Java DecimalFormat.format does not format the number?

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;
     }

Jon Lin · Accepted Answer · 2012-08-01 18:52:59Z

6

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.

answered Aug 1, 2012 at 18:52

Jon Lin

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2 Comments

davidfmatheson Over a year ago

@ametren Correct. If the poster wants a certain look to their double, they should really return a String in their method and then return formatter.format(area);.

ametren Over a year ago

Ya, sorry, I didn't intend that as a critique of your answer, just an observation.

OrangeDog · Accepted Answer · 2012-08-01 18:54:24Z

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);
 }

answered Aug 1, 2012 at 18:54

OrangeDog

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4 Comments

Marko Topolnik Over a year ago

Incredible, this is the only correct answer, and besides well-written, with a full code sample, and it got downvoted?? Sorry mate, but I can't give you more than one upvote. At least you're at zero now.

woliveirajr Over a year ago

@MarkoTopolnik: not the downvoter, so I'm only guessing: perhaps the problem was exactly because the question was "homework", and in general it's better to just point solutions, explain, and not provide the full code sample immediately. Teach how to, not "do it".

Marko Topolnik Over a year ago

@woliveirajr Yes, I see, that makes sense and I didn't notice the tag. Still, as OP already provided the full code and this answer only points out the single bit that is wrong with it, just in very clear terms, that DV was not deserved.

OrangeDog Over a year ago

Given the elementary mistake of the question, I thought "showing" as well as "telling" would contribute to the "teaching", as there's clearly some fundamental misunderstanding somewhere.

kosa · Accepted Answer · 2012-08-01 18:52:41Z

1

Formatters format method return String

String temp = formatter.format(area);

System.out.println(temp);

Now you will see formatted code.

answered Aug 1, 2012 at 18:52

kosa

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Comments

Habib · Accepted Answer · 2012-08-01 18:53:24Z

1

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

answered Aug 1, 2012 at 18:53

Habib

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Comments

Vikram · Accepted Answer · 2012-08-01 21:00:14Z

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.

Brian J · Accepted Answer · 2012-08-02 19:32:49Z

-1

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).

edited Aug 2, 2012 at 19:32

answered Aug 1, 2012 at 19:07

Brian J

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