Collectors partitioningBy() method in Java
Last Updated : 06 Dec, 2018
Collectors partitioningBy() method is a predefined method of java.util.stream.Collectors class which is used to partition a stream of objects(or a set of elements) based on a given predicate. There are two overloaded variants of the method that are present. One takes only a predicate as a parameter whereas the other takes both predicate and a collector instance as parameters.
partitioningBy(Predicate<? super T> predicate)
Syntax: public static <T> Collector<T, ?, Map<Boolean, List<T>>> partitioningBy(Predicate<? super T> predicate)
where,
- Interface Collector<T, A, R>: A mutable reduction operation that accumulates input elements into a mutable result container, optionally transforming the accumulated result into a final representation after all input elements have been processed. Reduction operations can be performed either sequentially or in parallel.
- T: The type of input elements to the reduction operation.
- A: The mutable accumulation type of the reduction operation.
- R: The result type of the reduction operation.
- Map<Boolean, List<T>>: The map containing the output.Keys are boolean values(true or false) and the corresponding values are lists containing elements of type T.
Parameters: This method takes a mandatory parameter
predicate which an instance of a Predicate Interface of type
T.
Return Value: This method returns a
Collector implementing the partitioning operation. Below is an example to illustrate partitioningBy() method:
Program: Java // Java code to show the implementation of // Collectors partitioningBy() function import java.util.List; import java.util.Map; import java.util.stream.Collectors; import java.util.stream.Stream; class Gfg { // Driver code public static void main(String[] args) { // creating an Integer stream Stream<Integer> s = Stream.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 10); // using Collectors partitioningBy() // method to split the stream of elements into // 2 parts, greater than 3 and less than 3. Map<Boolean, List<Integer> > map = s.collect( Collectors.partitioningBy(num -> num > 3)); // Displaying the result as a map // true if greater than 3, false otherwise System.out.println("Elements in stream " + "partitioned by " + "less than equal to 3: \n" + map); } } Output: Elements in stream partitioned by less than equal to 3: {false=[1, 2, 3], true=[4, 5, 6, 7, 8, 9, 10]} partitioningBy(Predicate<? super T>predicate, Collector<? super T, A, D> downstream)
Syntax: public static <T> Collector<T, ?, Map<Boolean, List<T>>> partitioningBy(Predicate<? super T>predicate, Collector<? super T, A, D> downstream)
where,
- Interface Collector<T, A, R>: A mutable reduction operation that accumulates input elements into a mutable result container, optionally transforming the accumulated result into a final representation after all input elements have been processed. Reduction operations can be performed either sequentially or in parallel.
- T: The type of input elements to the reduction operation.
- A: The mutable accumulation type of the reduction operation.
- R: The result type of the reduction operation.
- Map<Boolean, List<T>>: The map containing the output.Keys are boolean values(true or false) and the corresponding values are lists containing elements of type T.
Parameters: This method takes two parameters a
predicate which an instance of a Predicate Interface of type
T, and a collector which is used for implementing "downstream reduction" and producing the output.
Return Value: This method returns a
Collector implementing the partitioning operation. Below is an example to illustrate type 2 of partitioningBy() method:
Java // Java code to show the implementation of // Collectors partitioningBy() function import java.util.List; import java.util.Map; import java.util.stream.Collectors; import java.util.stream.Stream; class ArraytoArrayList { // Driver code public static void main(String[] args) { // creating an Integer stream Stream<Integer> s = Stream.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 10); // Using Collectors.counting() method // to count the number of elements in // the 2 partitions Map<Boolean, Long> map = s.collect( Collectors.partitioningBy( num -> (num > 3), Collectors.counting())); // Displaying the result as a map // true if greater than 3, false otherwise System.out.println("Elements in stream " + "partitioned by " + "less than equal to 3: \n" + map); } } Output: Elements in stream partitioned by less than equal to 3: {false=3, true=7}