How to multiply a BigDecimal by an integer in Java

Question

How do you multiply a BigDecimal by an integer in Java? I tried this but its not correct.

import java.math.BigDecimal;
import java.math.MathContext;

public class Payment {
    int itemCost;
    int totalCost = 0;

    public BigDecimal calculateCost(int itemQuantity,BigDecimal itemPrice){
        itemCost = itemPrice.multiply(itemQuantity);
        totalCost = totalCost + itemCost;
    return totalCost;
   }

I'd assume it's the compilation error since `multiply` takes another `BigDecimal`? — Dave Newton, Oct 17 '12 at 22:51
You should probably be using `BigDecimal` for `itemCost` and `totalCost`. After all, what if `itemPrice * itemQuantity` is more than will fit in an `int`? (And if that can never happen, why are you using `BigDecimal` to start with?) — Ted Hopp, Oct 17 '12 at 22:52
Define "it's not correct". Not an adequate problem statement. We can't tell whether you mean "compile error", "incorrect result", "computer exploded", ... — user207421, Oct 18 '12 at 00:46

Juvanis · Accepted Answer · 2021-06-09T14:01:48.707

106

You have a lot of type-mismatches in your code such as trying to put an int value where BigDecimal is required. The corrected version of your code:

public class Payment
{
    BigDecimal itemCost  = BigDecimal.ZERO;
    BigDecimal totalCost = BigDecimal.ZERO;

    public BigDecimal calculateCost(int itemQuantity, BigDecimal itemPrice)
    {
        itemCost  = itemPrice.multiply(BigDecimal.valueOf(itemQuantity));
        totalCost = totalCost.add(itemCost);
        return totalCost;
    }
}

edited Jun 09 '21 at 14:01

answered Oct 17 '12 at 22:53

Juvanis

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Heh. I was right about the `BigDecimal(int)` constructor. :P – Robert Harvey Oct 17 '12 at 22:55
14

Using `BigDecimal.valueOf(itemQuantity)`, instead of the constructor, will reuse BigDecimals zero through ten, avoiding probable new object construction. – Crunch Aug 28 '15 at 00:47

score 4 · Answer 2 · answered Oct 17 '12 at 23:19

If I were you, I would set the scale of the BigDecimal so that I dont end up on lengthy numbers. The integer 2 in the BigDecimal initialization below sets the scale.

Since you have lots of mismatch of data type, I have changed it accordingly to adjust.

class Payment   
{
      BigDecimal itemCost=new BigDecimal(BigInteger.ZERO,  2);
      BigDecimal totalCost=new BigDecimal(BigInteger.ZERO,  2);

     public BigDecimal calculateCost(int itemQuantity,BigDecimal itemPrice)
       { 
           BigDecimal   itemCost = itemPrice.multiply(new BigDecimal(itemQuantity)); 
             return totalCost.add(itemCost); 
       }
  }

BigDecimals are Object , not primitives, so make sure you initialize itemCost and totalCost , otherwise it can give you nullpointer while you try to add on totalCost or itemCost

+1 for emphasizing the issue about the scale. While you're at it, please note that your multiplication will end up with `itemCost` having a scale of `4`, not `2`: "Returns a BigDecimal whose value is (this × multiplicand), and whose scale is (this.scale() + multiplicand.scale())" https://docs.oracle.com/javase/8/docs/api/java/math/BigDecimal.html#multiply-java.math.BigDecimal- — pyb, Jan 13 '16 at 15:38

score 2 · Answer 3 · answered Oct 17 '12 at 22:52

First off, BigDecimal.multiply() returns a BigDecimal and you're trying to store that in an int.

Second, it takes another BigDecimal as the argument, not an int.

If you just use the BigDecimal for all variables involved in these calculations, it should work fine.

How to multiply a BigDecimal by an integer in Java

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