Remove all objects from list that does not exist in another list

Question

I have two list

    List<Map<String,Object>> list1 =  new ArrayList<Map<String,Object>>();
    List<Map<String,Object>> list2 =  new ArrayList<Map<String,Object>>();

    HashMap<String, Object> hm = new HashMap<String, Object>();
    hm.put("1", new Long(1L));
    HashMap<String, Object> hm2 = new HashMap<String, Object>();
    hm2.put("2", new Long(2L));
    HashMap<String, Object> hm3 = new HashMap<String, Object>();
    hm3.put("3", new Long(3L));
    HashMap<String, Object> hm4 = new HashMap<String, Object>();
    hm4.put("4", new Long(4L));

    list1.add(hm);
    list1.add(hm2);
    list1.add(hm3);
    list1.add(hm4);

    HashMap<String, Object> hm1 = new HashMap<String, Object>();
    hm1.put("1", new Long(1L));
    HashMap<String, Object> hm5 = new HashMap<String, Object>();
    hm5.put("2", new Long(2L));

    list2.add(hm1);
    list2.add(hm5);

I want to remove all objects from list1 that does not exist in another list2
My expected output is list1 -- [{2=2, 1=1}]

I can Iterate through list1 and can check if element present then do nothing else remove the element. But I want to know is there a better approach or simpler code?

Duplicate of http://stackoverflow.com/questions/16888265/remove-all-objects-in-an-arraylist-that-exist-in-another-arraylist? — austin wernli, May 12 '15 at 19:12
Do you mean to aggregate the contents of the HashMaps as well?, or did you intend the expected output to be `[{"2"=2L},{"1"=1L}]` ? — Morgen, May 12 '15 at 19:15
@austinwernli is not a duplicated, because the other question ask exactly the reverse case... — Mike D3ViD Tyson, Apr 17 '18 at 15:34

score 24 · Accepted Answer · answered May 12 '15 at 19:15

The Collection.retainAll method exists for exactly that purpose:

Retains only the elements in this collection that are contained in the specified collection. In other words, removes from this collection all of its elements that are not contained in the specified collection.

Usage would be:

list1.retainAll(list2);

It might not be any more efficient than a naive iteration approach, however, unless you use Sets instead of Lists.

Remove all objects from list that does not exist in another list

1 Answers1