Using Java BigDecimal still not correctly solving

Question

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?

Answer 1

Should I just use rounding?

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

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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

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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

Answer 2

Because the compiler will evaluate the sum first, it adds the two doubles first and hence you losing the precision, you essentially doing this

double value1 = 2789.45;
double value2 = 557.89;
double sum = value1 + value2; // precision lost here
System.out.println(BigDecimal.valueOf(sum));