public double evaluate(String expStr) throws Exception{
		double result;
		this.exp=preProcess(expStr);;
		this.expIndex=0;
		
		//ȡһ
		this.getToken();
		if(this.token.equals(EOE)){
			//ûбʽ쳣
			this.handleError(NOEXP_ERROR);
		}
		
		result=this.parseAddOrSub();		//Ӽ
		return result;
	}