JetBrains/intellij-community · error · UnexpectedFormElementException

unexpected element: {element}

Error message

unexpected element: {element}

What it means

Thrown by LwContainer.createComponent (via the createComponent factory in LwContainer) while parsing a form's XML: a child element inside a container matched none of the recognized component element names (component types like tabbed-pane, split-pane, toolbar, and their siblings). UnexpectedFormElementException signals that the .form XML contains an element the lightweight form reader does not know how to turn into an LwComponent.

Source

Thrown at java/compiler/forms-compiler/src/com/intellij/uiDesigner/lw/LwContainer.java:288

    }
    else if(UIFormXmlConstants.ELEMENT_SCROLLPANE.equals(name)) {
      String className = LwXmlReader.getOptionalString(child, UIFormXmlConstants.ATTRIBUTE_CLASS, "javax.swing.JScrollPane");
      component = new LwScrollPane(className);
    }
    else if(UIFormXmlConstants.ELEMENT_TABBEDPANE.equals(name)){
      String className = LwXmlReader.getOptionalString(child, UIFormXmlConstants.ATTRIBUTE_CLASS, "javax.swing.JTabbedPane");
      component = new LwTabbedPane(className);
    }
    else if(UIFormXmlConstants.ELEMENT_SPLITPANE.equals(name)){
      String className = LwXmlReader.getOptionalString(child, UIFormXmlConstants.ATTRIBUTE_CLASS, "javax.swing.JSplitPane");
      component = new LwSplitPane(className);
    }
    else if (UIFormXmlConstants.ELEMENT_TOOLBAR.equals(name)) {
      String className = LwXmlReader.getOptionalString(child, UIFormXmlConstants.ATTRIBUTE_CLASS, "javax.swing.JToolBar");
      component = new LwToolBar(className);
    }
    else{
      throw new UnexpectedFormElementException("unexpected element: "+child);
    }
    return component;
  }

  /**
   * 'xy' or 'grid'
   */
  protected final void readLayout(final Element element){
    myLayoutManager = element.getAttributeValue("layout-manager");
    if("xy".equals(element.getName())){
      myLayoutSerializer = XYLayoutSerializer.INSTANCE;
    }
    else if("grid".equals(element.getName())){
      createLayoutSerializer();
    }
    else{
      throw new UnexpectedFormElementException("unexpected element: "+element);
    }

View on GitHub (pinned to be881553f2)

Solutions

1. Open the .form file and inspect the element printed in the message; correct the tag name to the expected kebab-case element (e.g. 'tabbed-pane', 'split-pane', 'toolbar').
2. If the form came from a newer IDE, upgrade the forms-compiler/IDE reading it so the new element names are recognized.
3. Regenerate the form in the GUI designer instead of hand-editing the XML.
4. Delete the unrecognized element from the form if the component is not needed.

Example fix

<!-- before -->
<tabbedpane binding="myTabs"> ... </tabbedpane>

<!-- after -->
<tabbed-pane binding="myTabs"> ... </tabbed-pane>

// When generating form XML, whitelist container children before writing
Set<String> KNOWN = Set.of("component", "grid", "xy", "tabbed-pane", "split-pane", "toolbar");
if (!KNOWN.contains(childTagName)) throw new IllegalArgumentException("Unknown form element: " + childTagName);

try { Utils.loadFormXml(stream); } catch (UnexpectedFormElementException e) { /* message prints the offending element; fix or remove that tag in the .form file */ }

When it happens

Trigger: Parsing a .form file where a container element has a child whose tag is not in the known set — e.g. a typo'd or newer-version tag such as 'tabbedpane' instead of 'tabbed-pane', or an element introduced by a newer IDE format being read by an older forms-compiler. The else branch throws UnexpectedFormElementException with the offending element toString.

Common situations: Form files created by a newer IntelliJ version using new component tags, then opened/compiled by an older forms-compiler; hand-edited form XML with mistyped element names; third-party tools generating non-conforming form XML.

Related errors

• unexpected element: {element}
• subtag '{childName}' is required: {element}
• attribute '{attributeName}' is required: {element}
• String descriptor value required
• Cannot find group {groupName}

AI-assisted analysis of JetBrains/intellij-community@be881553f2 (2026-08-14). Data as JSON: /api/errors/c8fa5ad48bbe1620. Report an issue: GitHub.