How to manage currency values in Java 11.

I am on a quest to improve my knowledge of Java and also because it is required for the class I am taking called Software Engineering where the final project is building an Android app. I used Java last year to learn Hadoop but still found it complicated in comparison with Python.

Quick note to self:

String is an object. Use lower case for primitive types like byte, int, short, double, boolean.

Using jshell to learn Java.

Following the online course Java 11 Essential Training in Linkedin Learning I learned that managing currency values in Java is not as easy as I thought.

To make life even more enjoyable I started looking into Ethereum. Maybe it’s not too late for me. I was looking at the value of 1 USD to Ether 0.00037

I was trying to calculate 3 USD times the value of Ether but it showed too many decimals. Should I round the value? Or use the print format to display the precision? Or is that the wrong way of managing currency?

jshell> double dollarToEther = 0.00037;
dollarToEther ==> 3.7E-4

jshell> dollarToEther * 3;
$18 ==> 0.0011099999999999999

The course Java11 Essential Training recommends to use BigDecimal to manage currencies like this:

  1. Convert the value to String or pass the value as a String
  2. Create a BigDecimal instance
  3. Do calculations with this object
  4. Convert the result to a primitive

Convert the value to String

Convert the double value to String:

> var etherString = Double.toString(dollarToEther);
etherString ==> "3.7E-4"

Create a BigDecimal instance

Convert the String to a BigDecimal object:

> var etherBigDecimal = new BigDecimal(etherString);
etherBigDecimal ==> 0.00037

Calculations with BigDecimal

This is the part that I find the most annoying.

This doesn’t work:

> etherBigDecimal.multiply(3);
|  Error:
|  incompatible types: int cannot be converted to java.math.BigDecimal
|  etherBigDecimal.multiply(3);

The method follows this syntax:

BigDecimal.multiply(BigDecimal multiplicand)

The parameter in the method also must be BigDecimal.

Create the multiplicand. Let’s say this is the USD value as a string.

> var usdBigDecimal = new BigDecimal("3");
usdBigDecimal ==> 3

> var usdToEtherBigDecimal = etherBigDecimal.multiply(usdBigDecimal);
usdToEtherBigDecimal ==> 0.00111

Convert BigDecimal result to a primitive

Conver from BigDecimal to double:

> var usdToEther = usdToEtherBigDecimal.doubleValue();

Other BigDecimal methods

More about BigDecimal in the Oracle page here