Adding 0.39 Double variable in Java

Question

Need few suggestions on a strange behavior in adding a double with ONLY 0.39 (strange number). First snippet has proper example with 0.38 and the second with 0.39

// code snippet 1 : Adding xxxx.38 to xxxx.00

double b =1031.38;
double a =1587.00;
System.out.println ("using double     "+(a+b));

BigDecimal premium = BigDecimal.valueOf(a);
BigDecimal netToCompany = BigDecimal.valueOf(b);
double result =Double.parseDouble(premium.add(netToCompany).toString());
System.out.println("using BigDecimal "+result);

// correct Output xxxx.38 + xxxx.00 = xxxx.38

using double     2618.38
using BigDecimal 2618.38

***** ------------------------------ ******

// code snippet 2 : Adding xxxx.39 to xxxx.00

double b =1031.39;
double a =1587.00;
System.out.println("using double     "+(a+b));

BigDecimal premium = BigDecimal.valueOf(a);
BigDecimal netToCompany = BigDecimal.valueOf(b);
double result =   Double.parseDouble(premium.add(netToCompany).toString());

// wrong Output xxxx.39 + xxxx.00 = xxxx.3900000000003

using double  2618.3900000000003
using BigDecimal 2618.39

Answer 1

When you use BigDecimal, you want to use the version that takes a String. If you use double, then you have already lost precision.

BigDecimal premium = new BigDecimal("1031.38");
BigDecimal netToCompany = new BigDecimal("1587.00");

Which outputs (as expected)

2618.38

and for 2618.39

BigDecimal premium = new BigDecimal("1031.39");
BigDecimal netToCompany = new BigDecimal("1587.00");
System.out.println(premium.add(netToCompany));

but, if you need to use double, you might choose to use formatted output (which will round for you) like

double[] premiums = { 1031.38, 1031.39 };
double netToCompany = 1587.0;
for (double premium : premiums) {
    System.out.printf("%.2f%n", premium + netToCompany);
}