What is a "Multipart Widget"?

Normal widgets are renderd on a single slot but a multipart widget can be rendered over multiple slots.

Add a multipart widget to our GUI.

In this example we will fill the empty slots from our previus GUI with glass.

MultipartPlaceHolder multipartPlaceHolder = new MultipartPlaceHolder();
multipartPlaceHolder
  .setMaterial(Material.GLASS_PANE)                //Set the material of the placeholder
  .setSlots(Arrays.asList(
                            0,1,2,3,4,5,6,7,8,
                            9,10,11,12,14,15,16,17, //MultipartWidgets are renderd over multiple slots!
                            18,19,20,21,22,23,24,25,26
                          ))
  .addToGUI(gui);                                  //Adds the Placeholders to the GUI

Done

Now every slot (except for the button slot) is filled with glass.