Java Collectors Methods

The Collectors class in Java is a part of the Java Stream API, which provides a set of useful methods for performing mutable reduction operations on streams of elements. Collectors are primarily used with the Stream.collect method to accumulate elements into collections, calculate summary statistics, or perform other types of reduction operations.

This guide covers various methods available in the Collectors class. Each method is described in simple terms to help beginners understand how to use them. These methods enable a wide range of operations on streams, making it a powerful tool for handling collections and stream data in Java.

For more detailed information, please refer to the official Java SE Documentation. Additional resources and tutorials can provide deeper insights into practical applications of these methods.

Java Collectors Interface Methods

The table below contains various methods of the Java Collectors interface, each with a link to a detailed explanation, examples, and real-world uses. Click on the method names to learn more about how to use them effectively in your applications.

Method	Description
counting()	Returns a Collector that counts the number of input elements.
averagingInt()	Returns a Collector that calculates the arithmetic mean of integer values.
collectingAndThen()	Adapts a Collector to perform an additional finishing transformation.
filtering()	Returns a Collector that applies a filtering function to input elements before accumulation.
flatMapping()	Returns a Collector that performs a flat-mapping operation on input elements.
groupingBy()	Returns a Collector that groups elements by a classifier function.
groupingByConcurrent()	Returns a concurrent Collector that groups elements by a classifier function.
joining()	Returns a Collector that concatenates the input elements into a String.
mapping()	Returns a Collector that applies a mapping function to input elements before accumulation.
maxBy()	Returns a Collector that produces the maximal element according to a given comparator.
minBy()	Returns a Collector that produces the minimal element according to a given comparator.
partitioningBy()	Returns a Collector that partitions the input elements according to a predicate.
reducing()	Returns a Collector that performs a reduction on the input elements.
summarizingDouble()	Returns a Collector that produces a summary of double values.
summarizingInt()	Returns a Collector that produces a summary of integer values.
summingDouble()	Returns a Collector that sums double values.
summingLong()	Returns a Collector that sums long values.
teeing()	Returns a Collector that merges the results of two other Collectors.
toCollection()	Returns a Collector that accumulates the input elements into a new collection.
toConcurrentMap()	Returns a concurrent Collector that accumulates elements into a ConcurrentMap.
toList()	Returns a Collector that accumulates the input elements into a List.
toMap()	Returns a Collector that accumulates elements into a Map.
toSet()	Returns a Collector that accumulates the input elements into a Set.
toUnmodifiableList()	Returns a Collector that accumulates elements into an unmodifiable List.
toUnmodifiableMap()	Returns a Collector that accumulates elements into an unmodifiable Map.
toUnmodifiableSet()	Returns a Collector that accumulates elements into an unmodifiable Set.

References:

• Java SE Documentation