Collectors.partitioningBy method¶

Collectors.partitioningBy is a static method in the java.util.stream.Collectors class used to process a stream of elements and divide them into two groups based on a Predicate.^[600-developer-java-java8-java8-collect.md]

Method Signatures¶

There are two overloaded versions of the partitioningBy method: - partitioningBy(Predicate): Partitions the input elements based on the provided predicate.^[600-developer-java-java8-java8-collect.md] - partitioningBy(Predicate, Collector): Partitions the input elements based on the provided predicate and applies a downstream collector to the values in each partition.^[600-developer-java-java8-java8-collect.md]

Usage¶

The primary use case for this collector is to categorize stream elements by a boolean condition.^[600-developer-java-java8-java8-collect.md]

Basic Partitioning¶

The first variant accepts a Predicate lambda expression.^[600-developer-java-java8-java8-collect.md] For example, to split a list of students into those with high scores and others:

collect(Collectors.partitioningBy(student -> student.getScore() >= 90));

Partitioning with a Downstream Collector¶

The second variant allows an additional Collector to be passed, which is applied to the values in each partition to perform a reduction or other operation.^[600-developer-java-java8-java8-collect.md] For instance, to count the number of elements in each group:

collect(Collectors.partitioningBy(student -> student.getScore() >= 90, Collectors.counting()));

[[Stream API]]
[[Collectors.groupingBy]]

Sources¶

^[600-developer-java-java8-java8-collect.md]