aliplane aliplane - 4 months ago 32
Java Question

How to round 0.745 to 0.75 using BigDecimal.ROUND_HALF_UP?

I tried the following,

double doubleVal = 1.745;
double doubleVal1 = 0.745;
BigDecimal bdTest = new BigDecimal( doubleVal);
BigDecimal bdTest1 = new BigDecimal( doubleVal1 );
bdTest = bdTest.setScale(2, BigDecimal.ROUND_HALF_UP);
bdTest1 = bdTest1.setScale(2, BigDecimal.ROUND_HALF_UP);
System.out.println("bdTest:"+bdTest); //1.75
System.out.println("bdTest1:"+bdTest1);//0.74 problemmmm ????????????


but got weird results. Why?

Answer

Never construct BigDecimals from floats or doubles. Construct them from ints or strings. floats and doubles loose precision.

This code works as expected (I just changed the type from double to String):

public static void main(String[] args) {
  String doubleVal = "1.745";
  String doubleVal1 = "0.745";
  BigDecimal bdTest = new BigDecimal(  doubleVal);
  BigDecimal bdTest1 = new BigDecimal(  doubleVal1 );
  bdTest = bdTest.setScale(2, BigDecimal.ROUND_HALF_UP);
  bdTest1 = bdTest1.setScale(2, BigDecimal.ROUND_HALF_UP);
  System.out.println("bdTest:"+bdTest); //1.75
  System.out.println("bdTest1:"+bdTest1);//0.75, no problem
}