Should I use `strictfp` modifier when converting `float` to `double`?

Question

If I want to write method converting from float to double.

Is there any case the two would give back different result?

Which version should I use if I don't care about platform consistency?

What if I need it to behave consistently in different platforms?

  double f2d(float x) {
    return (double) x;
  }

or:

  strictfp double f2d(float x) {
    return (double) x;
  }

JLS 5.1.2:

A widening primitive conversion from float to double that is not strictfp may lose information about the overall magnitude of the converted value.

why will this conversion lose info?

Most likely you don't need this function. Can you give an example of how it would be used? — Peter Lawrey, Oct 13 '18 at 15:15
Possible duplicate of [When should I use the "strictfp" keyword in java?](https://stackoverflow.com/questions/517915/when-should-i-use-the-strictfp-keyword-in-java) — O.O.Balance, Oct 13 '18 at 15:36

score 1 · Accepted Answer · answered Oct 13 '18 at 19:57

strictfp will make no difference for float to double conversion. The reason is in Java Language Specification, 4.2.3. Floating-Point Types, Formats, and Values. In particular:

Note that the constraints in Table 4.2.3-A are designed so that every element of the float value set is necessarily also an element of the float-extended-exponent value set, the double value set, and the double-extended-exponent value set.

Since every element of the float value set is also an element of the double value set a float to double conversion will be exact regardless of whether it is FP-strict or not.

Should I use `strictfp` modifier when converting `float` to `double`?

1 Answers1