Jep

Jep is a Java library for parsing and evaluating mathematical expressions. With this package 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 270KB as jar archive)
* Fast evaluation
* High precision using BigDecimals
* Includes common math functions and operators
* Supports Boolean expressions
* Extendable and configurable architecture
* Support for strings, vectors and complex numbers
* Support for implicit multiplication
* Allows declared or undeclared variables
* Java 1.5 compatible
* Supports Unicode characters
* Extensive documentation
* Includes JavaCC grammar from which the main classes are generated