title: Android DecimalFormat crashes the app
body: <p>I'm having an issue with DecimalFormat in Android Studio.
  I'm following this tutorial to make a calculator app:
  <a href="http://innovativenetsolutions.com/2013/01/calculator-app/" rel="nofollow noreferrer">Tutorial</a></p>
  
  <p>I modified the code a bit and changed if statements to switch statements.
  The rest is the same. When I try to run the app it crashes immediately.
  I tried looking for a bug in the code and found out that this app crashes as long as this line is present:</p>
  
  <pre><code>DecimalFormat df = new DecimalFormat("@###########");
  </code></pre>
  
  <p>After removing this line (and all code containing this variable), the app starts but it doesn't really do much (it displays the numbers but I can't do any operations).</p>
  
  <p>My question is: what is wrong with that line? Should I change it in any way?
  Thanks for your help.</p>