NavigableMap<K, V> outer = Synchronized.navigableMap(inner, mutex);
    return outer;
  }

  static class TestEntry<K, V> extends ForwardingMapEntry<K, V> implements Serializable {
    private final Entry<K, V> delegate;
    private final Object mutex;

    TestEntry(Entry<K, V> delegate, Object mutex) {
      this.delegate = delegate;
      this.mutex = mutex;
    }

    @Override

        list.addLast("3");
        list.set(1, "4");
        assertThat(list.get(1), is("4"));
    }

    /**
     * @throws Exception
     */
    @Test
    public void testEntry() throws Exception {
        list.addLast("1");
        list.addLast("2");
        list.addLast("3");
        SLinkedList<String>.Entry e = list.getFirstEntry();
        assertThat(e.getPrevious(), is(nullValue()));