Java string to math equation [duplicate]

Question

I need to implement function public int eval(String infix) {...} and when I use this like this:

eval("3+2*(4+5)") 

I must receive 21.

The arithmetic expression can contain '+', '*' and parentheses.

So, how can I convert this to math equation? I can't use non-standard libs.

UPDATE: Solution found.

It is 2 way: Polish Notation and using ScriptEngine.

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Answer 1

Believe it or not, with JDK1.6, you can use the built-in Javascript engine. Customise to suit your needs.

Make sure you have these imports...

import javax.script.ScriptEngineManager;
import javax.script.ScriptEngine;

Code:

ScriptEngineManager mgr = new ScriptEngineManager();
ScriptEngine engine = mgr.getEngineByName("JavaScript");
String infix = "3+2*(4+5)";
System.out.println(engine.eval(infix));