ComboBoxEditorにJLayerを設定してプレースホルダ文字列を表示する

combo2.setEditor(new BasicComboBoxEditor() {
  private Component editorComponent;

  @Override public Component getEditorComponent() {
    editorComponent = Optional.ofNullable(editorComponent)
        .orElseGet(() -> {
          JTextComponent tc = (JTextComponent) super.getEditorComponent();
          return new JLayer<>(tc, new PlaceholderLayerUI<>("- Select type -"));
        });
    return editorComponent;
  }
});
combo2.setBorder(BorderFactory.createCompoundBorder(
    combo2.getBorder(),
    BorderFactory.createEmptyBorder(0, 2, 0, 0)));