Name" GridPane.columnIndex="0" GridPane.rowIndex="2" />
    <TextField fx:id="lastNameField" promptText="Enter your last name" GridPane.columnIndex="1" GridPane.rowIndex="2" />

    <Button text="OK" GridPane.columnIndex="1" onMouseEntered="#handleOkButtonOnMouseEntered" onMouseExited="#handleOkButtonOnMouseExited" onAction="#handleOkButtonOnAction"
            fx:id="okButton" GridPane.rowIndex="6" id="studentButton" alignment="BOTTOM_RIGHT" GridPane.halignment="RIGHT"/>

</GridPane>

资格等级

public class Qualification {

    @FXML
    private Label setName;

 public void initialize() {
        StudentDetails studentDetails = new StudentDetails();
        setName.setText(studentDetails.getFirstName());
        System.out.println(studentDetails.getFirstName() + " Q");
     }

}

Qualification.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.Button?> <?import javafx.scene.control.CheckBox?> <?import javafx.scene.control.Label?> <?import javafx.scene.layout.GridPane?>

<GridPane id="qualification" hgap="10" vgap="10" xmlns="http://javafx.com/javafx/8.0.121" xmlns:fx="http://javafx.com/fxml/1"
          fx:controller="sample.Qualification" stylesheets="@styleCareer.css" alignment="CENTER">

      <Label fx:id="setName" GridPane.halignment="LEFT" GridPane.rowIndex="0" GridPane.columnIndex="0" text=""/>

问题：

  

错误不断出现   “由：java.lang.NullPointerException       在sample.StudentDetails。（StudentDetails.java:23）       在sun.reflect.NativeConstructorAccessorImpl.newInstance0（本机方法）处       在sun.reflect.NativeConstructorAccessorImpl.newInstance（NativeConstructorAccessorImpl.java:62）       在sun.reflect.DelegatingConstructorAccessorImpl.newInstance（DelegatingConstructorAccessorImpl.java:45）       在java.lang.reflect.Constructor.newInstance（Constructor.java:423）       在java.lang.Class.newInstance（Class.java:442）       在sun.reflect.misc.ReflectUtil.newInstance（ReflectUtil.java:51）       在javafx.fxml.FXMLLoader $ ValueElement.processAttribute（FXMLLoader.java:927）       在javafx.fxml.FXMLLoader $ InstanceDeclarationElement.processAttribute（FXMLLoader.java:971）       在javafx.fxml.FXMLLoader $ Element.processStartElement（FXMLLoader.java:220）       在javafx.fxml.FXMLLoader $ ValueElement.processStartElement（FXMLLoader.java:744）       在javafx.fxml.FXMLLoader.processStartElement（FXMLLoader.java:2707）       在javafx.fxml.FXMLLoader.loadImpl（FXMLLoader.java:2527）       ...还有66个”

当尝试从StudentDetailClass中的textField检索值以及将值设置为限定类中的setName标签的文本时，我得到NPE（空指针异常）。