Android ::: How to set variable global / to avoid login form appearing after successful authentication

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?

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