final ConcurrentMap<String, Integer> delegate = new ConcurrentHashMap<>();

    @Override
    protected ConcurrentMap<String, Integer> delegate() {
      return delegate;
    }
  }

  public void testPutIfAbsent() {
    TestMap map = new TestMap();
    map.put("foo", 1);
    assertEquals(Integer.valueOf(1), map.putIfAbsent("foo", 2));
    assertEquals(Integer.valueOf(1), map.get("foo"));
    assertNull(map.putIfAbsent("bar", 3));