How do I make Java format a double like -3.2 rather than -3.1999999999999953?

Question

My app is producing doubles where Double.toString() produces "-3.1999999999999953" - whereas I would expect it to produce "-3.2".

I'm actually getting these doubles from JScience's Amount#getEstimatedValue().

I don't want to set an arbitrary number of digits for precision, since I don't know how many digits will be significant, but I don't want it to produce numbers that end in "99999999.*".

How can I convert Doubles to Strings without this problem?

Answer 1

Recommended solution

BigDecimal.valueOf (hisDouble).toPlainString ()

---

The hack provided later in the last section of this post was the first thing that came to mind when trying to solve OPs question.

Then a friend asked what I was doing and said that OP be better of using BigDecimal and I went into facepalm mode..

But I'll leave the hack in this post so that the world can see how stupid I can be sometimes.

---

When printing you can use System.out.format.

The snippet below will will round the value of yourDecimal to one decimal and then print the value.

Double yourDouble = -3.1999999999999953;
System.out.format ("%.1f", yourDouble);

output

-3.2

---

The most stupid hack ever written

  public static String fixDecimal (Double d) {
    String  str = "" + d;
    int    nDot = str.indexOf ('.');

    if (nDot == -1)
      return str;

    for (int i = nDot, j=0, last ='?'; i < str.length (); ++i) {
      j = str.charAt (i) == last ? j+1 : 0;

      if (j > 3)
        return String.format ("%."+(i-nDot-j-1)+"f", d);

      last = str.charAt (i);
    }

    return str;
  }

...

Double[] testcases = {
  3.19999999999953,
  3.145963219488888,
  10.4511111112,
  100000.0
};

for (int i =0; i < testcases.length; ++i)
  System.out.println (
    fixDecimal (testcases[i]) + "\n"
  );

output

3.2
3.1459632195
10.45
100000.0