Exception on Using Double. valueOf() Method

Why dont you catch that Exception?

try{
    mSolution.setText(s.solve(Double.valueOf(a),Double.valueOf(b), Double.valueOf(c)));
}
catch(NumberFormatException e){
    //whatever you feel necessary to do when en empty string is presented
}

Advantage: You don't have to check every String for its correctness. Disadvantage: You dont know which String is not correct. BUT: You could for example insert Arashs code in the catch, so that at that point you can check which string is faulty.

You got java.lang.NumberFormatException: Invalid double: "" because you trying to convert NULL value into Double

so before convert this value you must check the String is null or not like below:

 if(!a.equals("") && !b.equals("") && !c.equals("")){
  //Do your job
  }

Try this

try {

        if (a != null && a.length != 0 && b != null && b.length != 0
                && b != null && b.length != 0) {
            mSolution.setText(s.solve(Double.valueOf(a), Double.valueOf(b),
                    Double.valueOf(c)));
        }
    } catch (NumberFormatException e) {
        e.printStackTrace();
    }

You are getting java.lang.NumberFormatException: Invalid double: "" because the values of a',b, orc` is sometime empty i.e "".

Double.valueOf() will return NumberFormatException if it is empty. To avoid this you can give check points like below,

if(mA.getText().toString().equals("") ||mB.getText().toString().equals("")||mC.getText().toString().equals("") )
   {
   Toast.makeText(getApplicationContext(), "Edittext is empty", Toast.LENGTH_SHORT).show();
    }
   else
    {
      a = mA.getText().toString(); 
      b = mB.getText().toString(); 
      c = mC.getText().toString();
    }