App force closes when a textedit is left blank

Question

I am a new android programmer and I am trying to make a "Quadratic Equation Solver App". The user has to input 3 values in 3 respective TextEdit-s and those values are stored in doubles. If the user leaves one of the inputs blank , the app force closes. How can I set all values to 0 by default , so the app doesn't force close ?

public class MainActivity extends Activity {

EditText inputA, inputB, inputC;
TextView nrRoots, root1, root2, roots;
Button bStepByStep, calculate;
Double a, b, c, D, x1, x2, x, dRootNr;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    inputA = (EditText) findViewById(R.id.etParamA);
    inputB = (EditText) findViewById(R.id.etParamB);
    inputC = (EditText) findViewById(R.id.etParamC);
    nrRoots = (TextView) findViewById(R.id.tvNrRoots);
    root1 = (TextView) findViewById(R.id.tvRoot1);
    roots = (TextView) findViewById(R.id.tvRoots);
    root2 = (TextView) findViewById(R.id.tvRoot2);
    calculate = (Button) findViewById(R.id.bCalculate);
    bStepByStep = (Button) findViewById(R.id.bStepByStep);

    calculate.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View arg0) {
            a = Double.parseDouble(inputA.getText().toString());
            b = Double.parseDouble(inputB.getText().toString());
            c = Double.parseDouble(inputC.getText().toString());

            double D = Math.sqrt((b * b) - (4 * a * c));

            if (D == 0) {
                nrRoots.setText("The equation " + a + "x^2+" + b + "x+" + c
                        + "=0 has 1 distinct root.");
                x = ((-1) * b) / (2 * a);
                root1.setText("x=" + x);
                root2.setText("");
                roots.setText("");
            } else if (D > 0) {
                nrRoots.setText("The equation " + a + "x^2+" + b + "x+" + c
                        + "=0 has 2 distinct roots.");
                x1 = (((-1) * b) + D) / (2 * a);
                x2 = (((-1) * b) - D) / (2 * a);
                root1.setText("x1= " + x1);
                root2.setText("x2= " + x2);
            } else {
                nrRoots.setText("The equation " + a + "x^2+" + b + "x+" + c
                        + "=0 has no distinct roots.");
                root1.setText("");
                root2.setText("");
            }

        }

    });

Answer 1

this is NPE at line Double.parseDouble(..).

Create this function

public double getDouble(String str) {

    if (str != null) {

        if (str.equalsIgnoreCase("")) {
            return 0;
        } else {
            return Double.parseDouble(str);
        }
    } else {
        return 0;
    }
}

Then use this code

        a = getDouble(inputA.getText().toString());
        b = getDouble(inputB.getText().toString());
        c = getDouble(inputC.getText().toString());