Из одного Activity получить значение другого Activity

public class MainActivity extends Activity {
...
...
Intent intent = new Intent(getBaseContext(), DocsActivity.class);
startActivity(intent);

public DocsActivity(MainActivity mainActivity){
 
    
    }

new Class2(this);

public class Class2 extends Activity {
 
    public static MainActivity mainActivity;
 
    public Class2(MainActivity mainActivity) {
 
        this.mainActivity = mainActivity; //получаю активити первого класса
}

LayoutInflater layoutInflater = (LayoutInflater) getBaseContext().getSystemService(LAYOUT_INFLATER_SERVICE);

    // =========================================================
    // Показ всплывающего информационного окна
    // =========================================================
    public void ShowPopup(String message) {
 
        LayoutInflater layoutInflater = (LayoutInflater) getBaseContext()
                .getSystemService(LAYOUT_INFLATER_SERVICE);
 
        View popupView = layoutInflater.inflate(R.layout.popup, null);
 
        final PopupWindow popupWindow = new PopupWindow(popupView,
                LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
 
        TextView iTextView = (TextView) popupView.findViewById(R.id.iTextView);
        iTextView.setText(message);
 
        btnClose.setOnClickListener(new Button.OnClickListener() {
            @Override
            public void onClick(View v) {
                popupWindow.dismiss();
            }
        });
 
        popupWindow.showAtLocation(tvMain, Gravity.TOP, 10, 100);
        popupWindow.setFocusable(true);
        popupWindow.update();
    }

public PopupWindow ShowPopup(String message, Context context, TextView textView) {
 
        LayoutInflater layoutInflater = (LayoutInflater) context
                .getSystemService(LAYOUT_INFLATER_SERVICE);
 
        View popupView = layoutInflater.inflate(R.layout.popup, null);
 
        final PopupWindow popupWindow = new PopupWindow(popupView,
                LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
 
        textView.setText(message);
        
        popupWindow.showAtLocation(tvMain, Gravity.TOP, 10, 100);
        popupWindow.setFocusable(true);
        popupWindow.update();
    }

public static void ShowPopup(String message, Context context, View view) {
         
        LayoutInflater layoutInflater = (LayoutInflater) context
                .getSystemService(context.LAYOUT_INFLATER_SERVICE);
 
        View popupView = layoutInflater.inflate(R.layout.popup, null);
 
        final PopupWindow popupWindow = new PopupWindow(popupView,
                LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
 
        TextView iTextView = (TextView) popupView.findViewById(R.id.iTextView);
        iTextView.setText(message);
        
        popupWindow.showAtLocation(view, Gravity.TOP, 10, 100);
        popupWindow.setFocusable(true);
        popupWindow.update();
    }