java - Android Hashmap returning null -

i have expandable listview data stored in

hashmap<integer, list<mychildobject>>

and when expanding crashes saying list.size() null pointer in adapter.getchildrencount() simply:

public int getchildrencount(int groupposition) {     return mmap.get(groupposition).size(); } 

now put break point in ran debugger , saw mmap fine, i'm testing 1 element list of element contains 1 child object. can see in debugger element in fact list size = 1.

but testing list<mychildobject> list = mmap.get(0); returns null when can see in debugger mmap has element (<integer.valueof(0), non null list of objects>).

whats weirder when testing 4 hashmap elements:

<0, list> <1, list> <2, list> <3, list> 

it works fine , i'm able access non null list calling mmap.get(n)...

am not using hashmap correctly here integer keys? why unable retrieve elements?

odds pretty it's checking integer object boxes , unboxes correctly values between -128 , 127, because typically these saved. but, equals between these should same both given key value integer check underlying value rather memory location.

why using hashmap? mean doesn't form call out arraylist<arraylist<mychildobject> > ? , in case, should switch count function have null check anyhow.

public int getchildrencount(int groupposition) {     list<mychildobject> list = mmap.get(groupposition);     if (list == null) return 0;     return list.size(); } 

because odds if allow in 0 sized objects, you're off using null.