When should I use the “strictfp” keyword in java?

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Strictfp ensures that you get exactly the same results from your floating point calculations on every platform. If you don’t use strictfp, the JVM implementation is free to use extra precision where available.

From the JLS:

Within an FP-strict expression, all
intermediate values must be elements
of the float value set or the double
value set, implying that the results
of all FP-strict expressions must be
those predicted by IEEE 754 arithmetic
on operands represented using single
and double formats. Within an
expression that is not FP-strict, some
leeway is granted for an
implementation to use an extended
exponent range to represent
intermediate results; the net effect,
roughly speaking, is that a
calculation might produce “the correct
answer” in situations where exclusive
use of the float value set or double
value set might result in overflow or
underflow.

In other words, it’s about making sure that Write-Once-Run-Anywhere actually means Write-Once-Get-Equally-Wrong-Results-Everywhere.

With strictfp your results are portable, without it they are more likely to be accurate.

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