Java Jpanels and Jframe

I am completely disgusted right now.

I have a JFrame, and several complex JPanels that I want added to that JFrame, but I keep getting Null Exception errors.

I'm about to give up.
Who is Participating?
krakatoaConnect With a Mentor Commented:
As your code is not properly presented - i.e. inside code tags here on EE -  and difficult to see how the snippets fit together, it is nigh impossible to say what's going on. But one of the first things I would look at is whether you are calling the constructors in the right order - i.e are you sure you have an "myUpdateSegmentPanel" ready to "add()" a component to - as that would be a prime candidate for an NPE.
MarkLoveExExAuthor Commented:
Let me try to post some code segments.
All my programs start with a class called "StartHere":
In StartHere (below the class, but above "main", I declare a JFrame and 2 JPanels:
public static TopLevelFrame myTopLevelFrame;
public static BasinSelectorPanel myBasinSelectorPanel;
public static UpdateSegmentPanel myUpdateSegmentPanel;

Within main,
myTopLevelFrame = new TopLevelFrame();
myBasinSelectorPanel = new BasinSelectorPanel();
myUpdateSegmentPanel = new UpdateSegmentPanel();             ****Fails****, with Exception in thread "main" java.lang.NullPointerException

Now, for my TopLevelFrame JFrame:
public class TopLevelFrame extends JFrame implements ActionListener {
JTabbedPane tabbedPane = new JTabbedPane();

public TopLevelFrame() throws Exception {
super blah blah
setSize blah blah

These seem to work.  I get the tabs on the TopLevelFrame, but they have no contents
tabbedPane.addTab("LAST UPDATE", StartHere.myUpdateSegmentPanel);
tabbedPane.addTab("VIEW HYDROGRAPHS", StartHere.myBasinSelectorPanel);

Now, my UpdateSegmentPanel:
public class UpdateSegmentPanel extends JPanel implements ActionListener, ListSelectionListener {

DefaultListModel<String> locationId_model_Chowan = new DefaultListModel<String>();
JList<String> JList_Chowan = new JList<String>(locationId_model_Chowan);

public UpdateSegmentPanel() throws Exception {
StartHere.myUpdateSegmentPanel.add(new JScrollPane(JList_Chowan));    ****Fails****, I think the NullPointerException is coming from here
Free Tool: ZipGrep

ZipGrep is a utility that can list and search zip (.war, .ear, .jar, etc) archives for text patterns, without the need to extract the archive's contents.

One of a set of tools we're offering as a way to say thank you for being a part of the community.

MarkLoveExExAuthor Commented:
I've also tried to create a new Jpanel within TopLevelFrame:
JPanel panelHydrographViewer = new JPanel();

Then, add the StartHere.myBasinSelectorPanel to THAT jPanel.
I can't add it. Another Null Pointer Exception
MarkLoveExExAuthor Commented:
Should the constructors of the panels be constructed before the main JFrame that holds them?
Well if you try to add something to a component that doesn't exist, that will fail, as would adding something that doesn't exist to a component that does.

( I should add that your JFrame is probably not the one at fault, since if it isn't assigned then the compiler will tell you).
MarkLoveExExAuthor Commented:
You were correct.  I had to be more careful on the order of my constructors...  Thanks, Mark
Very pleased to hear you got there.
Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

All Courses

From novice to tech pro — start learning today.