If/Else Validation in Android App?

I am creating a simple App for a college prep assignment using android studio

I have been able to develop most of the App but I need help with the following function:

• Make sure all fields are entered

if yes (do maths calculation)

if no (do no maths calculation, have a popup returning "missing field")

like this

Toast.makeText(getApplicationContext(), "Missing Value", Toast.LENGTH_SHORT).show();

I was thinking an If/Else statement but I am new to all this and cant seem to get it to work

Heres the all the java code I have showing the maths functions etc

//Inside the OnCreate
//Linking the Variables
Numet0 = (EditText) findViewById(R.id.editText);
Numet1 = (EditText) findViewById(R.id.editText2);
Numet2 = (EditText) findViewById(R.id.editText3);
b = (Button) findViewById(R.id.button);
tv = (TextView) findViewById(R.id.textView2);

//Creating Function On Button 1
b.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        Double Num0 = Double.parseDouble(Numet0.getText().toString());
        Double Num1 = Double.parseDouble(Numet1.getText().toString());
        Double Num2 = Double.parseDouble(Numet2.getText().toString());
        //Result Output
            result = ((Num0 - Num1) / Num2);
            tv.setText("Your monthly payment = €" + result);
            //Popup After Button Press
            Toast.makeText(getApplicationContext(), "Result Complete", Toast.LENGTH_SHORT).show();
    }
});