Android SQLite Query DB field against a Texfield value ?

I'm currently coding my login activity. I want to test to see if the username the user has entered is in the database. So far i have...

        EditText email = (EditText) findViewById(R.id.tf_email);
        EditText pass = (EditText) findViewById(R.id.tf_password);

        String username = email.getText().toString();
        String password = pass.getText().toString();

        DBAdapter db = new DBAdapter(this);

        db.open();
        Cursor u = db.getAllUsers();
        if (username = .....)

Thank you in advance.