Working with Stream Collectors
1. Collecting to List
Method Result Collectors.toList()Mutable List (impl-specific) Collectors.toUnmodifiableList()Immutable stream.toList() JAVA 16+ Unmodifiable, allows null
2. Collecting to Set
Method Result Collectors.toSet()HashSet Collectors.toUnmodifiableSet()Immutable Collectors.toCollection(TreeSet::new)Custom Set impl
3. Collecting to Map
Example: toMap
Map< Long , User > byId = users. stream () . collect (Collectors. toMap (User :: id, Function. identity ())); Map< String , Integer > sumByDept = employees. stream () . collect (Collectors. toMap ( Employee :: dept, Employee :: salary, Integer :: sum // merge function for duplicates ));
Overload Use 2 args Throws on duplicate keys 3 args Merge function for duplicates 4 args Custom map supplier
4. Joining Strings
Method Result joining()Concat with no delimiter joining(", ")With delimiter joining(", ", "[", "]")With prefix/suffix
5. Grouping Elements
Example: groupingBy
Map< String , List< Employee >> byDept = employees. stream () . collect (Collectors. groupingBy (Employee :: dept)); Map< String , Long > countByDept = employees. stream () . collect (Collectors. groupingBy (Employee :: dept, Collectors. counting ())); Map< String , Map< String , Long >> nested = employees. stream () . collect (Collectors. groupingBy (Employee :: dept, Collectors. groupingBy (Employee :: role, Collectors. counting ())));
Form Detail groupingBy(classifier)Classifier → List groupingBy(classifier, downstream)Custom downstream collector groupingBy(classifier, mapFactory, downstream)Custom map type
6. Partitioning Elements
Method Result partitioningBy(predicate)Map<Boolean, List<T>>With downstream partitioningBy(p, counting())
7. Counting Elements
Method Result Collectors.counting()Long totalstream.count()long
8. Finding Statistics
Collector Result summingInt/Long/Double(fn)Sum averagingInt/Long/Double(fn)Mean minBy(cmp) / maxBy(cmp)Optional summarizingInt(fn)IntSummaryStatistics (count, sum, min, max, avg)
9. Reducing Elements
Example: reduce
int sum = nums. stream (). reduce ( 0 , Integer :: sum); Optional< Integer > max = nums. stream (). reduce (Integer :: max); String concat = strs. stream (). reduce ( "" , (a, b) -> a + b);
Form Returns reduce(identity, op)T reduce(op)Optional<T>reduce(id, accumulator, combiner)U (parallel-safe)
10. Creating Custom Collectors
Component Detail Supplier New container Accumulator Add element to container Combiner Merge two containers (parallel) Finisher Transform container to result Factory Collector.of(supplier, accumulator, combiner, finisher)
11. Collecting to Unmodifiable Collections
Method Result toUnmodifiableList()Immutable list toUnmodifiableSet()Immutable set toUnmodifiableMap(k, v)Immutable map
Warning: Unmodifiable collectors do NOT allow null elements/keys/values.
12. Using Downstream Collectors
Example: Downstream chaining
Map< String , Set< String >> emailsByDept = employees. stream () . collect (Collectors. groupingBy ( Employee :: dept, Collectors. mapping (Employee :: email, Collectors. toSet ()) ));
Collector Use mapping(fn, downstream)Transform before collecting filtering(p, downstream)Filter before collecting flatMapping(fn, downstream)Flatten before collecting collectingAndThen(c, finisher)Post-process result