What’s the difference between java.lang.Math and java.lang.StrictMath?

The Javadoc for the Math class provides some information on the differences between the two classes:

Unlike some of the numeric methods of
class StrictMath, all implementations
of the equivalent functions of class
Math are not defined to return the
bit-for-bit same results. This
relaxation permits better-performing
implementations where strict
reproducibility is not required.

By default many of the Math methods
simply call the equivalent method in
StrictMath for their implementation.
Code generators are encouraged to use
platform-specific native libraries or
microprocessor instructions, where
available, to provide
higher-performance implementations of
Math methods. Such higher-performance
implementations still must conform to
the specification for Math.

Therefore, the Math class lays out some rules about what certain operations should do, but they do not demand that the exact same results be returned in all implementations of the libraries.

This allows for specific implementations of the libraries to return similiar, but not the exact same result if, for example, the Math.cos class is called. This would allow for platform-specific implementations (such as using x86 floating point and, say, SPARC floating point) which may return different results.

(Refer to the Software Implementations section of the Sine article in Wikipedia for some examples of platform-specific implementations.)

However, with StrictMath, the results returned by different implementations must return the same result. This would be desirable for instances where the reproducibility of results on different platforms are required.