JavaFX，为什么TableColumn为null？

Question

As you will see I am a massive noob when it comes to Java and Javafx. I have spent a lot of time reading around (various forum posts, and tuts) and trying to figure out myself where I am getting this issue but it has come to the point for me to post for feedback from someone who knows their business.

When replying, please could you take the time to also explain why something isn't working and some general pointers? Here is what I have so far (my FXML and my two classes) any pointers would be fantastic!!

My FXML;

<Pane id="myScene" fx:id="myScene" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity"
      prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1"
        fx:controller="sample.TestController">
   <children>
      <TableView id="employeesTable" fx:id="employeesTable" layoutX="131.0" layoutY="64.0" prefHeight="200.0" prefWidth="360.0">
        <columns>
          <TableColumn id="colFirstName" fx:id="colFirstName" prefWidth="75.0" text="First Name" />
          <TableColumn id="colLastName" fx:id="colLastName" prefWidth="75.0" text="Last Name" />
            <TableColumn id="colEmail" fx:id="colEmail" prefWidth="75.0" text="email" />
        </columns>
      </TableView>
   </children>
</Pane>

Now the Employee class I have;

public class Employee {

private StringProperty firstName;
private StringProperty lastName;
private StringProperty email;


public Employee(String a, String b, String c) {
    this.firstName = new SimpleStringProperty(a);
    this.lastName = new SimpleStringProperty(b);
    this.email = new SimpleStringProperty(c);
}

public Employee() {
}


public String getFirstName() {
    return firstName.get();
}

public StringProperty firstNameProperty() {
    return firstName;
}

public void setFirstName(String firstName) {
    this.firstName.set(firstName);
}

public String getLastName() {
    return lastName.get();
}

public StringProperty lastNameProperty() {
    return lastName;
}

public void setLastName(String lastName) {
    this.lastName.set(lastName);
}

public String getEmail() {
    return email.get();
}

public StringProperty emailProperty() {
    return email;
}

public void setEmail(String email) {
    this.email.set(email);
}

}

And finally the class for my test controller is

public class TestController {
public Label login;
public TextField loginUserName;
public PasswordField loginPassword;
public TextField testOutput;
@FXML TableView<Employee> employeesTable;
@FXML TableColumn<Employee, String> colFirstName;
@FXML TableColumn<Employee, String> colLastName;
@FXML TableColumn<Employee, String> colEmail;
@FXML Pane myScene;

//public javafx.scene.control.TableView employeesTable;
private ObservableList<Employee> myData;
private MainController MainController;

public void loadEmployeeForm(ActionEvent actionEvent) throws IOException, SQLException, ClassNotFoundException {
    myData = FXCollections.observableArrayList(DBCON.getEmployees());
    System.out.println(myData.size());
    Parent root = FXMLLoader.load(getClass().getResource("frmEmployees.fxml"));
    Scene myScene = new Scene( root );
    sample.MainController.setScene(myScene);
    colFirstName.setCellValueFactory(new PropertyValueFactory<Employee, String>("firstName"));
    colLastName.setCellValueFactory(new PropertyValueFactory<Employee, String>("lastName"));
    colEmail.setCellValueFactory(new PropertyValueFactory<Employee, String>("email"));
    employeesTable.setItems(null);
    employeesTable.setItems(myData);
    employeesTable.setVisible(true);
}

I get a null pointer exception when I go to set colFirstName to the property value factory which make me think I haven't initialized something somewhere but I am utterly clueless on how to go about adding that.

If I add in lines such as; TableColumn colFirstName = new TableColumn("firstName");

for each of my columns and the tablename it works (ie it doesn't throw a load of error messages at me) that way but then I don't get any data loading into the tableview because I think that's me creating a new tableView not using the one generated from the FXML?

I have a feeling it will be very simple, but as I said I am a massive noob and any points would be much obliged.

Thanks Mark

Update 1;

The method for load employee form is called from a button on myMain.fxml;

    <GridPane alignment="CENTER" hgap="10" prefHeight="300" prefWidth="300" vgap="10" xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/8"
          fx:controller="sample.TestController" stylesheets="/sample/myFirst.css">
    <children>
       <Button onAction="#login" text="Login" GridPane.halignment="CENTER" GridPane.rowIndex="5" GridPane.valignment="CENTER" />
        <Button text="GoEmployees" onAction="#loadEmployeeForm" GridPane.halignment="CENTER" GridPane.rowIndex="3" GridPane.valignment="CENTER" />
       <Label fx:id="login" GridPane.rowIndex="1" />
      <Label text="UserName" GridPane.columnIndex="0" GridPane.rowIndex="1" />
      <Label text="Password" GridPane.columnIndex="0" GridPane.rowIndex="2" />
      <TextField fx:id="loginUserName" GridPane.rowIndex="1" GridPane.columnIndex="1" />
      <PasswordField fx:id="loginPassword" GridPane.rowIndex="2" GridPane.columnIndex="1" blendMode="OVERLAY" />
        <TextField fx:id="testOutput" GridPane.rowIndex="4" GridPane.columnIndex="0" GridPane.columnSpan="3" />
   </children>
   <columnConstraints>
      <ColumnConstraints prefWidth="125.0" />
   </columnConstraints>
   <rowConstraints>
      <RowConstraints prefHeight="50.0" />
   </rowConstraints>
   <padding>
      <Insets bottom="10.0" left="9.0" right="10.0" top="10.0" />
   </padding>

</GridPane>

Is having my testController control two different FXMLs a problem/a no go?

Answer 1

When the FXMLLoader tries to load an fxml file, it will create new instance of the controller class defined with fx:controller in fxml file. Then it creates and maps the @FXML annotated fields with fx:id components in fxml file. Finally, it calls the controller's initialize() method. You can get the instantiated controller with fxmlloader.getController() after fxmlloader.load().

According to this basic work flow, the pitfall in your code is:
myMain.fxml's controller is TestController, but myMain.fxml does not contain TableColumns with fx:id colFirstName etc. So these fields are null, when the myMain.fxml has been loaded. As a result, there will be NPE in loadEmployeeForm() while trying to use these fields.

Move the TableView and TableColumns to frmEmployees.fxml's controller, and configure them (setCellValueFactory, initial data etc.) in this controller's initialize() method.

Answer 2

You never use your Employee(String, String, String) constructor. This constructor initializes the firstName , lastName , and email . Otherwise, your references will point to nothing.