code: @SuppressWarnings({ "unchecked" })
      @SequentialOnly
      public <KK> EntryStream<KK, V> selectByKey(Class<KK> clazz) {
          if (isParallel()) {
              return (EntryStream<KK, V>) sequential().filterByKey(Fn.instanceOf(clazz)).parallel(maxThreadNum(), splitor());
          } else {
              return (EntryStream<KK, V>) filterByKey(Fn.instanceOf(clazz));
          }
      }
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