Double vs BigDecimal

Question

I have to calculate some floating point variables and my colleague suggest me to use BigDecimal instead of double since it will be more precise  But I want to know what it is and how to make most out of BigDecimal

User · Answer

If you write down a fractional value like 1 / 7 as decimal value you get

1/7 = 0.142857142857142857142857142857142857142857...

with an infinite sequence of 142857. Since you can only write a finite number of digits you will inevitably introduce a rounding (or truncation) error.

Numbers like 1/10 or 1/100 expressed as binary numbers with a fractional part also have an infinite number of digits after the decimal point:

1/10 = binary 0.0001100110011001100110011001100110...

Doubles store values as binary and therefore might introduce an error solely by converting a decimal number to a binary number, without even doing any arithmetic.

Decimal numbers (like BigDecimal), on the other hand, store each decimal digit as is (binary coded, but each decimal on its own). This means that a decimal type is not more precise than a binary floating point or fixed point type in a general sense (i.e. it cannot store 1/7 without loss of precision), but it is more accurate for numbers that have a finite number of decimal digits as is often the case for money calculations.

Java's BigDecimal has the additional advantage that it can have an arbitrary (but finite) number of digits on both sides of the decimal point, limited only by the available memory.

User · Answer

A BigDecimal is an exact way of representing numbers  A Double has a certain precision  Working with doubles of various magnitudes  say d1 1000 0 and d2 0 001  could result in the 0 001 being dropped alltogether when summing as the difference in magnitude is so large  With BigDecimal this would not happen   The disadvantage of BigDecimal is that it s slower  and it s a bit more difficult to program algorithms that way  due to   -   and   not being overloaded    If you are dealing with money  or precision is a must  use BigDecimal  Otherwise Doubles tend to be good enough   I do recommend reading the javadoc of BigDecimal as they do explain things better than I do here

User · Answer

My English is not good so I ll just write a simple example here       double a   0 02      double b   0 03      double c   b - a      System out println c        BigDecimal  a   new BigDecimal  0 02        BigDecimal  b   new BigDecimal  0 03        BigDecimal  c    b subtract  a       System out println  c     Program output   0 009999999999999998 0 01   Somebody still want to use double

User · Answer

BigDecimal is Oracle s arbitrary-precision numerical library  BigDecimal is part of the Java language and is useful for a variety of applications ranging from the financial to the scientific  that s where sort of am    There s nothing wrong with using doubles for certain calculations  Suppose  however  you wanted to calculate Math Pi   Math Pi   6  that is  the value of the Riemann Zeta Function for a real argument of two  a project I m currently working on   Floating-point division presents you with a painful problem of rounding error   BigDecimal  on the other hand  includes many options for calculating expressions to arbitrary precision  The add  multiply  and divide methods as described in the Oracle documentation below  take the place  of       and   in BigDecimal Java World   http   docs oracle com javase 7 docs api java math BigDecimal html  The compareTo method is especially useful in while and for loops   Be careful  however  in your use of constructors for BigDecimal  The string constructor is very useful in many cases  For instance  the code  BigDecimal onethird   new BigDecimal  0 33333333333     utilizes a string representation of 1 3 to represent that infinitely-repeating number to a specified degree of accuracy  The round-off error is most likely somewhere so deep inside the JVM that the round-off errors won t disturb most of your practical calculations  I have  from personal experience  seen round-off creep up  however  The setScale method is important in these regards  as can be seen from the Oracle documentation

User · Answer

If you are dealing with calculation  there are laws on how you should calculate and what precision you should use  If you fail that you will be doing something illegal   The only real reason is that the bit representation of decimal cases are not precise  As Basil simply put  an example is the best explanation  Just to complement his example  here s what happens    static void theDoubleProblem1         double d1   0 3      double d2   0 2      System out println  Double  t 0 3 - 0 2        d1 - d2         float f1   0 3f      float f2   0 2f      System out println  Float  t 0 3 - 0 2        f1 - f2         BigDecimal bd1   new BigDecimal  0 3        BigDecimal bd2   new BigDecimal  0 2        System out println  BigDec  t 0 3 - 0 2        bd1 subtract bd2         Output    Double   0 3 - 0 2   0 09999999999999998 Float    0 3 - 0 2   0 10000001 BigDec   0 3 - 0 2   0 1   Also we have that    static void theDoubleProblem2         double d1   10      double d2   3      System out println  Double  t 10   3        d1   d2         float f1   10f      float f2   3f      System out println  Float  t 10   3        f1   f2            Exception       BigDecimal bd3   new BigDecimal  10        BigDecimal bd4   new BigDecimal  3        System out println  BigDec  t 10   3        bd3 divide bd4         Gives us the output    Double   10   3   3 3333333333333335 Float    10   3   3 3333333 Exception in thread  main  java lang ArithmeticException  Non-terminating decimal expansion   But    static void theDoubleProblem2         BigDecimal bd3   new BigDecimal  10        BigDecimal bd4   new BigDecimal  3        System out println  BigDec  t 10   3        bd3 divide bd4  4  BigDecimal ROUND HALF UP         Has the output    BigDec   10   3   3 3333

User · Answer

There are two main differences from double    Arbitrary precision  similarly to BigInteger they can contain number of arbitrary precision and size Base 10 instead of Base 2  a BigDecimal is n 10 scale where n is an arbitrary large signed integer and scale can be thought of as the number of digits to move the decimal point left or right   The reason you should use BigDecimal for monetary calculations is not that it can represent any number  but that it can represent all numbers that can be represented in decimal notion and that include virtually all numbers in the monetary world  you never transfer 1 3   to someone

[java] Double vs. BigDecimal?

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