title: java.lang.NumberFormatException: Invalid double: ""
body: <p>When I try running my Android App, I get an error saying: java.lang.NumberFormatException: Invalid double: "". This is my code: </p>
  
  <pre><code>public class MainActivity extends Activity {
  
  double score;
  EditText gpa;
  EditText sat;
  EditText act;
  Button calc;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
  
      gpa = (EditText) findViewById(R.id.gpa);
      String gpaString = gpa.getText().toString();
      final double gpaDouble = Double.parseDouble(gpaString);
  
      sat = (EditText) findViewById(R.id.sat);
      String satString = sat.getText().toString();
      final int satInt = Integer.parseInt(satString);
  
      act = (EditText) findViewById(R.id.act);
      String actString = act.getText().toString();
      final int actInt = Integer.parseInt(actString);
  
  
      calc = (Button) findViewById(R.id.calc);
  
      calc.setOnClickListener(new View.OnClickListener() {
  
          @Override
          public void onClick(View v) {
              // TODO Auto-generated method stub
              if(actInt/36&gt;satInt/2400){
                  score = (0.6*gpaDouble*25)+(0.4*(actInt/36)*100);
              }else{
                  score = (0.6*gpaDouble*25)+(0.4*(satInt/2400)*100);
              }
  
  
          }
      });
  }
  
  
  }
  </code></pre>
  
  <p>I essentially want to get numbers from three EditTexts, make one of them into a double and the other two into ints. Then I would use these variables to set the value for another double variable. I am not getting errors before I run the app. I feel that when the EditText field is blank, it will not parse correctly, but I am unsure how to solve this. What is the problem? </p>