title: request.getParameter don't have null values when they are expected
body: <p>I found this issue so perplexing (but also interesting) that I'd like to ask people here for insights.</p>
  
  <p>I've been teaching myself JSP and associated technologies. What I'm tring to do is to retrieve parameters from a JSP to a servlet, then use them with If( ).
  Here is part my coding.</p>
  
  <pre><code>if ((request.getParameter("ID_A") != null || request.getParameter("Password_A") != null) &amp;&amp;
      (request.getParameter("ID_B") != null || request.getParameter("Password_B") != null)) {
  
          errorMessage = "Information is detected for the both side";
          request.setAttribute("errorMessage", errorMessage);
          request.getRequestDispatcher("4_enter_personal_info.jsp").forward(request, response);
      } // Other conditions...
  </code></pre>
  
  <p>Here is part of a JSP(The previous step)</p>
  
  <pre><code>&lt;form action="PersonalInfor_to_confirmation_servlet" method="POST"&gt;
      &lt;h2&gt;For an existing customer&lt;/h2&gt;
          &lt;p&gt;Customer ID&lt;/p&gt;
          &lt;input type="text" name="ID_A" value="" /&gt;
          &lt;p&gt;Password&lt;/p&gt;
          &lt;input type="text" name="Password_A" value="" /&gt;
          &lt;br&gt;
      &lt;h2&gt;For a new customer&lt;/h2&gt;
          &lt;p&gt;Set your customer ID&lt;/p&gt;
          &lt;input type="text" name="ID_B" value="" /&gt;
          &lt;p&gt;Set your password&lt;/p&gt;
          &lt;input type="text" name="Password_B" value="" /&gt;
      //There are other lines
  &lt;/form&gt;
  </code></pre>
  
  <p>I'm tring to ensure that, when a client entered information on the both sides (<code>For an existing customer/For a new customer</code>), there will appear the above message <code>"Information is detected for the both side"</code> on a JSP.</p>
  
  <p>However, even if all the text boxes are blank, the error message appears. So all the <code>request.getParameter( )</code> methods above don't contain null values even if I make them empty.</p>
  
  <p>Even if I can come up with other algorithms, I'd like to know the reason this phenomenon happens.</p>
  
  <p>Any advice will be appreciated.</p>