Group a list of objects by an attribute
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by Er Satya
All > Tech Blogger | Java | Spring Boot | HTML | CSS | MySQL > Java | Spring Boot
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HashMap with locationID as key.HashMap> hashMap = new HashMap >>();HashMap:if (!hashMap.containsKey(locationId)) {
List list = new ArrayList >();
list.add(employee );
hashMap.put(locationId, list);
} else {
hashMap.get(locationId).add(employee );
}
hashMap.get(locationId);which will get you all the employee with the same the location ID.
Java 8 groupingBy Collector
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 questionMapEmployee >> map = groupBy(employee list, Employee::getLocation);
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