Explain Java8 Collectors partitioningBy method.

Java 8 Collectors.partitioningBy method accepts a predicate and returns a Collector by partitioning the element of the stream using Predicate so it will have 2 keys at most in Map. The key of the map can be only true and false and each key's value is a list of elements.

public static <T> Collector<T,?,Map<Boolean,List<T>>> 
                                         partitioningBy(Predicate<? super T> predicate)

Consider the below example. This program checks if each item of list1 exists in list2 and partition with predicate.

public class PartitioningByEx1 {

	public static void main(String[] args) {

		List<String> list1 = Arrays.asList("A", "B", "C", "D");
		List<String> list2 = Arrays.asList("A", "D", "E");

		list1.stream().collect(Collectors.partitioningBy(item -> list2.contains(item)))
				.forEach((key, val) -> System.out.println(key + "-->>" + val));
	}

}

Output:

false-->>[B, C]
true-->>[A, D]

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