How to parse a mathematical expression given as a string and return a number?

i recently developed a expression parser and released it under the apache license. you can grab it at http://projects.congrace.de/exp4j/index.html

hope that helped

You can pass it to a BeanShell bsh.Interpreter, something like this:

Interpreter interpreter = new Interpreter();
interpreter.eval("result = 5+4*(7-15)");
System.out.println(interpreter.get("result"));

You'll want to ensure the string you evaluate is from a trusted source and the usual precautions but otherwise it'll work straight off.

If you want to go a more complicated (but safer) approach you could use ANTLR (that I suspect has a math grammar as a starting point) and actually compile/interpret the statement yourself.