title: Java string to math equation
body: <p>I need to implement function <code>public int eval(String infix) {...}</code>
  and when I use this like this:</p>
  
  <pre><code>eval("3+2*(4+5)") 
  </code></pre>
  
  <p>I must receive 21. </p>
  
  <p>The arithmetic expression can contain '+', '*' and parentheses.</p>
  
  <p>So, how can I convert this to math equation? <strong>I can't use non-standard libs.</strong></p>
  
  <p><strong>UPDATE:
  Solution found.</strong></p>
  
  <p><strong>It is 2 way: Polish Notation and using ScriptEngine.</strong></p>
  
  <hr>