java - Empty arrayList and empty string throws StringIndexOutOfBounds exception -

i getting stringindexoutofbounds exception in part of code.
field captures mylist optional in ui.this error occurs when mylist/myurl string empty. how handle it?

can correct me doing wrong here?

    if (mylist != null) {              (int r = 0; r < mylist.size(); r++) {                 myurl = myurl + mylist.tostring() + ",";             }              myurl = myurl.substring(0, myrssurl.length() - 1);             myurl = myurl.replace("[", "").replace("]", "");          }    else     {         rsslist.clear();         rssurl=null;         system.out.println("inside else >>>>");     } 

if length 0, myurl = myurl.substring(0, myrssurl.length() - 1); evaluate myurl = myurl.substring(0,-1); getting out of bounds error.

to fix, should check if arraylist.isempty() or if length 0.