Java floating point math - (conversion for feet/meters)

Question

Pretty basic question I think - I'm performing this function:

private double convertMetersToFeet(double meters)
{
  //function converts Feet to Meters.
      double toFeet = meters;
      toFeet = meters*3.2808;  // official conversion rate of Meters to Feet
      return toFeet;
}

Problem is the output; for example I get 337.36080000000004 from an input of 101. What's the appropriate practice for truncating the floating points?

As the answers below assumed, I'd want 4 significant figures to remain consistent with my conversion ratio.

Answer 1

Use java.math.BigDecimal for decimal arithmetic.

Answer 2

You can use a NumberFormat instance.

NumberFormat nf = NumberFormat.getInstance(Locale.UK);
nf.setMinimumFractionDigits(4);
nf.setMaximumFractionDigits(4);
System.out.println(nf.format(feet));

Or you can use DecimalFormat.

DecimalFormat df = new DecimalFormat("0.0000");
System.out.println(df.format(feet));

The latter (DecimalFormat), is to be used when you explicitly want to state the format and the former (NumberFormat), when want localized settings.

For four consistent fractional figures, there is no need to drag in BigDecimal, if your aren't working with really long distances.

Answer 3

If you need precise calculations, use BigDecimal instead of float. If you just want to truncate on printing, use DecimalFormat.

DecimalFormat df = new DecimalFormat ("0.00");
System.out.println(df.format(convertMetersToFeet(101)));

Answer 4

I'm answering my own question for posterity's sake. I used the DecimalFormat answer above, but the answers failed to take into account the return type of the method.

Here's the finished code:

  private double convertMetersToFeet(double meters)
{
  //function converts Feet to Meters.
      double toFeet = meters;
      toFeet = meters*3.2808;  // official conversion rate of Meters to Feet
      String formattedNumber = new DecimalFormat("0.0000").format(toFeet); //return with 4 decimal places
      double d = Double.valueOf(formattedNumber.trim()).doubleValue();
      return d;
}

Answer 5

If it is not for printing you should not care about the error. 337.36080000000004 is just as correct as 337.3608 since you only have 5 significant digits in the factor and only 3 in the test input. (And I certainly hope that the answer your method gave was 331 and not 337)

However, this line fetched from another question seems to do the trick as well.

double toFeet = ((int)(meters*3.2808*10000))/10000.0;

The only problem is that is overflows a lot faster.