How use stream filter for list of hashmap? ArrayList<HashMap<String, Object>>

Question

I want to filter an array list containing hash maps using the stream API.

ArrayList<HashMap<String, Object>> resultslist = new ArrayList<HashMap<String, Object>>();   


ResultSet rs = pstmt.executeQuery();

while (rs.next()) {
    HashMap<String, Object> returndatas = new HashMap<String, Object>();
    int colcnt = rs.getMetaData().getColumnCount();

    for (int i = 1; i <= colcnt; i++) {
        if (rs.getObject(i) instanceof BigInteger) {
            returndatas.put(rs.getMetaData().getColumnName(i), rs.getInt(i));
        } else {
            returndatas.put(rs.getMetaData().getColumnName(i), rs.getObject(i));
        }
    }
    resultslist.add(returndatas);
}

The above method gives the resultslist . Now I want to filter this list by some condition and collect a new array list of hash maps.

For example, I want to select hash maps that have a "null" value and string have "parent_item_id".

I tried the following.

    ArrayList<HashMap<String, Object>> reducedList = resultslist .stream()
    .filter((HashMap<String, Object> hasmap) -> hasmap.entrySet().stream().anyMatch(e -> e.getKey().equals("parent_item_id") && e.getValue().equals(0)))
    .collect(Collectors.toList());

Finally, it's working

I used this. Thankyou for all

List<HashMap<String, Object>> reducedList = resultslist.stream()
                    .filter(hashmap ->((hashmap.get("parent_item_id")) == null ))
                    .collect(Collectors.toList());

Answer 1

It you are just trying to reduce your list of hashmaps based on the presence of a value being == to 0 this will do the job. However, mapping with String, Object makes it seem like you really should be creating an object instead of a map.

List<Map<String, Object>> reducedList = access_collections.stream()
                .filter(hashmap -> hashmap.containsKey("parent_item_id"))
                .filter(hashmap -> (Integer) hashmap.get("parent_item_id") == 0)
                .collect(Collectors.toList());

Based on your snippet above I am assuming parent_item_id is an integer. Another reason to switch whatever is in your map to an actual object instead of trying to model it in a map where we have to guess which type each of your values are.

Answer 2

Well the question is not clear, hence another solution for filtering maps with null :

// If you are sure the key "parent_item_id" exists:
List<HashMap<String, Object>> reducedList = 
        access_collections.stream()
                          .filter(m -> m.get("parent_item_id") == null)
                          .collect(toList());

// If you are not sure then:
List<HashMap<String, Object>> reducedList = 
        access_collections.stream()
                          .filter(m -> m.containsKey("parent_item_id") && m.get("parent_item_id") == null)
                          .collect(toList());

Answer 3

Similar to other but you can use Predicate .

Predicate<HashMap<String, Object>> containsKey = hashmap -> hashmap.containsKey("parent_item_id");
Predicate<HashMap<String, Object>> isNull = hashmap -> hashmap.get("parent_item_id") == null;

List<Map<String, Object>> reducedList = access_collections.stream()
        .filter(containsKey.and(isNull))
        .collect(Collectors.toList());

Or you can use this predicat during while (rs.next()) { and:

if (containsKey.and(isNull).test(returndatas)) {
    resultslist.add(returndatas);
}

UPDATE

Below is only generalization that may blur the first solution (as exercise that not added anything to solution): Change Predicate to BiPredicate and have more generic solution.

BiPredicate<String, HashMap<String, Object>> containsKey = 
        (label, hashmap) -> hashmap.containsKey(label);
BiPredicate<String, HashMap<String, Object>> isNull = 
        (label, hashmap) -> hashmap.get(label) == null;

Function<String, Predicate<HashMap<String, Object>>> checkFor =
        (label) -> (hashmap) -> containsKey.and(isNull).test(label, hashmap);

Predicate<HashMap<String, Object>> checkParentItemId = checkFor.apply("parent_item_id");

List<Map<String, Object>> reducedList = access_collections.stream()
        .filter(checkParentItemId)
        .collect(Collectors.toList());

and

if (checkFor.apply("parent_item_id").test(returndatas)) {
    resultslist.add(returndatas);
}