java.util.stream · java.base · since Java 1.8

Collectors

declaration

public final class Collectors

The standard Collector recipes: containers (toList/toSet/toMap), strings (joining), statistics, and the grouping/partitioning family with composable downstreams.

Key methods

`static Collector toList() / toSet() / toUnmodifiableList()`	Container terminals.
`static Collector toMap(keyFn, valueFn[, mergeFn])`	Throws on duplicate keys without a merge function!
`static Collector groupingBy(classifier[, downstream])`	SQL GROUP BY — Map<K, List<T>> or aggregated.
`static Collector partitioningBy(predicate[, downstream])`	Boolean split, both keys guaranteed.
`static Collector joining(delim, prefix, suffix)`	Delimited strings.
`static Collector counting() / summingInt(…) / averagingDouble(…)`	Downstream aggregates.
`static Collector mapping(fn, downstream) / filtering(p, downstream)`	Adapt elements before a downstream.
`static Collector teeing(c1, c2, merger)`	Two collectors, one pass (Java 12).

Example

Java

Map<String, Long> byDept = staff.stream()
        .collect(groupingBy(Employee::dept, counting()));