Comparing BigDecimals with Round and SetScale

In Java financial programming, you need the precision BigDecimals afford. However, due to the great precision of this value, comparisons are very error prone. For example:

0.000000000000000000000000000000001 != 0.0

For most cases, we want to round when the calculations are complete to the desired precision, rather than during intermediate steps.

DON’T USE round() as it sets the number of significant digits, not the number of decimals. It will lead to odd behavior if you do, as shown below. Note that

// unexpected round() method behavior - don't use it
MathContext myContext = new MathContext( 2, RoundingMode.HALF_UP);
System.out.println( new BigDecimal( "4.55" ).round( myContext ).toString() ); // 4.6
System.out.println( new BigDecimal( "4.5" ).round( myContext ).toString() ); // 4.5
System.out.println( new BigDecimal( "45.983" ).round( myContext ).toString() ); // 45

Instead, use setScale(), which sets just the number of decimal places as shown below. Imagine your own values for someBD(1-4).

// constant
int myNumDecimals = 2;

// do some math
BigDecimal myValA = someBD1.multiply(someBD2)
BigDecimal myValB = someBD3.divide( someBD4);

// round to desired number decimal places
myValA.setScale( myNumDecimals, RoundingMode.HALF_UP);
myValB.setScale( myNumDecimals, RoundingMode.HALF_UP);

// compare - has three int returns: [-1,0,1] -> [<,==,>]
if( myValA.compareTo(myValB) == 0 ) {
 System.out.println("They match!");
}

Or, more succinctly

// constant
int myNumDecimals = 2;

// do some math, round to desired number decimal places
BigDecimal myValA = someBD1.multiply(someBD2).setScale( myNumDecimals, RoundingMode.HALF_UP);
BigDecimal myValB = someBD3.divide( someBD4).setScale( myNumDecimals, RoundingMode.HALF_UP);

// compare - has three int returns: [-1,0,1] -> [<,==,>]
if( myValA.compareTo(myValB) == 0 ) {
 System.out.println("They match!");
}