code: @Override
      public boolean containsKey(int x, int y, int z) {
          long key = Int21TripleHashed.key(x, y, z);
          return map.containsKey(key);
      }
docs: Returns true if this map contains a mapping for the specified key. More formally, returns <code>true</code> if and only if this map contains a mapping for a key <code>k</code> such that
  <code>key.equals(k)</code>. (There can be at most one such mapping.)
  
  @param x an <code>int</code> value
  @param y an <code>int</code> value
  @param z an <code>int</code> value
  @return <code>true</code> if this map contains a mapping for the specified <code>key(x, y, z)</code>.