Using Math.round to round to one decimal place?

Question

I have these two variables

double num = 540.512
double sum = 1978.8

Then I did this expression

double total = Math.round((         


        

Answer 1

The Math.round method returns a long (or an int if you pass in a float), and Java's integer division is the culprit. Cast it back to a double, or use a double literal when dividing by 10. Either:

double total = (double) Math.round((num / sum * 100) * 10) / 10;

or

double total = Math.round((num / sum * 100) * 10) / 10.0;

Then you should get

27.3

Answer 2

Double toBeTruncated = new Double("2.25");

Double truncatedDouble = new BigDecimal(toBeTruncated).setScale(1, BigDecimal.ROUND_HALF_UP).doubleValue();

it will return 2.3

Answer 3

try this

for example

DecimalFormat df = new DecimalFormat("#.##");
df.format(55.544545);

output:

55.54

Answer 4

DecimalFormat decimalFormat = new DecimalFormat(".#");
String result = decimalFormat.format(12.763);                // -->  12.7

Answer 5

A neat alternative that is much more readable in my opinion, however, arguably a tad less efficient due to the conversions between double and String:

double num = 540.512;
double sum = 1978.8;

// NOTE: This does take care of rounding
String str = String.format("%.1f", (num/sum) * 100.0); 

If you want the answer as a double, you could of course convert it back:

double ans = Double.parseDouble(str);

Answer 6

If you need this and similar operations more often, it may be more convenient to find the right library instead of implementing it yourself.

Here are one-liners solving your question from Apache Commons Math using Precision, Colt using Functions, and Weka using Utils:

double value = 540.512 / 1978.8 * 100;
// Apache commons math
double rounded1 = Precision.round(value, 1);
double rounded2 = Precision.round(value, 1, BigDecimal.ROUND_HALF_UP);
// Colt
double rounded3 = Functions.round(0.1).apply(value)
// Weka
double rounded4 = Utils.roundDouble(value, 1)

---

Maven dependencies:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-math3</artifactId>
    <version>3.5</version>
</dependency>

<dependency>
    <groupId>colt</groupId>
    <artifactId>colt</artifactId>
    <version>1.2.0</version>
</dependency>

<dependency>
    <groupId>nz.ac.waikato.cms.weka</groupId>
    <artifactId>weka-stable</artifactId>
    <version>3.6.12</version>
</dependency>