[android] How to create a popup window (PopupWindow) in Android

This an example from my code how to address a widget(button) in popupwindow

View v=LayoutInflater.from(getContext()).inflate(R.layout.popupwindow, null, false);
    final PopupWindow pw = new PopupWindow(v,500,500, true);
    final Button button = rootView.findViewById(R.id.button);
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            pw.showAtLocation(rootView.findViewById(R.id.constraintLayout), Gravity.CENTER, 0, 0);


        }
    });
    final Button popup_btn=v.findViewById(R.id.popupbutton);

    popup_btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            popup_btn.setBackgroundColor(Color.RED);
        }
    });

Hope this help you