Expr

or within a program:
Hashtable vars = new Hashtable();
vars.put("x", new Double(2));
vars.put("y", new Double(10.0));
double result = Expr.eval("(x+1) * (y%4) + cond(x<>y, 10, 0)", vars);

Available functions are: + - * / ^ ? % sqrt() sin() cos() tan() asin() acos() atan() exp() ln() cond(,,) min(,) max(,)

cond works somewhat like an if/then/else, or rather like the "a ? b : c" construct in C or Java. The 1st argument should evaluate to true or false (the numerical comparison operators can be used); if it's true, cond evaluates to the 2nd argument, otherwise to the 3rd.
? returns a random number between 0 and 1.
Variables can be upper- or lowercase; they override functions of the same name. "PI" and "E" are predefined to be the corresponding mathematical constants.

Changes:

• May 1997: ported to Oberon and small enhancements
• November 1997: ported to Java
• May 1999: now accepts an arbitrary number of variables
• May 1999: added cond and % functions, PI and E constants
• June 2001: added min, max, tan, asin, acos functions, and ? (random number)
• March 2002: enhanced min and max to accept an arbitrary number of arguments;
  added AND, OR and NOT operators: AND and OR accept an arbitrary number of arguments -
  a number whose absolute value is below 1.0e-6 is considered to be logically false, otherwise true

The source code is available here.