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

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 2

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;
}