title: Exception on using Double.valueOf() method
body: <p>I'm a beginner in Android development. I am making a basic square equations solver. 
  I use the code below to get strings from <code>EditText</code> elements:</p>
  
  <pre><code>a = mA.getText().toString(); 
  b = mB.getText().toString(); 
  c = mC.getText().toString(); 
  </code></pre>
  
  <p>Then I use <code>Double.valueOf()</code> method to convert them into doubles and solve the equatation(<code>s</code> is a Solver instance):</p>
  
  <pre><code>mSolution.setText(s.solve(Double.valueOf(a),
                         Double.valueOf(b), Double.valueOf(c)));
  </code></pre>
  
  <p>If <code>EditText</code> is empty, an attempt to solve it gives me an exception</p>
  
  <blockquote>
    <p>java.lang.NumberFormatException: Invalid double: ""</p>
  </blockquote>
  
  <p>I guess, the <code>.toString()</code> method returns <code>null</code> or <code>""</code>, but the if statement doesn't work here :c.</p>