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javamathdoublebigdecimal

Using Java BigDecimal still not correctly solving

I have a method with two double values being inputted. When trying to add them together I get incorrect values over a certain threshold so I started using BigDecimal.

However even with BigDecimal I still have incorrect values?

double value1 = 2789.45;
double value2 = 557.89;
System.out.println(BigDecimal.valueOf(value1 + value2));

prints 

3347.3399999999997

when It should read as

3347.34

How can I do this correctly even if value1 and value2 could be higher than current scope? (they are calculated in a separate method).

Should I just use rounding?

Solution

  • Should I just use rounding?

    NOPE, what you are experiencing is lose of precision of double sum.

    You are suming the doubles first (value1 + value2) and after converting the double sum (which has lost precision) to a BigDecimal.

    To avoid this use this instead:

    double value1 = 2789.45;
double value2 = 557.89;
System.out.println(BigDecimal.valueOf(value1).add(BigDecimal.valueOf(value2)));

    OUTPUT:

    3347.34

    Working IDEONE DEMO here

    UPDATE

    -1. Instead of using BigDecimal.valueOf, which is still fragile, you should be using BigDecimal from the beginning and create them with new BigDecimal("2789.45"), new BigDecimal("557.89"). As soon as you've used a double literal, you've introduced imprecision. BigDecimal.valueOf tries to get it back, but it doesn't always work. – Louis Wasserman

    Actually I'm not totally agree with this. You cannot hardcode the values creating new decimals, what I can be agree is to read the values (if possible for the OP) in a String directly to avoid double precision lost:

    String value1 = "2789.45"; 
BigDecimal one = new BigDecimal(value1);
String value2 = "557.89";
BigDecimal two = new BigDecimal(value2);
System.out.println(one.add(two));

    OUTPUT:

    3347.34

    This will avoid the problems Louis Wasserman is noticing.

    NEW WORKING DEMO