Java DecimalFormat.format不格式化数字？

Question

I have the method below, but for some reason it is not formatting the area to have only one decimal. All help is appreciated.

public double beraknaArea()
    {
       DecimalFormat formatter = new DecimalFormat("#0.0");
       double area = 0;
       area = radie*radie*3.14;
       formatter.format(area);
       return area;
     }

Answer 1

You are returning the same double, and not the output of the formatter.

The format() method does not alter the double that you pass in, it returns a String format of the double.

Answer 2

No methods in Java may change the type of a declared variable, or mutate any primitive value (or indeed String ). You need to return what formatter.format(area) returns, not the unchanged value of area .

public String beraknaArea()
{
   DecimalFormat formatter = new DecimalFormat("#0.0");
   double area = radie*radie*3.14;
   return formatter.format(area);
 }

Answer 3

Formatters format method return String

String temp = formatter.format(area);

System.out.println(temp);

Now you will see formatted code.

Answer 4

your statement formatter.format(area); is not modifying the variable area , the variable is still holding the same value as before, you may return a string if you want fromated output

Answer 5

The problem is that instead of returning the formatted area, you are returning the area which you computed, but which is not formatted.

Refer following:

public String beraknaArea()
    {
       DecimalFormat formatter = new DecimalFormat("#0.0");
       double area = 0;
       area = radie*radie*3.14;
       String formattedArea = formatter.format(area);
       return formattedArea;
     }



/************ OR **************/



public double beraknaArea()
    {
       DecimalFormat formatter = new DecimalFormat("#0.0");
       double area = 0;
       area = radie*radie*3.14;
       String formattedArea = formatter.format(area);
       return Double.parseDouble(formattedArea);
     }

The first solution returns the formattedArea as a String, whereas second one returns double value of formattedArea.

Answer 6

You could multiply the number by 10, cast it as an integer, and then divide it by 10.0 to get a double with exactly one decimal point.

area = ((int) (area * 10)) / 10.0;

Instead of casting to an int, you could use Math.floor

area = Math.floor(area * 10) / 10.0;

Now, further arithmetic could make the number have a different number of decimal places.

Edit:

This is assuming the OP wants to keep the return value as a double instead of a String (as other answers have provided).