How to nicely format floating numbers to string without unnecessary decimal 0’s

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In short:

If you want to get rid of trailing zeros and locale problems, then you should use:

double myValue = 0.00000021d;

DecimalFormat df = new DecimalFormat("0", DecimalFormatSymbols.getInstance(Locale.ENGLISH));
df.setMaximumFractionDigits(340); //340 = DecimalFormat.DOUBLE_FRACTION_DIGITS

System.out.println(df.format(myValue)); //output: 0.00000021

Explanation:

Why other answers did not suit me:

  • Double.toString() or System.out.println or FloatingDecimal.toJavaFormatString uses scientific notations if double is less than 10^-3 or greater than or equal to 10^7

     double myValue = 0.00000021d;
 String.format("%s", myvalue); //output: 2.1E-7

  • by using %f, the default decimal precision is 6, otherwise you can hardcode it, but it results in extra zeros added if you have fewer decimals. Example:

     double myValue = 0.00000021d;
 String.format("%.12f", myvalue); // Output: 0.000000210000

  • by using setMaximumFractionDigits(0); or %.0f you remove any decimal precision, which is fine for integers/longs but not for double

     double myValue = 0.00000021d;
 System.out.println(String.format("%.0f", myvalue)); // Output: 0
 DecimalFormat df = new DecimalFormat("0");
 System.out.println(df.format(myValue)); // Output: 0

  • by using DecimalFormat, you are local dependent. In the French locale, the decimal separator is a comma, not a point:

     double myValue = 0.00000021d;
 DecimalFormat df = new DecimalFormat("0");
 df.setMaximumFractionDigits(340);
 System.out.println(df.format(myvalue)); // Output: 0,00000021

    Using the ENGLISH locale makes sure you get a point for decimal separator, wherever your program will run.

Why using 340 then for setMaximumFractionDigits?

Two reasons:

  • setMaximumFractionDigits accepts an integer, but its implementation has a maximum digits allowed of DecimalFormat.DOUBLE_FRACTION_DIGITS which equals 340
  • Double.MIN_VALUE = 4.9E-324 so with 340 digits you are sure not to round your double and lose precision

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