Java Swing: Error when dynamically adding JScrollpane to JPanel

Question

Update: If I move the code to be within the mouseclick event (ie instead of calling initMarksScreen() I just put the code directly in) it works as expected. So my problem is calling the code in its own method. Does this mean if I want to perform the same steps at another point or on a different button that I have to have the code directly in there instead of in a method I can call?

---

I am fairly new to Java and trying to create a button that adds a JScrollPane which contains a JTable.

It is called by:

    public void mouseClicked(java.awt.event.MouseEvent evt) {
            initMarksScreen();
        }

The code is:

public final void initMarksScreen() {
    String[] columnNames = {"Student ID",
                            "Last Name",
                            "Firstname",
                            "Status",
                            "Degree",
                            "Candidate No.",
                            "Stage",
                            "Year",
                            "Code",
                            "Title",
                            "Grade Mode",
                            "Mark",
                            "Result"};
    Object[][] data = {
            {"100123456", "Cooper","Sheldon", "Signed Up", "BSc Physics","1201234","1","12","PH1001","Blackholes and Revelations","D",new Integer(99),"P"},
            {"100123456", "Cooper","Sheldon", "Signed Up", "BSc Physics","1201234","1","12","PH1025","Astrophysics","D",new Integer(95),"P"}
        };
    JTable tMarks = new JTable(data, columnNames);
    JScrollPane scrollPane = new JScrollPane(tMarks);
    tMarks.setFillsViewportHeight(true);
    panelCentral.add(scrollPane); --!!ERROR AT THIS LINE!!
    panelCentral.revalidate();
}   

The panel is declared as:

private static final Container panelCentral = null;

As I'm quite new to Java I imagine there is an easy fix but if I need to add an SSCCE.

Everything displays fine until I click the button then the error I get is:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at GUI.mainScreen.initMarksScreen(mainScreen.java:312)
at GUI.mainScreen$2.mouseClicked(mainScreen.java:183)
at java.awt.AWTEventMulticaster.mouseClicked(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

if panelCentral is JPanel and its not initialized, initialize before add

 panelCentral = new JPanel();
 panelCentral.add(scrollPane);