CollectorsgroupingBy

Collectors.groupingBy is a static method in the Java standard library (java.util.stream.Collectors) that implements a downstream collector to group elements of a stream by a classifier function and collect the results into a map. It is commonly used to categorize data, such as grouping objects by a property like city, category, or type. By default, when invoked with a classifier function, it yields a map from each classification key to a list of elements that share that key.

Overloads and behavior: groupingBy supports several variants. The simplest form takes a classifier function and returns a Map<K, List<T>> where K is the type produced by the classifier and T is the stream element type. Other overloads allow customizing the result: you can supply a downstream collector to define how values are produced for each key (for example, counting, summing, or joining), or provide a map factory to choose the map implementation (such as a LinkedHashMap). There is also a groupingByConcurrent variant designed for parallel streams that produces a concurrent map.

Downstream usage: The basic usage groups elements into lists per key. A common pattern combines groupingBy with a downstream collector to produce different aggregate results, such as groupingBy(Person::getCity, Collectors.counting()) to count elements per city, or groupingBy(Person::getDepartment, Collectors.maxBy(Comparator.comparing(Person::getSalary))) to select a representative element per group.

Examples and considerations: Typical usage includes grouping by a property to drive reporting or categorization. When introducing a downstream collector, the value type in the resulting map changes accordingly. For parallel processing, groupingByConcurrent can improve performance but has different behavior with regards to thread-safety and map type. Overall, groupingBy is a versatile tool for data aggregation within streams. See also Collectors and related grouping utilities.