title: check the input null
body: <p>i've found this line of code got error if the input not an number</p>
  
  <pre><code>int sum = Integer.parseInt(request.getParameter("sum"));
  </code></pre>
  
  <p>the error message is</p>
  
  <pre><code>type Exception report
  
  message
  
  descriptionThe server encountered an internal error () that prevented it from fulfilling this request.
  
  exception
  
  org.apache.jasper.JasperException: java.lang.NumberFormatException: For input string: "a"
  root cause
  
  java.lang.NumberFormatException: For input string: "a"
  </code></pre>
  
  <p>how to handle the input if the input is a string or null?</p>
  
  <p>thanks</p>