(Moderator note: the poster didn't include a URL, but it appears to be
http://www.singularsys.com/jep/ )

JEP is a Java API for parsing and evaluating mathematical expressions.
With this library you can allow your users to enter an arbitrary
formula as a string, and instantly evaluate it. JEP supports user
defined variables, constants, and functions. A number of common
mathematical functions and constants are included.

Features
- Small size (only 75KB as jar archive)
- Supports Boolean expressions (!, &&, ||, <, >, !=, ==, >=, and <=)
- Fast evaluation (the expression can be evaluated quickly for
different variable values)
- Includes common math functions
- Extendable through user defined functions
- Predefined constants such as 'pi' and 'e'
- Support for strings, complex numbers, and vectors
- Support for implicit multiplication (allows use of expressions such
as "3x" instead of "3*x")
- Allows choice between declared and undeclared variables
- Java 1.2 compatible (cross-compiled with Sun JDK 1.2.2)
- Supports Unicode characters (including Greek symbols)
- Extensive documentation
- Includes JavaCC grammar from which the main classes are generated