How to transport information from one class to another in order to show it without using static?

Question

I am programming an application that deals with orders from a database. It has several pages, a navigation, a header that always should show information about the actual order you are working with and a content area, in which the details of said order get shown:

My MainProgram extends a JFrame and contains a CardLayout, in which the other pages are hosted, so when the user clicks on the page in the navigation, only the view of the content-area changes. Logo, header and the navigation stay the same. The header keeps displaying the order number.

As there are several different pages that contain details about the same order, I need to "send / transfer" information about the order from one page to the other so I can show some information in the header and in the content area from the order object.

But I am not getting this to work as intended, mostly to my misunderstand of static and when to use it, where objects get created exactly and also the complexity of my program: I am using a class that is intended for the navigation and therefore should also handle the information transfer from one page to the other.

Since I am using a database, creating a MVCE will be hard, so instead I will show the important parts of my program.

---

MainProgram.java

Here the navigation and the content panel (centerPanel) get created, also the CardLayout. centerPanel and the CardLayout are static, so I can call this from other classes and switch the page that is shown (probably not a good idea?):

NavigationPanel navigationPanel = new NavigationPanel();

public static JPanel centerPanel = new JPanel();
public static CardLayout contentCardsLayout = new CardLayout();

I create the pages and put them into my CardLayout:

OverviewPage overviewPage = new OverviewPage();
BasicDataPage basicDataPage = new BasicDataPage();

centerPanel.setLayout(contentCardsLayout);

overviewPage.setName("overviewPage");
basicDataPage.setName("basicDataPage");

centerPanel.add(overviewPage, "overviewPage");
centerPanel.add(basicDataPage, "basicDataPage");

The main method, where I create a MainProgram object:

public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                UIManager.setLookAndFeel("com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel");
                MainProgram window = new MainProgram();
                window.setVisible(true);
                window.initialize();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}

---

OverviewPage.java

The overview page contains a JTable which gets populated from a database. If the user double-clicks an entry, he gets transfered to the BasicDataPage where he can see the details of the order.

But in order to show the details, I need to somehow transfer the information of the order object into the target class and thats the point I am struggling with!

// I tried several things like object into constructor, static object, creating a method etc...
if (mouseEvent.getClickCount() == 2 && row != -1) {
    String workNumberOfOrderObject = (String) table.getValueAt(row, 0);
    OrderObject orderObject = GetOrderObject.getOrderObjectFromDatabase(workNumberOfOrderObject);
    BasicDataPage basicDataPage = new BasicDataPage();
    basicDataPage.recieveOrderObject(orderObject);
    workNumberPanel.recieveOrderObject(orderObject);
    workNumberPanel.setTxtWorkNumber(workNumberOfOrderObject);
    MainProgram.contentCardsLayout.show(MainProgram.centerPanel, "basicDataPage");
}

I tried "sending" the order object to the BasicDataPage via the constructor and set the text in the JTextFields in the BasicDataPage accordingly. This did not work, the textfields simply stayed empty altough I can System.out.println(orderObject.toString()) the recieved object.

---

BasicDataPage.java

I also tried creating a method receiveOrderObject that I use in the OverviewPage, which should set the textfields of the basicDataPage AND the workNumberPanel, but the fields stay empty:

WorkNumberPanel workNumberPanel = new WorkNumberPanel();

JTextField txtCarWidth = new JTextField(TEXTFIELD_LENGTH);
JTextField txtCarDepth = new JTextField(TEXTFIELD_LENGTH);
JTextField txtCarHeight = new JTextField(TEXTFIELD_LENGTH);

public void recieveOrderObject(OrderObject orderObject){
    txtCarDepth.setText(orderObject.getCar_depth());
}

Before posting my question I've read several Q/As here on SO like this:

Accessing UUID from another class in Java ... suggesting to use static for global variables.

I know that static variables are class variables, that all instances can use and only one version exists of. So I tried to send a static object from one class to the other.

But since I am using JTextFields, I had to mix static and non-static content, which either did not work at all or the textfields disappeared.

I have the feeling that I am getting a very basic concept in java wrong, so any help, no matter in which direction, is appreciated!

EDIT:

Based on Reşit Dönüks answer, I was able to fill the textfields by making BasicDataPage and loadBasicData(orderObject) in MainProgram static. Now I can do MainProgram.loadBasicData(orderObject); ... and the textfields in the BasicDataPage get filled as intended.

Is this a valid approach or do I get problems for using static for GUI-Elements? ..... Don't!

Answer 1

I realized that, your are creating BasicDataPage in each double click.

if (mouseEvent.getClickCount() == 2 && row != -1) {
    String workNumberOfOrderObject = (String) table.getValueAt(row, 0);
    OrderObject orderObject = GetOrderObject.getOrderObjectFromDatabase(workNumberOfOrderObject);
    BasicDataPage basicDataPage = new BasicDataPage();

This is the main problem. Do not create BasicDataPage there, just reach the created instance and set the order object to that. My solution is below.

public class MainProgram implements OrderView{

    //remove statics here
    private JPanel centerPanel = new JPanel();
    private CardLayout contentCardsLayout = new CardLayout();

    private BasicDataPage basicPage;

    public MainProgram() {
        //other codes
        OverviewPage overviewPage = new OverviewPage();
        basicPage = new BasicDataPage();

        centerPanel.setLayout(contentCardsLayout);

        overviewPage.setName("overviewPage");
        basicDataPage.setName("basicDataPage");

        centerPanel.add(overviewPage, "overviewPage");
        centerPanel.add(basicPage, "basicDataPage");
        //oher codes
    }

    @Override
    public void loadOrder(OrderObject order) {
        basicPage.recieveOrderObject(orderObject);

        contentCardsLayout.show(centerPanel, "basicDataPage");
    }
}

public interface OrderView {

    public void loadOrder(OrderObject order);
}

public class OverviewPage {

    OrderView orderView;

    public OverviewPage(OrderView orderView) {
        this.orderView = orderView;
    }

    //in ActionPerformed
    if (mouseEvent.getClickCount() == 2 && row != -1) {
        String workNumberOfOrderObject = (String) table.getValueAt(row, 0);
        OrderObject orderObject = GetOrderObject.getOrderObjectFromDatabase(workNumberOfOrderObject);
        orderView.loadOrder(orderObject);
        workNumberPanel.recieveOrderObject(orderObject);
        workNumberPanel.setTxtWorkNumber(workNumberOfOrderObject);
    }
}

Answer 2

As pointed already, Singleton is the way to go. I would just like to point out a mistake in the code provided in the answer before.

private static MainFrameinstance = null;

Rename MainFrameinstance to instance or vice-versa; because the same variable is checked by the getInstance() method.