@Override
  public V putIfAbsent(K k, V v) {
    return mInternalMap.putIfAbsent(new IdentityObject<>(k), v);
  }
@Test
  public void putIfAbsent() {
    String t = new String("test");
    assertNull(mMap.putIfAbsent(t, "v1"));
    assertEquals(1, mMap.size());
    assertEquals("v1", mMap.get(t));
    assertFalse(mMap.containsKey("test"));
    assertEquals("v1", mMap.putIfAbsent(t, "v2"));
    assertEquals("v1", mMap.get(t));
  }