为什么在删除大量（〜80％）密钥后HashMap的大小没有改变？

Question

I need to use a HashMap whose keys are of Long datatype and values are some user-objects, defined as:

HashMap <Long,SomeClass> dummy=new HashMap<>();

Initially this dummy hashmap contains about 10 million <key,value> pairs, added using

  dummy.add(SomeLong,new SomeClass(SomeParameters);

The memory consumed is 7-8GB. After creating this map, 80-90% of entries are to be removed:

for (Iterator<Entry<Long, SomeClass>> it = collisionMap.entrySet().iterator(); it.hasNext();) {
    Map.Entry<Long,SomeClass> entry = it.next();         
    if(SomeCondition) {
       it.remove(); 
    }
 }  

The memory being used is still same after removing these entries. I checked with Runtime.getRuntime().totalMemory() and Runtime.getRuntime().freeMemory() .

Now the problem is why the memory has not been reclaimed after this remove() operation? I am creating this type of hashmap about 1000-2000 times during the course of program. And it is giving java.lang.OutOfMemoryError: GC overhead limit exceeded error :(

Can anyone help? Thanks

***************** Update/Additional information ****************

The SomeClass is defined as:

Class SomeClass {
   private ArrayList <Integer> list1;
   private ArrayList <Integer> list2;    
   public SomeClass(int l,List <Integer> l2) {
     list2=l2;
     list1=new ArrayList<>();
     list1.add(l);
   }     
   public void addList1(int l) { list1.add(l); } 
   public ArrayList <Integer> getList1() { return list1; } 
   public ArrayList <Integer> getList2() { return list2; }    
}

For some reasons, I had earlier planned to use BitSet datatype instead of ArrayList list1 . And if I replace list1' by a BitSet variable and set the l^th bit instead of adding l to list1' and then create dummy HashMap with the objects of this class, then the results about memory are different. Surprisingly, the memory is reclaimed after remove operation on HashMap. Eg After removing about 80% of entries in HahsMap memory being used is also reduced by 40% approx.

Is it that memory allocated to an ArrayList is not being reclaimed? :( :(

Answer 1

java.lang.OutOfMemoryError: GC overhead limit exceeded

This indicates that you're not necessarily using up too much memory (although the map may consume up to 80MB for its internal nodes array in your situation) but that the GC does not have enough breathing room (max heap limit) or CPU time (GC time limit) available to do its work given your allocation profile.

You most likely have to tune your GC parameters to avoid this issue.