private double parseAddOrSub() throws Exception{
		char op;	//
		double result;	//
		double partialResult;	//ӱʽĽ
		
		result=this.parseMulOrDiv();	//ó˷㵱ǰӱʽֵ
		//ǰǵĵһĸǼӼţмӼ
		while((op=this.token.charAt(0))=='+'||op=='-'){
			this.getToken();	//ȡһ
			//ó˷㵱ǰֱʽֵ
			partialResult=this.parseMulOrDiv();
			switch(op){
			case '-':
				//ǼѴӱʽֵȥǰӱʽֵ
				result=result-partialResult;
				break;
			case '+':
				//ǼӷѴӱʽֵϵǰӱʽֵ
				result=result+partialResult;
				break;
			}
		}
		return result;
	}