Missing JTable header

JTableHeader will not be displayed unless you put it in a scroll pane. The reason is that the table manually checks if it is being put into a JScrollPane, and then adds the header row in the row header space of the scroll pane.

This is a picture of a table without the enclosing JScrollPane:

Same table with the enclosing JScrollPane:

Here is a snippet from JTable that shows this ugly operation:

public void addNotify() {
        super.addNotify();
        configureEnclosingScrollPane();
}
 
protected void configureEnclosingScrollPane() {
      Container p = getParent();
      if (p instanceof JViewport) {
          Container gp = p.getParent();
            if (gp instanceof JScrollPane) {
                JScrollPane scrollPane = (JScrollPane)gp;
                // Make certain we are the viewPort's view and not, for
                // example, the rowHeaderView of the scrollPane -
                // an implementor of fixed columns might do this.
                JViewport viewport = scrollPane.getViewport();
                if (viewport == null || viewport.getView() != this) {
                    return;
                }
                scrollPane.setColumnHeaderView(getTableHeader());
		//  scrollPane.getViewport().setBackingStoreEnabled(true);
                Border border = scrollPane.getBorder();
                if (border == null || border instanceof UIResource) {
                    scrollPane.setBorder(UIManager.getBorder("Table.scrollPaneBorder"));
                }
            }
        }
 }

The reason for this is probabily the incompletenes of the Scrollable interface - that does not define a header component. So, there are two solutions: put JTable into a JScrollPane, or if you do not want to use JScrollPane, install the header by hand above table body - however, remember to align it properly. In my experience, it is much easier to put a JTable in the JScrollPane - you can hide the scroll bars if they are not needed.

See also Avoid Double Scroll, Always Put JTable In JScrollPane.