背景

今天生产报了个问题，在申报和预算相同的金额的时候，后台返回超出预算。一开始以为是判断逻辑的问题，找了个数据试了下发现重现不出来。又是改数据又是找前端传参最后发现是接受参数处理的问题。

double转Bigdecimal，调用compare方法没有返回预期结果

部分代码如下

balanceFee.compareTo(new BigDecimal(amt)) >= 0

debug尝试

下面两个图是分别对100和100.6
debug如图


debug看上去都好像相同, 但是结果是, 100相同, 100.6不相同

测试代码

 public static void main(String[] args) {
        double d1 = 6.66;
        double d2 = 666.0 / 100.0;
        double d3 = Double.parseDouble("6.66");
        String s1 = "6.66";
        testDoubleBigdecimal(d1, d2,d3, s1);
    }

    private static void testDoubleBigdecimal(double d1, double d2, double d3, String s1) {
        BigDecimal bg1 = new BigDecimal(d1);
        BigDecimal bg2 = new BigDecimal(d2);
        BigDecimal bg3 = new BigDecimal(d3);
        BigDecimal bg4 = new BigDecimal(s1);
        System.out.println("double = " + bg1);
        System.out.println("double / = " + bg2);
        System.out.println("str 2 double = " + bg3);
        System.out.println("str = " + bg4);
        System.out.println("bg1.compareTo(bg2) = " + bg1.compareTo(bg2));
        System.out.println("bg1.compareTo(bg3) = " + bg1.compareTo(bg3));
        System.out.println("bg1.compareTo(bg4) = " + bg1.compareTo(bg4));
        System.out.println("bg2.compareTo(bg3) = " + bg2.compareTo(bg3));
        System.out.println("bg2.compareTo(bg4) = " + bg2.compareTo(bg4));
        System.out.println("bg3.compareTo(bg4) = " + bg3.compareTo(bg4));
    }

结果

分别用666.6和66.66测试

• 666.6结果：

double = 666.6000000000000227373675443232059478759765625
double / = 666.6000000000000227373675443232059478759765625
str 2 double = 666.6000000000000227373675443232059478759765625
str = 666.6
bg1.compareTo(bg2) = 0
bg1.compareTo(bg3) = 0
bg1.compareTo(bg4) = 1
bg2.compareTo(bg3) = 0
bg2.compareTo(bg4) = 1
bg3.compareTo(bg4) = 1

• 66.66结果

double = 66.659999999999996589394868351519107818603515625
double / = 66.659999999999996589394868351519107818603515625
str 2 double = 66.659999999999996589394868351519107818603515625
str = 66.66
bg1.compareTo(bg2) = 0
bg1.compareTo(bg3) = 0
bg1.compareTo(bg4) = -1
bg2.compareTo(bg3) = 0
bg2.compareTo(bg4) = -1
bg3.compareTo(bg4) = -1

总结

其实double有精度问题,这个是大家都知道的. 解决精度问题用BigDecimal也是很常见的手段, 但是这里不知道为什么controller的接口使用了double来接收.

1. double有精度问题
2. 使用BigDecimal最好用string来转化

最坑是在debug的时候, 由于代码是balanceFee.compareTo(new BigDecimal(amt)) >= 0
第一个balanceFee是字符串转化计算而来的,是准确的, 第二个amt是double, 在debug模式下浮窗显示的值也是精准的, 然而最重要的new BigDecimal(amt)却没有浮窗显示出值.
这样也就不难理解为什么debug的时候图上看上去是一样的值, 比较的结果却不是0了