Convert double to BigDecimal and set BigDecimal Precision

Question

In Java  I want to take a double value and convert it to a BigDecimal and print out its String value to a certain precision     import java math BigDecimal  public class Main       public static void main String   args            double d - 00012          System out println d        This prints -1 2E-4          double c 47 48000          BigDecimal b   new BigDecimal c           System out println b toString                This prints 47 47999999999999687361196265555918216705322265625            It prints this huge thing      47 47999999999999687361196265555918216705322265625   and not     47 48   The reason I m doing the BigDecimal conversion is sometimes the double value will contain a lot of decimal places  i e  - 000012  and the when converting the double to a String will produce scientific notation -1 2E-4   I want to store the String value in non-scientific notation    I want to have BigDecimal always have two units of precision like this   47 48    Can BigDecimal restrict precision on conversion to string

User · Answer

BigDecimal b   new BigDecimal c  setScale 2 BigDecimal ROUND HALF UP

User · Answer

In Java 9 the following is deprecated   BigDecimal valueOf d  setScale 2  BigDecimal ROUND HALF UP    instead use   BigDecimal valueOf d  setScale 2  RoundingMode HALF UP     Example       double d   47 48111       System out println BigDecimal valueOf d      Prints  47 48111      BigDecimal bigDecimal   BigDecimal valueOf d  setScale 2  RoundingMode HALF UP       System out println bigDecimal     Prints  47 48

User · Answer

It s printing out the actual  exact value of the double   Double toString    which converts doubles to Strings  does not print the exact decimal value of the input -- if x is your double value  it prints out exactly enough digits that x is the closest double to the value it printed   The point is that there is no such double as 47 48 exactly   Doubles store values as binary fractions  not as decimals  so it can t store exact decimal values    That s what BigDecimal is for

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The String format syntax helps us convert doubles and BigDecimals to strings of whatever precision   This java code   double dennis   0 00000008880000d  System out println dennis   System out println String format    7f   dennis    System out println String format    9f   new BigDecimal dennis     System out println String format    19f   new BigDecimal dennis       Prints   8 88E-8 0 0000001 0 000000089 0 0000000888000000000

User · Answer

The reason of such behaviour is that the string that is printed is the exact value - probably not what you expected  but that s the real value stored in memory - it s just a limitation of floating point representation   According to javadoc  BigDecimal double val  constructor behaviour can be unexpected if you don t take into consideration this limitation      The results of this constructor can be somewhat unpredictable  One   might assume that writing new BigDecimal 0 1  in Java creates a   BigDecimal which is exactly equal to 0 1  an unscaled value of 1  with   a scale of 1   but it is actually equal to   0 1000000000000000055511151231257827021181583404541015625  This is because 0 1 cannot be represented exactly as a double  or  for that   matter  as a binary fraction of any finite length   Thus  the value   that is being passed in to the constructor is not exactly equal to   0 1  appearances notwithstanding    So in your case  instead of using  double val   77 48  new BigDecimal val     use  BigDecimal valueOf val     Value that is returned by BigDecimal valueOf is equal to that resulting from invocation of Double toString double

User · Answer

You want to try String format   f   d   which will print your double in decimal notation  Don t use BigDecimal at all   Regarding the precision issue  You are first storing 47 48 in the double c  then making a new BigDecimal from that double  The loss of precision is in assigning to c  You could do  BigDecimal b   new BigDecimal  47 48     to avoid losing any precision

User · Answer

Why not    b   b setScale 2  RoundingMode HALF UP

User · Answer

It prints 47 48000 if you use another MathContext   BigDecimal b   new BigDecimal d  MathContext DECIMAL64     Just pick the context you need

[java] Convert double to BigDecimal and set BigDecimal Precision

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