PropertyValueFactory cannot read from unreadable property

[Luna Cruz]

Hello again,

Thank you for the all the replies on my previous post, I managed to make some progress with my application but I found myself stuck once again. So I'm working on this student management application that stores and retrieves data from a database. I have 2 tables, students and modules. My first table 'Students' was working as intended, I was able to get the user to input student details and store them on the database and then display the table to the user using a TableView, now I added the second table 'Modules' and while I'm still able to insert data onto the table, when I try to display it back to the user using the same method as I do for table now, the student ID column is not displaying any data and I'm getting that annoying PropertyValueFactory error again. I don't know if this is relevant but student_id is a foreign key from students table. I feel like I mistyped something somewhere but I triple checked my code and I don't seem to find anything. The student_id is no longer displaying anything on the student tableView either since I added the second table insert statements, I almost forgot to mention.

My code
Student Class

public class Student {
    protected StringProperty student_id;
    private StringProperty name;
    private SimpleObjectProperty<LocalDate> dateOfBirth;
    private SimpleIntegerProperty semester;

    public Student(String student_id, String name, LocalDate dateOfBirth, int semester) {
        this.student_id = new SimpleStringProperty(student_id);
        this.name = new SimpleStringProperty(name);
        this.dateOfBirth = new SimpleObjectProperty<>(dateOfBirth);
        this.semester = new SimpleIntegerProperty(semester);
    }

    public Student() {
    	
    }

Module Class

Student student = new Student();
	
	protected StringProperty student_id = student.student_id;
	private StringProperty code;
	private StringProperty module_name;
	private SimpleIntegerProperty semester;
	private StringProperty grade;
	
	public Module(String student_id, String code, String module_name, String grade, int semester) {
		this.student_id = new SimpleStringProperty(student_id);
		this.code = new SimpleStringProperty(code);
        this.module_name = new SimpleStringProperty(module_name);
        this.semester = new SimpleIntegerProperty(semester);
        this.grade = new SimpleStringProperty(grade);
    }
	
	public Module() {
        
    }

The show modules method

public void showTable() throws SQLException {
        TableColumn<Module, String> idColumn = new TableColumn<>("Student ID");
        idColumn.setCellValueFactory(new PropertyValueFactory<>("student_id"));

        TableColumn<Module, String> codeColumn = new TableColumn<>("Module Code");
        codeColumn.setCellValueFactory(new PropertyValueFactory<>("code"));

        TableColumn<Module, LocalDate> nameColumn = new TableColumn<>("Module Name");
        nameColumn.setCellValueFactory(new PropertyValueFactory<>("module_name"));
        
        TableColumn<Module, Integer> gradeColumn = new TableColumn<>("Grade");
        gradeColumn.setCellValueFactory(new PropertyValueFactory<>("grade"));

        TableColumn<Module, Integer> semesterColumn = new TableColumn<>("Semester");
        semesterColumn.setCellValueFactory(new PropertyValueFactory<>("semester"));

        Main.showRecordsTable.getColumns().addAll(idColumn, codeColumn, nameColumn, gradeColumn, semesterColumn);  
       
        /*
         * "A generic array of tableColumn is created for a varargs parameter" 
			I don't understand what this means but the table is displaying correctly.
         * 
         */
        
        Main.showRecordsTable.getItems().clear();
        Main.showRecordsTable.getItems().addAll(getAllModules());

    }

The error message

May 11, 2023 10:27:58 PM javafx.scene.control.cell.PropertyValueFactory getCellDataReflectively
WARNING: Can not retrieve property 'student_id' in PropertyValueFactory: javafx.scene.control.cell.PropertyValueFactory@3e99404d with provided class type: class application.Module
java.lang.IllegalStateException: Cannot read from unreadable property student_id
	at javafx.base@20/com.sun.javafx.property.PropertyReference.get(PropertyReference.java:167)
	at javafx.controls@20/javafx.scene.control.cell.PropertyValueFactory.getCellDataReflectively(PropertyValueFactory.java:183)
	at javafx.controls@20/javafx.scene.control.cell.PropertyValueFactory.call(PropertyValueFactory.java:153)
	
	...
	at javafx.graphics@20/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:185)
	at java.base/java.lang.Thread.run(Thread.java:833)

[Lou Hamers]

   protected StringProperty student_id = student.student_id;
   private StringProperty code;
   private StringProperty module_name;
   private SimpleIntegerProperty semester;
   private StringProperty grade;


Do all of these fields in Module have public getters? I believe the default behavior should be able to find those automatically. Also since you're using a StringProperty, you MIGHT need that to return a plain String instead of the property (I forget). Usually FX property getters are named something like getStudentIdProperty() if I'm recalling that convention correctly.

(Also student_id would normally be named studentId to follow more standard camelCase Java coding conventions, and same idea for module_name.)

[Lou Hamers]

Yeah, or name it studentIdProperty() (after refactoring student_id to studentId). See the Javadoc below which explains what it's doing. The getStudentId() is just a fallback so I'd just return the property object you've got.

https://docs.oracle.com/javase/8/javafx/api/javafx/scene/control/cell/PropertyValueFactory.html

In this example, the "firstName" string is used as a reference to an assumed firstNameProperty() method in the Person class type (which is the class type of the TableView items list). Additionally, this method must return a Property instance. If a method meeting these requirements is found, then the TableCell is populated with this ObservableValue. In addition, the TableView will automatically add an observer to the returned value, such that any changes fired will be observed by the TableView, resulting in the cell immediately updating.

If no method matching this pattern exists, there is fall-through support for attempting to call get<property>() or is<property>() (that is, getFirstName() or isFirstName() in the example above). If a method matching this pattern exists, the value returned from this method is wrapped in a ReadOnlyObjectWrapper and returned to the TableCell. However, in this situation, this means that the TableCell will not be able to observe the ObservableValue for changes (as is the case in the first approach above).