Java字符串用逗号拼接

		String str = "";
                String tn_select = "";
                for(AddTnBean itemBean : list){
                    if(itemBean.isSelect()){
                        str+=(itemBean.getPosition()+1)+",";
                        tn_select = str.substring(0,str.length()-1);
                    }
                }
                if(str.equals("")){
                }else{
//                    Toast.makeText(mActivity,str,Toast.LENGTH_SHORT).show();
                    tn_Select= tn_select;
                }