CODEWITHSUNDEEP
javadoublebigdecimal
Diego
Diego

Reputation: 17150

BigDecimal from Double incorrect value?

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?

Upvotes: 5

Views: 4026

Answers (4)

Criscach Cachaña
Criscach Cachaña

Reputation: 29

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

Upvotes: 1

Nir Alfasi
Nir Alfasi

Reputation: 53565

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

Upvotes: 1

Steve B.
Steve B.

Reputation: 57333

You can give a big decimal a specified precision. e.g. 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"

Upvotes: 0

Jim Garrison
Jim Garrison

Reputation: 86774

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.

Upvotes: 12

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