Error while using global variable in android

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Android

i am trying to implement a login screen where username and password are validated with the help of a rest service.

i have created a global class like this

public class GlobalClass extends Application{
    String url="myRestService";
    String newURL;

    public String getName() {
        return url;
    }

    public void setName(String aName) {
        url = aName;
    }

    public String getNewURL() {
        return newURL;
    }

    public void setNewURL(String bName) {
        newURL = bName;
    }
}

I have created a screen where the superadmin can change the rest service like
this class is intented to update the rest service

final GlobalClass globalVariable = new GlobalClass();
        String restMethod=globalVariable.getName();
        et.setText(restMethod);
        b= (Button) findViewById(R.id.button);

        b.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String userEnteredMethod=et.getText().toString();
                globalVariable.setNewURL(userEnteredMethod);
                Toast.makeText(getApplicationContext(),userEnteredMethod,Toast.LENGTH_SHORT).show();
                Intent login=new Intent(getApplicationContext(),Login.class);
                startActivity(login);
            }
        });

Now in my login
i called the global class like this and get the url

final GlobalClass gb = new GlobalClass();
       userEnteredMethod = gb.getNewURL();

String newURL = userEnteredMethod.concat("/MethodResult");
        AsyncHttpClient client = new AsyncHttpClient();
        client.get(newURL, params, new AsyncHttpResponseHandler() {
            @Override
            public void onSuccess(String response) {
                try {
                   // prgDialog.hide();
                    JSONObject obj = new JSONObject(response);
                    if (obj != null) {
                        JSONArray data = obj.getJSONArray("ValidateSubscriberLoginResult");
                        for (int i = 0; i < data.length(); i++) {
                            JSONObject user = data.getJSONObject(i);
                            //
                            if (!user.getString("ID").equals("0")) {
                               Toast.makeText(getApplicationContext(),"success",Toast.LENGTH_SHORT).show();
                            } else
                            {
                                Toast.makeText(getApplicationContext(),"Check again",Toast.LENGTH_SHORT).show();
                            }
                        }
                    }
                } catch (JSONException e) {
                    Toast.makeText(getApplicationContext(), "Error Occured [Server's JSON response might be invalid]!", Toast.LENGTH_LONG).show();

                    e.printStackTrace();
                }
                super.onSuccess(response);
            }

on executing the code i'm getting a null pointer exception in String newURL = userEnteredMethod.concat("/MethodResult");
while debuging i found out that i am getting a null value at userEnteredMethod = gb.getNewURL();

please help me to rectify the problem