Java 8- Lambda expression and streams to iterate over HashMap of HashMap

Question

I need to iterate over HashMap of HashMap using lambda expressions and filter out few undesired entries. I tried a few approaches but it doesn't seem to work. Below is the structure of the map.

Map<String, Map<Date, String>> sensor_tags = new HashMap<String, Map<Date,String>>();

From this map, I need to remove the entries where sensor data is older than certain date (date is key of inner map). Below is the sample data for the map sensor_tags-

String tagName = "zoneSensor";
Map<Date, String> values= new HashMap<Date, String>();

// (1st entry for tag 1) --> date is day before yesterday
Calendar datekey1 = Calendar.getInstance();
datekey1.set(2018, 12, 01, 12, 30, 45);
values.put(datekey1.getTime(), "ON");

// (2nd entry for tag 1) --> date is yesterdys date
Calendar datekey = Calendar.getInstance();
datekey.set(2018, 12, 02, 12, 30, 45);
values.put(datekey.getTime(), "OFF");

// (3rd entry for tag 1) --> date is today
Calendar instance = Calendar.getInstance();
instance.set(2018, 12, 03, 12, 30, 45);
values.put(instance.getTime(), "ON");

//(4th entry for tag 1)--> date is today + 10 sec
instance.add(Calendar.MILLISECOND, 10000);
sensor_tags.put(tagName, values);

values.put(instance.getTime(), "ON");
sensor_tags.put(tagName, values);

// 5th entry for tag2
tagName = "zoneSensor1";
values= new HashMap<Date, String>();
values.put(Calendar.getInstance().getTime(), "NORMAL");
sensor_tags.put(tagName, values);

//code in java 7
for (final Entry<String, Map<Date, String>> entry : sensor_tags.entrySet()) {
            final Iterator<Date> iter = entry.getValue().keySet().iterator();
            while (iter.hasNext()) {
                final Date date = iter.next();
                if (date.before(givendate)) {
                    iter.remove();
                }
        }
}

// code tried in java 8 (I am a beginner here :-) )
 sensor_tags
            .entrySet()
            .stream()
            .map(Map.Entry::getValue) // get an inner map
            .filter(value -> ((Date)value).before(datekey.getTime()));

How can I iterate over and filter data for inner maps? Also, once the data is filtered, I need to retain structure of the map as is, hence the output needs to be collected in Map<String, Map<Date, String>> only.

Are you trying to modify the existing map or produce a filtered copy? — shmosel
– shmosel, Commented Dec 3, 2018 at 23:46
Why did givendate mutate to datekey.getTime() upon switching from loop to Stream code? — Holger
– Holger, Commented Dec 4, 2018 at 8:10

shmosel · Accepted Answer · 2018-12-03 23:49:12Z

5

If you want to modify the existing map, you don't need streams. Just loop and call removeIf():

sensor_tags.values().forEach(
        m -> m.keySet().removeIf(datekey.getTime()::after));

answered Dec 3, 2018 at 23:49

shmosel

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Comments

João Esperancinha · Accepted Answer · 2018-12-04 01:11:06Z

I have made these two methods for you. The first creates a new hashmap and the second changes your Hashmap on the fly with the desired filtering. I didn't check for corner cases but they seem to work. Hope this helps:

Keeps your hashmap

private Map<String, Map<Date, String>> getFilteredResultsKeeping(Map<String, Map<Date, String>> sensor_tags,
        Date givendate) {
    return sensor_tags.entrySet().stream()
            .map(sensorValue -> new SimpleEntry<>(sensorValue.getKey(),
                    sensorValue.getValue().entrySet().stream()
                            .filter(sensor -> !sensor.getKey().before(givendate))
                            .map(sensor -> sensor.getKey())
                            .collect(toMap(date -> date, date -> sensorValue.getValue().get(date)))))
            .collect((toMap(SimpleEntry::getKey, SimpleEntry::getValue)));
}

Changes your hashmap

private Map<String, Map<Date, String>> getFilteredResultsRemoving(Map<String, Map<Date, String>> sensor_tags,
        Date givendate) {
    return sensor_tags.entrySet().stream()
            .peek(sensorValue -> {
                List<Date> invalidDates = sensorValue.getValue().entrySet().stream()
                        .map(sensor -> sensor.getKey())
                        .filter(date -> date.before(givendate)).collect(Collectors.toList());
                invalidDates.forEach(date -> sensorValue.getValue().remove(date));
            })
            .collect((toMap(Entry::getKey, Entry::getValue)));
}

Nikolas · Accepted Answer · 2018-12-04 15:26:56Z

2

if you don't want to modify the existing map then there's no need for a stream, you could simplify your imperative approach to:

for (final Map<Date, String> v : sensor_tags.values()) {
      v.keySet().removeIf(d -> d.before(datekey.getTime()));
}

if you don't want to modify the existing map then you can do:

Map<String, Map<Date, String>> result = sensor_tags.entrySet()
     .stream()
     .collect(toMap(
         Map.Entry::getKey, 
         e -> e.getValue().entrySet().stream()
                                     .filter(a -> !a.getKey().before(datekey.getTime()))
                                     .collect(toMap(Map.Entry::getKey, 
                                                    Map.Entry::getValue))));

edited Dec 4, 2018 at 15:26

Nikolas

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answered Dec 3, 2018 at 23:58

Ousmane D.

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