兩種格式化float的方式，不同的輸出

        float mAverageNumber = sum/12;
        NumberFormat numFormat = NumberFormat.getNumberInstance();
        numFormat.setMaximumFractionDigits(2);
        String str = numFormat.format(mAverageNumber);
        //above code output :$1,581.42
        String str = String.valueOf((new BigDecimal(mAverageNumber)).setScale(2,BigDecimal.ROUND_HALF_UP));
        //that output :$1581.42
	System.out.println("$"+str);

詳細解釋後面再補充