JAVA - Getting a Date from JDatePicker - class java.util.GregorianCalendar cannot be cast to class java.util.Date

Question

I'm building a very simple expense tracker:

public class ExpenseTracker {

JFrame frame;
JPanel mainPanel;
JPanel inputPanel;
JPanel registerPanel;
DefaultTableModel tableModel;
JDatePicker datePicker;
JTextField nameTextField;
JTextField amountTextField;
SimpleDateFormat sdf;
Date date;


public static void main(String[] args) {
    new ExpenseTracker().buildGUI();
}

public void buildGUI() {

    mainPanel = new JPanel();
    inputPanel = new JPanel();
    inputPanel.setLayout(new BoxLayout(inputPanel, BoxLayout.Y_AXIS));
    
    JLabel nameLabel = new JLabel("Name: ");
    nameLabel.setFont(new Font("Arial", Font.ITALIC, 25));
    nameTextField = new JTextField();
    nameTextField.setDocument(new JTextFieldLimit(10));
    JLabel dateLabel = new JLabel("Date: ");
    dateLabel.setFont(new Font("Arial", Font.ITALIC, 25));
    datePicker = new JDatePicker();


    JLabel amountLabel = new JLabel("Amount: ");
    amountLabel.setFont(new Font("Arial", Font.ITALIC, 25));
    amountTextField = new JTextField();
    amountTextField.setDocument(new JTextFieldLimit(5));

    
    JButton addExpense = new JButton("Add");
    addExpense.addActionListener(new addExpenseListener());

    inputPanel.add(nameLabel);
    inputPanel.add(nameTextField);
    inputPanel.add(dateLabel);
    inputPanel.add(datePicker);
    inputPanel.add(amountLabel);
    inputPanel.add(amountTextField);
    inputPanel.add(addExpense);
    
    
    registerPanel = new JPanel();
    tableModel = new DefaultTableModel();
    JTable register = new JTable(tableModel);
    tableModel.addColumn("Name");
    tableModel.addColumn("Date");
    tableModel.addColumn("Amount");
    register.setBackground(Color.LIGHT_GRAY);
    registerPanel.add(register);
    
    
    
    mainPanel.add(BorderLayout.NORTH, inputPanel);
    mainPanel.add(BorderLayout.CENTER, registerPanel);
    frame.add(mainPanel);
    frame.setVisible(true);
    
}


public void addToRegister() {
    String name = nameTextField.getText();
    sdf = new SimpleDateFormat();
    Date dateToFormat = (Date) datePicker.getModel().getValue();
    String date = sdf.format(dateToFormat);
    String amount = amountTextField.getText();
    tableModel.addRow(new Object[] {name, date, amount});
}

Leaving out a bit of code. I get the exception:

class java.util.GregorianCalendar cannot be cast to class java.util.Date (java.util.GregorianCalendar and java.util.Date are in module java.base of loader 'bootstrap')

and I don't know how do I change this. I know the thing is how to get the date from the DatePicker and the class clash there. Using jdatepicker libraries.

Answer 1

java.time

The java.util Date-Time API and their formatting API, SimpleDateFormat are outdated and error-prone. It is recommended to stop using them completely and switch to the modern Date-Time API.

First, make the following change (check Calendar#getTime to learn more)

Date dateToFormat = datePicker.getModel().getValue().getTime();

and then switch to java.time API as follows:

Instant instant = dateToFormat.toInstant();
ZonedDateTime zdt = instant.atZone(ZoneId.systemDefault());

// A sample formatting
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss", Locale.ENGLISH);
String formattedDateStr = zdt.format(formatter);

Now, you can use formattedDateStr in your components.

An alternative way to get ZonedDateTime can be as suggested by Ole V.V.:

GregorianCalendar selected = (GregorianCalendar) datePicker.getModel().getValue();
ZonedDateTime zdt = selected.toZonedDateTime();

Learn more about the modern Date-Time API from Trail: Date Time.