spring - In JSP, Field Path Uses HashMap<String,ArrayList<String>> (SpringMVC Form) -

suppose model field hashmap of strings arraylist's of strings.

hashmap<string,arraylist<string>> map = new hashmap<string,arraylist<string>>(); 

for field binding, need point i-th item of arraylist of specific key in jsp. how do it? double-bracket notation work? assume 'key' , 'index' known jstl variables in jsp.

<form:checkbox path="map[${key}][${index}]" /> 

what i'm doing is, field value, store path e.g.

map['testkey'][4] 

which point to

map.get("testkey").get(4); 

that doesn't work:

org.springframework.beans.nullvalueinnestedpathexception: invalid property 'map[testkey][0]'  

note according (binding map of lists in spring mvc), problem because sub-objects not auto-growable, , need implement lazylist or growable list? arraylist growable spring mvc's forms, when used sub-collection, it's not? tricky.

i solved problem following lazylist/lazyset implementation in link above, binding map of lists in spring mvc

from example,

public class prsdata {    private map<string, list<prscddata>> prscddata;    public prsdata() {       this.prscddata = maputils.lazymap(new hashmap<string,list<object>>(), new factory() {            public object create() {               return lazylist.decorate(new arraylist<prscddata>(),                               factoryutils.instantiatefactory(prscddata.class));           }        });   }  } 

once had data structure mapped jsp path, field binding magically started working , make path e.g. map["key"][index] in jsp.

as replied, springmvc auto-converts simple lists , maps lazy collections, not when collection subobject.