China will tighten policy if measures already taken fail to cool the overheating economy, China Daily reported, quoted the China Securities Journal.   "The central bank will wait and see whether the signs of a slowing trend evident in the June economic indicators continues or not. If not, the central bank will take stronger measures," Liu Lida, head of the monetary policy and theoretical research division of the central bank's research bureau, was quoted as saying by the official newspaper.   Although the latest tightening was less aggressive than the markets had anticipated, some investors expected more tightening steps in coming month, wrote the papers.   "It's clear investors are nervous," said Zheng Weigang, senior stock analyst at Shanghai Securities.   "The economy is racing at such a pace, so mild measures such as hikes in bank reserve requirements are not enough. The markets expect a possible interest rate hike."