BigDecimal round to 0 or 5

Question

I'm tinkering with BigDecimal and currency formatting in Android, and was wondering if it was possible to do the following using BigDecimal:

What I desire:

  64.99 --> 65.00 (Rounded Up)
  64.99 --> 60.00 (Rounded Down)
  65.01 --> 70.00 (Rounded Up)
  65.01 --> 65.00 (Rounded Down)

At present, with my code below, I'm only able to round to zeros. For example:

What I get:

  64.99 --> 70.00 (Rounded Up)
  64.99 --> 60.00 (Rounded Down)
  65.01 --> 70.00 (Rounded Up)
  65.01 --> 60.00 (Rounded Down)

Is there a way using BigDecimal to achieve what I desire?

My code:

private static void printRoundedValues() {
    NumberFormat currencyFormat = NumberFormat.getCurrencyInstance();
    BigDecimal valueUp = new BigDecimal(64.50, new MathContext(1,RoundingMode.UP));
    BigDecimal valueDown = new BigDecimal(64.50, new MathContext(1,RoundingMode.DOWN)); 

    System.out.println("Value Up: " +  currencyFormat.format(valueUp));
    System.out.println("Value Down: " +  currencyFormat.format(valueDown));
}

score 4 · Accepted Answer · answered Feb 19 '14 at 17:16

You can multiply by 2, then round to the nearest 10, then divide by 2. The precision is 2 for these BigDecimals; it may need to be different for BigDecimals of different scale.

BigDecimal[] bds = {new BigDecimal("64.99"), new BigDecimal("65.01")};
BigDecimal two = new BigDecimal("2");
MathContext mcUp = new MathContext(2, RoundingMode.UP);
MathContext mcDown = new MathContext(2, RoundingMode.DOWN);
NumberFormat currency = NumberFormat.getCurrencyInstance();
for (BigDecimal bd : bds)
{
    System.out.println("Test: " + bd);
    BigDecimal roundUp5 = bd.multiply(two).round(mcUp).divide(two);
    System.out.println("Round up: " + currency.format(roundUp5));
    BigDecimal roundDown5 = bd.multiply(two).round(mcDown).divide(two);
    System.out.println("Round down: " + currency.format(roundDown5));
}

Output:

Test: 64.99
Round up: $65.00
Round down: $60.00
Test: 65.01
Round up: $70.00
Round down: $65.00

And never use a `new BigDecimal(65.99)` but `new BigDecimal("65.99")`. Doubles are an approximation, a sum of (negative) powers of 2. And with a String the precision is known for BigDecimal. — Joop Eggen, Feb 19 '14 at 17:20
This is a nice solution plus it gives sensible values. Thank you. @JoopEggen thank you for the tip, that actually saves me some cycles as I was parsing to float then creating a BigDecimal, now I can just pass in the string. — TEK, Feb 19 '14 at 17:27

score 0 · Answer 2 · answered Feb 19 '14 at 17:21

    NumberFormat currencyFormat = NumberFormat.getCurrencyInstance();
    BigDecimal valueUp = new BigDecimal(64.50, new MathContext(2, RoundingMode.UP));
    BigDecimal valueDown = new BigDecimal(64.50, new MathContext(2, RoundingMode.DOWN));

    System.out.println("Value Up: " +  currencyFormat.format(valueUp));
    System.out.println("Value Down: " +  currencyFormat.format(valueDown));

Here is how MathContext precision works:

creates a value (64.5)
set the precision to given value (2) which results in 64.5
round this value using given RoundingMode (UP), which gives you the value of 65

You probably need to set scale rather than precision:

    BigDecimal valueUp = new BigDecimal(64.50).setScale(0, RoundingMode.UP);
    BigDecimal valueDown = new BigDecimal(64.50).setScale(0, RoundingMode.DOWN);

score 0 · Answer 3 · edited May 23 '17 at 12:28

0

The function you are looking for is setScale and is part of BigDecimal.

Here is a good answer to the same question: Java BigDecimal: Round to the nearest whole value

edited May 23 '17 at 12:28

Community

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1

answered Feb 19 '14 at 17:53

Raanan

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BigDecimal round to 0 or 5

3 Answers3