如何在Java中舍入一个大的十进制值？

Question

I'm trying in the following code to round a decimal number based on the decimal format that I set at the beginning:

DecimalFormat df = new DecimalFormat("#.000"); 
df.setRoundingMode(RoundingMode.FLOOR);

double a = Double.parseDouble(df.format(43.473684210526315));
double b = Math.pow(a, 10);
double c = Double.parseDouble(df.format(b));

System.out.println(a + " ** " + b  +  " ** " + c);

The result that I got is:

43.473 ** 2.4109939006965688E16 ** 2.4109939006965688E16

As you see, the value of a is formatted properly. While the other values are not. After spending a long time trying to understand what is going on, I found out the value 2.4109939006965688E16 can not be formatted. I tested the same value after removing E16 and it worked.

My question is how can I round such a large decimal so that it works as the a ?

Answer 1

对于“大”小数，可以使用以下格式：

DecimalFormat df2 = new DecimalFormat("#.000E0"); 

Answer 2

I see several issues here:

• You do not specify a locale for your DecimalFormat. So the combination of DecimalFormat.format and Double.parseDouble is error prone, as eg with german locale, DecimalFormat.format will use a comma as decimal separator, which is unsupported by Double.parseDouble , and will throw a NumberFormatException . You should use both format and parse of your DecimalFormat.
• Also, you use your double values directly for System.out.println . Why not concatenating the results of DecimalFormat.format , to achieve exactly what you want? Eg System.out.println(df.format(a) + " ** " + df.format(b) + " ** " + df.format(c));
• If you want to do real mathematical rounding (eg needing the double value for additional calculations), use Math.round . You can multiply and divide by 10, 100, 1000 etc. to achieve the desired precision. Note that, with the double data type, you can't do exact rounding.

Answer 3

It's pure luck that a works. If the initial value was 43.4701 you would be seeing 43.47 . And in some cases you would see more than 3 decimal places due to the inaccuracy of double .

You only use the DecimalFormatter to create a String briefly before turning back into a double again. You want to keep and use that string.

DecimalFormat df = new DecimalFormat("#.000");

double b = Math.pow(43.473684210526315, 10);

String bFormatted = df.format(b);

System.out.println(bFormatted);

Gives you your desired output 24115485538109308.000

Answer 4

For large numbers, you'll need to use BigInteger or BigDecimal

DecimalFormat df = new DecimalFormat("#.000");
df.setRoundingMode(RoundingMode.FLOOR);

BigDecimal a = new BigDecimal(43.473);
BigDecimal b = a.pow(10);

System.out.println(df.format(a) + " ** " + b.doubleValue() + " ** " + df.format(b));

This results in the following output

43.472 ** 2.4109939006965688E16 ** 24109939006965686.655