代码如下，java用BigDecimal四舍五入怎么还有问题呀？哪种方式是完全没有问题的？

Question

1.为什么不是1.51？输出的是1.50

2.decimalformat括号里的字符串代表了什么意思？有模板吗？如果是别的格式应该怎么写

3.为什么要用#号而不是0.00？如果是#好的话，当a是0.505的时候有bug

        double a =1.505;
        BigDecimal bd = new BigDecimal(a);
        System.out.println(new DecimalFormat("#.00").format(bd));

Answer 1

First of all, point out a problem: construction BigDecimal 的时候，尽量不要使用浮点数（double，float), because there is a lack of precision in the storage of floating point numbers in computers. For example, the code you wrote:

public static void main(String[] args) throws Exception {
    double a = 1.505;
    BigDecimal bd = new BigDecimal(a);

    System.out.println("bd: " + bd.toString());
}

Run results:

You can see that the floating point number 1.505 cannot be stored in the computer - if you don’t know the specific reason, please search for it yourself "The problem of missing precision of floating point numbers"

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Second, if you need to output 1.51, which is often called "rounding", then you need to specify the rounding mode of DecimalFormat:

public static void main(String[] args) throws Exception {
    BigDecimal bd = new BigDecimal("1.505");
    System.out.println("bd: " + bd.toString());

    DecimalFormat df = new DecimalFormat("#.00");
    df.setRoundingMode(RoundingMode.HALF_UP);

    System.out.println("format: " + df.format(bd));
}

Run results:

If you don’t understand the usage of RoundingMode, please search for "Usage of RoundingMode"

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Third, regarding the meaning of the DecimalFormat strings in brackets, please search "Usage of DecimalFormat"

Answer 2

    System.out.println(String.format("%.2f", a));
    System.out.println(new Formatter().format("%.2f", a));
    
    这两种是可以的。
  
  double d = 5.505;
  System.out.println(d)
  输出的是1.504xxxxxxxxx，所以java在做四舍五入的时候舍掉了后面的4xxxxxx。
  
  
  等待大神解答