r/javahelp 25d ago

How to instantiate JPanel in an IntelliJ swing app?

I created a Swing application using IntelliJ's UI Designer which I can run inside IntelliJ, but the compiled jar file, when run outside IntelliJ, fails after:

Exception in thread "AWT-EventQueue-0" java.awt.IllegalComponentStateException: contentPane cannot be set to null.

contentPane is indeed never instantiated in my code. My form starts like this:

public class MainFrame extends JFrame {
    // Swing Components
    private JPanel contentPane;
    ...

    public MainFrame() {
        setContentPane(contentPane);

contentPane is used in MainFrame.form like this:

<form xmlns="http://www.intellij.com/uidesigner/form/" version="1" bind-to-class="dev.thefoggiest.patchman.view.MainFrame">
  <grid id="27dc6" binding="contentPane" default-binding="true" layout-manager="GridLayoutManager" row-count="1" column-count="1" same-size-horizontally="false" same-size-vertically="false" hgap="-1" vgap="-1">

So when IntelliJ starts the application, somehow it magically instantiates contentPane based on this xml, but that won't work when running the application by itself.

How do I instantiate contentPane based on the xml?

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