How to declare global variables in Android?

I am creating an application which requires login. I created the main and the login activity.

In the main activity onCreate method I added the following condition:

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        ...
        
        loadSettings();
        if(strSessionString == null)
        {
            login();
        }
        ...
    }

The onActivityResult method which is executed when the login form terminates looks like this:

    @Override
    public void onActivityResult(int requestCode,
                                 int resultCode,
                                 Intent data)
    {
        super.onActivityResult(requestCode, resultCode, data);
        switch(requestCode)
        {
            case(SHOW_SUBACTICITY_LOGIN):
            {
                if(resultCode == Activity.RESULT_OK)
                {
                    
                    strSessionString = data.getStringExtra(Login.SESSIONSTRING);
                    connectionAvailable = true;
                    strUsername = data.getStringExtra(Login.USERNAME);
                }
            }
        }

The problem is the login form sometimes appears twice (the login() method is called twice) and also when the phone keyboard slides the login form appears again, and I guess the problem is the variable strSessionString.

Does anyone know how to set the variable global in order to avoid login form appearing after the user already successfully authenticates?