Double 中的 BigDecimal 值不正确？

Question

I am trying to make a BigDecimal from a string. Don't ask me why, I just need it: This is my code:

Double theDouble = new Double(".3");
System.out.println("The Double: " + theDouble.toString());
BigDecimal theBigDecimal = new BigDecimal(theDouble);
System.out.println("The Big: " + theBigDecimal.toString());

This is the output I get?

The Double: 0.3
The Big: 0.299999999999999988897769753748434595763683319091796875

Any ideas?

Answer 1

When you create a double, the value 0.3 cannot be represented exactly. You can create a BigDecimal from a string without the intermediate double, as in

new BigDecimal("0.3")

A floating point number is represented as a binary fraction and an exponent. Therefore there are some number that cannot be represented exactly. There is an analogous problem in base 10 with numbers like 1/3, which is 0.333333333..... Any decimal representation of 1/3 is inexact. This happens to a DIFFERENT set of fractions in binary, and 0.3 is one of the set that is inexact in binary.

Answer 2

Since new Double(".3") can't be represented exactly, the nearest value is 0x1.3333333333333P-2 or .299999999999999988897769753748434595763683319091796875 , what would be need to is this:

Double theDouble = new Double(".3");
System.out.println("The Double: " + theDouble.toString());
BigDecimal theBigDecimal = new 
BigDecimal(theDouble).setScale(2, RoundingMode.CEILING);  // <-- here
System.out.println("The Big: " + theBigDecimal.toString());

This will print:

The Double: 0.3
The Big: 0.30

Answer 3

You can give a big decimal a specified precision. eg append to your example:

Double theDouble = new Double(".3");
theBigDecimal = new BigDecimal(theDouble, new MathContext(2));
System.out.println("The Big: " + theBigDecimal.toString());

This will print out "0.30"

Answer 4

Another way is to use MathContext.DECIMAL32 which guarantees 7 digit precision (which is good enough in our case):

Double theDouble = new Double(".3");
System.out.println("The Double: " + theDouble.toString());
BigDecimal theBigDecimal = new BigDecimal(theDouble, MathContext.DECIMAL32);  // <-- here
System.out.println("The Big: " + theBigDecimal.toString());

OUTPUT

The Double: 0.3
The Big: 0.3000000