android - how can i generate the total amount automatically? -

i new android , trying generate total amount automatically after user input quantity in buying page. first product price activity , make edit text quantity. after page auto show total amount. has experience this.

my code:

public class pagebuy extends activity { textview lblpid; textview lblname; textview lblamount; edittext quantity; integer total_amount;  private static final string key_error_msg = "error_msg"; private static final string tag_name = "product_name"; private static final string tag_pid = "product_id"; private static final string tag_amount = "amount"; private static final string key_quantity = "quantity";    private static final string key_uid = "uid"; private static final string key_created_at = "created_at";  /** called when activity first created. */ @override public void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         setcontentview(r.layout.pagebuy);          intent gg = getintent();           string product_id = gg.getstringextra(tag_pid);         string product_name = gg.getstringextra(tag_name);         string amount = gg.getstringextra(tag_amount);          lblpid = (textview) findviewbyid(r.id.product_id_label);         lblname = (textview) findviewbyid(r.id.product_name_label);         lblamount = (textview) findviewbyid(r.id.amount_label);          lblpid.settext(product_id);         lblname.settext(product_name);         lblamount.settext(amount);          quantity = (edittext) findviewbyid(r.id.quantity);          string quan = quantity.gettext().tostring().trim();         system.out.println(quan +"short");         //userfunctions userfunction = new userfunctions();         //jsonobject json = userfunction.rebuy(product_id, amount, quan);          button btn_confirm = (button) findviewbyid(r.id.btn_confirm);         button btn_home = (button) findviewbyid(r.id.btn_home);            btn_confirm.setonclicklistener(new view.onclicklistener() {               public void onclick(view view) {                   databasehandler db = new databasehandler(getapplicationcontext());                          string uid = db.getcurrentid();                          system.out.println("shortalex" + uid);                          string product_id = lblpid.gettext().tostring().trim();                         string amount = lblamount.gettext().tostring();                         string quan = quantity.gettext().tostring().trim();                          userfunctions userfunction = new userfunctions();                         jsonobject json = userfunction.rebuy(product_id, amount, quan,uid);                         //databasehandler2 db = new databasehandler2(getapplicationcontext());                         jsonobject json_user;                         try {                                system.out.println("shorty");                             system.out.println(json.tostring());                             json_user = json.getjsonobject("userbuy");                          } catch (jsonexception e) {                             // todo auto-generated catch block                             e.printstacktrace();                         }                            toast.maketext(pagebuy.this, "purchase success", toast.length_short).show();                         intent = new intent(getapplicationcontext(), pagepurchaserecord.class);                         startactivity(i);                  }                        });         btn_home.setonclicklistener(new view.onclicklistener() {               public void onclick(view view) {                      intent = new intent(getapplicationcontext(), main.class);                     startactivity(i);                 }         });      } } 

you need textwatcher this

            quantity.addtextchangedlistener(new textwatcher() {                  @override                 public void ontextchanged(charsequence s, int start, int before, int count) {                     // update quanttiti here                  }                  @override                 public void beforetextchanged(charsequence s, int start, int count,                         int after) {                     // todo auto-generated method stub                  }                  @override                 public void aftertextchanged(editable s) {                     // todo auto-generated method stub                  }             });