JavaFX TableView how to get cell's data?

Assuming you know something is selected, you can do

TablePosition pos = table.getSelectionModel().getSelectedCells().get(0);
int row = pos.getRow();

// Item here is the table view type:
Item item = table.getItems().get(row);

TableColumn col = pos.getTableColumn();

// this gives the value in the selected cell:
String data = (String) col.getCellObservableValue(item).getValue();

Assuming you haven't selected any row but know where and what you want....

// Item here is the table view type:
    Unpaid item = userTable.getItems().get(0);

    TableColumn col = userTable.getColumns().get(3);

    // this gives the value in the selected cell:
    String data = (String) col.getCellObservableValue(item).getValue();
    JOptionPane.showMessageDialog(null, data);