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How do I make Java format a double like -3.2 rather than -3.1999999999999953?
BigDecimal.valueOf (hisDouble).toPlainString ()

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

-3.2

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"
  );

3.2
3.1459632195
10.45
100000.0

System.out.println(BigDecimal.valueOf(-3.2d).toPlainString());

-3.2