public static PrinterQueue getInstance() {
    if (Objects.isNull(currentInstance)) {
      currentInstance = new PrinterQueue();
    }
    return currentInstance;
  }
@Test
  @Timeout(1000)
  void singletonTest() {
    PrinterQueue printerQueue1 = PrinterQueue.getInstance();
    PrinterQueue printerQueue2 = PrinterQueue.getInstance();
    assertSame(printerQueue1, printerQueue2);
  }