Added missing album art icon.

src/main/java/musicplayer/LibraryTreeCellRenderer.java

@@ -10,6 +10,7 @@ import java.awt.*;
 class LibraryTreeCellRenderer implements TreeCellRenderer {
 
     private final JLabel label;
+    private static Icon missingIcon = new ImageIcon(LibraryTreeCellRenderer.class.getClassLoader().getResource("missing.gif"));
 
     LibraryTreeCellRenderer() {
         label = new JLabel();
@@ -23,6 +24,7 @@ class LibraryTreeCellRenderer implements TreeCellRenderer {
             label.setText(o.toString());
             if (o instanceof Album) {
                 Album album = (Album) o;
+                label.setIcon(missingIcon);
                 album.getAlbumArt().ifPresent(x -> label.setIcon(new ImageIcon(x)));
             }
         }