MainActivity.java sqlite

EditText firstname,lastname;
    TextView textView;
    DB_Controller controller;




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

        firstname=(EditText)findViewById(R.id.firstname_input);
        lastname=(EditText)findViewById(R.id.lastname_input);
        textView=(TextView)findViewById(R.id.textView);
        controller =new DB_Controller(this,"",null,1);




    }




    public void btn_click(View view) {

        switch (view.getId()){
            case R.id.btn_add:
                try {
                    controller.insert_student(firstname.getText().toString(),lastname.getText().toString());

                }catch (SQLiteException e){

                    Toast.makeText(MainActivity.this,"ALREDY EXIST",Toast.LENGTH_LONG).show();

                }

                break;
            case R.id.btn_del:
                controller.delete_student(firstname.getText().toString());
                break;
            case R.id.btn_update:
                AlertDialog.Builder dialog=new AlertDialog.Builder(MainActivity.this);
                dialog.setTitle("ENTER NEW FIRSTNAME");
                final EditText new_firstname=new EditText((this));
                dialog.setView(new_firstname);


                dialog.setPositiveButton("OK", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialogInterface,int i){
                        controller.update_student(firstname.getText().toString(),new_firstname.getText().toString());

                    }
                });
                dialog.show();


                break;
            case R.id.btn_show:
                controller.list_all_student(textView);
                break;



        }
    }
}