Filter Map<String,List<Object>> Using Java Streams

Question

class Custom{
   String itemId,
   long createdTS
   //constructor
   public Custom(String itemId,long createdTS)
}

I have two maps which are

Map<String,Long> itemIDToFilterAfterTS;
Map<String,List<Custom>> itemIDToCustoms;

I want to filter 2nd map itemIDToCustoms values using 1st map itemIDToTimestampMap value for item using java streams. eg.

itemIDToFilterAfterTS = new HashMap();
itemIDToFilterAfterTS.put("1",100);
itemIDToFilterAfterTS.put("2",200);
itemIDToFilterAfterTS.put("3",300);

itemIDToCustoms = new HashMap();
List<Custom> listToFilter = new ArrayList();
listToFilter.add(new Custom("1",50));
listToFilter.add(new Custom("1",90));
listToFilter.add(new Custom("1",120));
listToFilter.add(new Custom("1",130));
itemIDToCustoms.put("1",listToFilter)

Now I want to use java streams and want the filtered result map for which getKey("1") gives filtered list of Custom object which has createdTS > 100 (100 will be fetched from itemIDToFilterAfterTS.getKey("1"))

Map<String,List<Custom>> filteredResult will be 
Map{
   "1" : List of (Custom("1",120),Custom("1",130))
}

azro · Accepted Answer · 2021-12-13 19:35:46Z

2

The stream syntax here is a bit too much

itemIDToCustoms = itemIDToCustoms.entrySet().stream().collect(Collectors.toMap(e -> e.getKey(),
            e -> e.getValue().stream().filter(val -> val.createdTS > itemIDToFilterAfterTS.get(e.getKey())).collect(Collectors.toList())));

1 Comment

rahulpareekyx Over a year ago

Thanks for your help, much appreciated..It worked for me!!

Hadi · Accepted Answer · 2021-12-13 21:35:26Z

2

You can use Map#computeIfPresent method like this:

which allows you to compute value of a mapping for specified key if key is already associated with a value

public Object computeIfPresent(Object key,BiFunction remappingFunction)

So you can perform it like:

itemIDToFilterAfterTS.forEach((key, value) ->
            itemIDToCustoms.computeIfPresent(key, (s, customs) -> customs.stream()
                    .filter(c -> c.getCreatedTS() > value)
                    .collect(Collectors.toList())));

answered Dec 13, 2021 at 21:35

Hadi

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