code: @SuppressWarnings({ "unchecked" })
      public <KK> EntryStream<KK, V> selectKeys(Class<KK> clazz) {
          return (EntryStream<KK, V>) filter(e -> clazz.isInstance(e.getKey()));
      }
docs: Returns a stream consisting of the elements of this stream which keys are
  instances of given class.
  
  <p>
  This is an <a href="package-summary.html#StreamOps">intermediate</a>
  operation.
  
  @param <KK> a type of keys to select.
  @param clazz a class to filter the keys.
  @return the new stream