Group a list of objects by an attribute

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Group a list of objects by an attribute

This will add the employee object to the HashMap with locationID as key.

HashMap> hashMap = new HashMap>>();

Iterate over this code and add an employee to the HashMap:

if (!hashMap.containsKey(locationId)) {
    List list = new ArrayList >();
    list.add(employee );

    hashMap.put(locationId, list);
} else {
    hashMap.get(locationId).add(employee );
}

If you want all the employee with particular location details then you can use this:

hashMap.get(locationId);

which will get you all the employee with the same the location ID.

Java 8 groupingBy Collector

You can just use the Collectors.groupingBy() bypassing the grouping logic as a function parameter and you will get the split list with the key parameter mapping. Note that using Optional is used to avoid unwanted NPE when the provided list is null

public static  Map> groupBy(List list, Function keyFunction) {
    return Optional.ofNullable(list)
            .orElseGet(ArrayList::new)
            .stream()
            .collect(Collectors.groupingBy(keyFunction));
}
Now you can groupBy anything with this. For the use case here in the question
MapEmployee>> map = groupBy(employee list, Employee::getLocation);