CheckBox Enable

i have a problem and i dont know how to fix it .I want to make a basic begginer calculator app.I want only one checkbox to be checkable at time but i cant figure out ,stil can check all .And how can i cast strings from edittext to doubl?.App is not finished yet . Sorry for english.

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.top_fragment);

     double result ;

    EditText number_1 = (EditText)findViewById(R.id.number_1);
    EditText number_2 = (EditText)findViewById(R.id.number_2);
    CheckBox add_box = (CheckBox)findViewById(R.id.add_box);
    CheckBox subtract_box = (CheckBox)findViewById(R.id.subtract_box);
    CheckBox divide_box = (CheckBox)findViewById(R.id.divide_box);
    CheckBox product_box = (CheckBox)findViewById(R.id.product_box);
    final Button button = (Button)findViewById(R.id.button);

   number_1.getText().toString();
   number_2.getText().toString();

    if(add_box.isChecked())
    {
        subtract_box.setEnabled(false);
        divide_box.setEnabled(false);
        product_box.setEnabled(false);
    }else if(subtract_box.isChecked())
    {
        product_box.setEnabled(false);
        divide_box.setEnabled(false);
        add_box.setEnabled(false);
    }else if(divide_box.isChecked())
    {
        subtract_box.setEnabled(false);
        product_box.setEnabled(false);
        add_box.setEnabled(false);
    }else if(product_box.isChecked())
    {
        subtract_box.setEnabled(false);
        divide_box.setEnabled(false);
        add_box.setEnabled(false);
    }
}

}