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Error: while trying to format decimal output in Java

I am writing this program as an assignment for school. The program takes input in the form of 'sex' and 'age' from the user, and gives back the average age of all men and/or women.
The program has worked fine up until my mom beta tested it and we found a problem by happenstance. If by any chance the user were to input a number of individuals where the sum of their ages is not divisible by the number of individuals inputted, the output will give an answer with 15 decimal places.
For example if I input 3 men with the ages 98, 1 and 1, the program divides 100 by 3 and I get the output:
33.333333333333336.
So I took to SO to find a solution to this problem, and found this which I implemented in my program like below so that it would trim down the answer to a maximum of 3 decimal places:
/*
This method takes two values. The first value is divided by the second value to get the average. Then it trims the
answer to output a maximum of 3 decimal places in cases where decimals run amok.
*/
public static double average (double a, double b){
double d = a/b;
DecimalFormat df = new DecimalFormat("#.###");
return Double.parseDouble(df.format(d));
I wrote the code in the bottom of my program, in its own method, which I call in the main method at lines 76 and 77:
// Here we calculate the average age of all the people and put them into their respective variable.
double yAverage = average(yAge, men);
double xAverage = average(xAge, women);
However. I get this error message when I try to run the program, and I don't understand the error message. I tried googling the error, but found nothing.
Please keep in mind that I'm a beginner, and I need as simple an answer as anyone can give me.
Thank you in advance!

The problem is that DecimalFormat honors you Locale setting, formatting the number according to your language setting.
E.g. in US English the result is 33.333, but in Germany the result is 33,333.
However, Double.parseDouble(String s) is hardcoded to only parse US English formatting.
A few options to fix it:
Don't round the value. Recommended
Use a DecimalFormat wherever the value needs to be displayed, but keep the full precision of the value itself.
Force DecimalFormat to use US English formatting symbols.
DecimalFormat df = new DecimalFormat("#.###", DecimalFormatSymbols.getInstance(Locale.US));
Use the DecimalFormat to re-parse the value.
DecimalFormat df = new DecimalFormat("#.###");
try {
return df.parse(df.format(d)).doubleValue();
} catch (ParseException e) {
throw new AssertionError(e.toString(), e);
}
Don't convert to/from string to round to 3 decimal places.
Use Math.round(double a).
return Math.round(d * 1000d) / 1000d;
Use BigDecimal (and stick with it). Recommended
return BigDecimal.valueOf(d).setScale(3, RoundingMode.HALF_UP);
Use BigDecimal (temporarily).
return BigDecimal.valueOf(d).setScale(3, RoundingMode.HALF_UP).doubleValue();

Try this code
public static double average(double a, double b) {
double d = a / b;
DecimalFormat df = new DecimalFormat(
"#.###",
DecimalFormatSymbols.getInstance(Locale.ENGLISH)
);
return Double.parseDouble(df.format(d));
}
You're using a formatting with the point as decimal separator ("#.###"). Depending on the location where you run your program, the Java runtime uses a different localisation setting, e.g. in Germany, where a comma is used as decimal separator.
When you use new DecimalFormat("#.###") the default locale is used to interpret the string #.### which may work in some places, but won't in others. Luckily, there is another constructor for DecimalFormat where you can specify what symbols should be used. By using DecimalFormatSymbols.getInstance(Locale.ENGLISH) as second parameter you specify that you want the English formatting conventions ("." as decimal separator, "," for thousands).

Double with specific format and two decimals

I'm trying to format the numbers to look like a certain way.
So, I have like the number 1007,2, and I want it to look like 1 007,20
This has two factors needed:
Thousands format
Two decimal places
I have a code that sets the thousands format:
Double total_value = Double.valueOf(1007,2);
String formatedValue = NumberFormat.getNumberInstance(Locale.CANADA_FRENCH).format(total_value);
And the output is:
1 007,2€
And I have the code for the two decimal places:
Double total_value = Double.valueOf(1007,2);
String formatedValue = String.format("%.2f", total_value);
The problem is, for using this two format methods at the same time they get always problems, because the two of them return Strings, and both need to receive the values for formatting in Double.
If I receive one in String, when I try to parse the String to Double like String value = Double.parseDouble(formatedValue);or String value = Double.valueOf(formatedValue) they always get an error. I've already tried the DecimalFormat to but it returns a String too.
So, I dont know how to do to conjugue the two methods to work together!
If you have any idea please comment it :)

You don't want to combine NumberFormat and String.format().
You can further configure your NumberFormat object to tell it to use two decimal places:
NumberFormat numberFormat = NumberFormat.getNumberInstance(Locale.CANADA_FRENCH);
numberFormat.setMinimumFractionDigits(2);
assertThat(numberFormat.format(1007.2), is("1 007,20"));
(and possibly setMaximumFractionDigits() etc., depending on your needs -- see the Javadoc)
Take care - NumberFormat.format() is not thread-safe.
Alternatively you can use String.format(locale, format, args):
assertThat(String.format(Locale.CANADA_FRENCH, "%,.2f", 1007.2), is("1 007,20"));
The , flag in the format tells the formatter to use a thousands-separator, and the locale tells it that the separator is a space.

After setting the thousand format you could do someting like:
String[] splitter = formatedValue.split("\\,");
int decimalDigits = 0;
if (splitter.length > 1) {
formatedValue = splitter[1].length();
} else {
formatedValue += ",";
}
for (int i = 0; i < 2 - decimalDigits; i++) {
formatedValue += "0";
}
I have not tested this, and it is not the really pretty, but i am using something similar for the english format.
To make sure you have only two decimal digits you should probably round your total value at the beginning.

Method to Insert Commas into Large Numbers

Is there a method that already exists and if not can a method be written that can format large numbers and insert commas into them?
100 = 100
1000 = 1,000
10000 = 10,000
100000 = 100,000
1000000 = 1,000,000
public String insertCommas(Integer largeNumber) {
}

With NumberFormat you can do this easily:
NumberFormat format = NumberFormat.getInstance(Locale.US);
System.out.println(format.format(100));
System.out.println(format.format(1000));
System.out.println(format.format(1000000));
will ouput:
100
1,000
1,000,000

You can use NumberFormat#getNumberInstance with Locale.US:
A Locale object represents a specific geographical, political, or
cultural region. An operation that requires a Locale to perform its
task is called locale-sensitive and uses the Locale to tailor
information for the user. For example, displaying a number is a
locale-sensitive operation— the number should be formatted according
to the customs and conventions of the user's native country, region,
or culture.
System.out.println(NumberFormat.getNumberInstance(Locale.US).format(10000000));
This will print:
10,000,000
Side note: In Java 7, you can write an int with underscores: 1_000_000.

Have you looked at DecimalFormat (a derivation of the abstract NumberFormat)? You can customise the format to your needs.
DecimalFormat myFormatter = new DecimalFormat("###,###.###");
String output = myFormatter.format(value);
Note also that you can determine formatting based upon the locale.

Have you tried java.text.NumberFormat?

I tried to be very clear with what i'm doing, it could be done in much less lines.
The algorithm is simple, i reverse the input string, then i split the number using a regex, in each match we add a comma.
If the number size module 3 is zero (for example 123456) we have to remove the last comma.
Now we restore original string order by reversing it again, and voilá.
public String insertCommas(Integer largeNumber) {
String result;
String reversedNum = new StringBuilder(""+largeNumber).reverse().toString();
String reversedResult = "";
Pattern pattern = Pattern.compile("\\d{3}");
Matcher matcher = pattern.matcher(reversedNum);
int lastIndex = reversedNum.length();
while(matcher.find()){
reversedResult += matcher.group()+",";
lastIndex = matcher.end();
}
String remaining = reversedNum.substring(lastIndex);
reversedResult += remaining;
result = new StringBuilder(reversedResult).reverse().toString();
if(remaining.isEmpty()){
result = new StringBuilder(reversedResult).reverse().toString().substring(1);
}else{
result = new StringBuilder(reversedResult).reverse().toString();
}
return result;
}

How to get this number format in java?

Lets say I am having a number as follows :
long number = 32301672060;
I want the following the format for the number :
323.016.720,60
i.e the last two digits should be separated by comma and then dot between every three digits.
Suppose the another number is :
long number = 139454
then output should be
1.394,54

try Formatter
long number = Integer.MAX_VALUE;
System.out.printf(Locale.GERMAN, "%,.2f", new Double(number/100d) );
output
21.474.836,47

Cast the value to a double, divide it by 100 (to get the 2 decimal points) and then set the current locale to something like de_DE and use NumberFormat to format it for you.
Edit: As noted by Behrang in the comments, after converting the long to a double, it should only be used for display purposes as further calculations using this might result in loss of precision.

long number = 32301672060L;
NumberFormat nb = NumberFormat.getNumberInstance(Locale.GERMAN);
nb.setMinimumFractionDigits(2);
System.out.println(nb.format((double)number/100));
This should work for you. The German Local is important to have the point as decimal point and the comma at the last 2 digits.

Use decimals.
final BigDecimal dec = new BigDecimal(BigInteger.valueOf(32301672060L), 2);
System.out.println(new DecimalFormat("###,##0.00").format(dec));
or instead of the pattern, better to use locale's formats, e.g.
System.out.println(NumberFormat.getCurrencyInstance(Locale.US).format(dec));
or
System.out.println(NumberFormat.getNumberInstance(Locale.US).format(dec));

How can I format a String number to have commas and round?

What is the best way to format the following number that is given to me as a String?
String number = "1000500000.574" //assume my value will always be a String
I want this to be a String with the value: 1,000,500,000.57
How can I format it as such?

You might want to look at the DecimalFormat class; it supports different locales (eg: in some countries that would get formatted as 1.000.500.000,57 instead).
You also need to convert that string into a number, this can be done with:
double amount = Double.parseDouble(number);
Code sample:
String number = "1000500000.574";
double amount = Double.parseDouble(number);
DecimalFormat formatter = new DecimalFormat("#,###.00");
System.out.println(formatter.format(amount));

This can also be accomplished using String.format(), which may be easier and/or more flexible if you are formatting multiple numbers in one string.
String number = "1000500000.574";
Double numParsed = Double.parseDouble(number);
System.out.println(String.format("The input number is: %,.2f", numParsed));
// Or
String numString = String.format("%,.2f", numParsed);
For the format string "%,.2f" - "," means separate digit groups with commas, and ".2" means round to two places after the decimal.
For reference on other formatting options, see https://docs.oracle.com/javase/tutorial/java/data/numberformat.html

Given this is the number one Google result for format number commas java, here's an answer that works for people who are working with whole numbers and don't care about decimals.
String.format("%,d", 2000000)
outputs:
2,000,000

Once you've converted your String to a number, you can use
// format the number for the default locale
NumberFormat.getInstance().format(num)
or
// format the number for a particular locale
NumberFormat.getInstance(locale).format(num)

I've created my own formatting utility. Which is extremely fast at processing the formatting along with giving you many features :)
It supports:
Comma Formatting E.g. 1234567 becomes 1,234,567.
Prefixing with "Thousand(K),Million(M),Billion(B),Trillion(T)".
Precision of 0 through 15.
Precision re-sizing (Means if you want 6 digit precision, but only have 3 available digits it forces it to 3).
Prefix lowering (Means if the prefix you choose is too large it lowers it to a more suitable prefix).
The code can be found here. You call it like this:
public static void main(String[])
{
int settings = ValueFormat.COMMAS | ValueFormat.PRECISION(2) | ValueFormat.MILLIONS;
String formatted = ValueFormat.format(1234567, settings);
}
I should also point out this doesn't handle decimal support, but is very useful for integer values. The above example would show "1.23M" as the output. I could probably add decimal support maybe, but didn't see too much use for it since then I might as well merge this into a BigInteger type of class that handles compressed char[] arrays for math computations.

you can also use the below solution
public static String getRoundOffValue(double value){
DecimalFormat df = new DecimalFormat("##,##,##,##,##,##,##0.00");
return df.format(value);
}

public void convert(int s)
{
System.out.println(NumberFormat.getNumberInstance(Locale.US).format(s));
}
public static void main(String args[])
{
LocalEx n=new LocalEx();
n.convert(10000);
}

You can do the entire conversion in one line, using the following code:
String number = "1000500000.574";
String convertedString = new DecimalFormat("#,###.##").format(Double.parseDouble(number));
The last two # signs in the DecimalFormat constructor can also be 0s. Either way works.

Here is the simplest way to get there:
String number = "10987655.876";
double result = Double.parseDouble(number);
System.out.println(String.format("%,.2f",result));
output:
10,987,655.88

The first answer works very well, but for ZERO / 0 it will format as .00
Hence the format #,##0.00 is working well for me.
Always test different numbers such as 0 / 100 / 2334.30 and negative numbers before deploying to production system.

According to chartGPT
Using DecimalFormat:
DecimalFormat df = new DecimalFormat("#,###.00");
String formattedNumber = df.format(yourNumber);
Using NumberFormat:
NumberFormat nf = NumberFormat.getNumberInstance();
nf.setGroupingUsed(true);
String formattedNumber = nf.format(yourNumber);
Using String.format():
String formattedNumber = String.format("%,.2f", yourNumber);
Note: In all the above examples, "yourNumber" is the double value that you want to format with a comma. The ".2f" in the format string indicates that the decimal places should be rounded to 2 decimal places. You can adjust this value as needed.