Java 8 - Streams - Collectors

Java 8 - Streams - Collectors - Partition By

partitionBy operation on Collectors provides a way to aggregate the result of a stream operation into a map.
The map contains two items which are the result partitions based on a predicate function.

stream of employees


Stream employees = Stream.of(
            new Employee("Joe", "Blogs", "Sales", 100.0),
            new Employee("Richardo", "Banks", "Engineering", 50.0),
            new Employee("Hewlet", "Packer", "Warehouse", 40.0)
    );

USAGE


Map<Boolean, List<Employee>> employeePartitions = employees
                .collect(Collectors.partitioningBy(employee -> employee.getSalary() > 50));

Comments

Java 8 - Streams - Introduction

Streams provide extended functionality to collections. All collection operations except operations on Map can be extended using streams. Streams can either be sequential or parallel . Operations on sequential streams are performed on a single thread. Operations on parallel streams are performed concurrently on multiple threads, and so are faster than sequential streams (although item order is affected). NOTE that parallel streams are CPU intensive and should be used with care as it can lead to starvation of other processes in your application. Operations on streams are grouped into intermediate ( transformations ) and terminal ( actions ). Stream processing is triggered only by terminal operations, while intermediate operations return a new stream. an empty stream Stream emptyStream = Stream.empty(); creating a stream from array Stream<Integer> streamFromArray = Stream.of(new Integer[]{1,2,3}); Stream<Integer> streamFromArray2 = Arrays.stream(new In

Java 8 - Streams - Parallel Stream

Operations on parallel streams are performed concurrently on multiple threads and so are faster than sequential streams (although item order is affected) However, operations on parallel streams can be ordered using ' stream.forEachOrdered ' parallel Integer Stream Stream<Integer> parallelIntegerStream = new ArrayList<>(Arrays.asList(1,2,3)).parallelStream(); another parallel Integer Stream Stream<Integer> parallelIntegerStream2 = IntStream.range(0, 10).parallel().boxed(); parallel String Stream Stream<String> parallelStringStream = Arrays.asList("ab","acd","db","dab","ea").parallelStream(); USAGE 1 Consumer<Stream> orderedParallelStreamConsumer = stream -> stream.forEachOrdered(item -> log.info(item.toString())); orderedParallelStreamConsumer.accept(parallelIntegerStream); orderedParallelStreamConsumer.accept(parallelIntegerStream2); USAGE 2 Consumer&

Java 8 - Streams - Actions - Reduce

reduce operation on stream provides a way to accumulate the result of an operation which traverses all items in the stream. The traversal starts from the leftmost item in the stream. There are three implementations of the reduce operation. The first implementation uses the first item in the stream as initial accumulator. The second implementation expects an initial accumulator input. The third implementation provides a way to return an accumulator which has a different type from the stream items. This is similar to foldLeft in other functional languages e.g. Scala. int stream one to three IntStream oneToThree = IntStream.of(1, 2, 3); int stream one to two IntStream oneToTwo = IntStream.of(1, 2); string stream Stream stringStream = Stream.of("a", "ab", "abc", "abcd"); USAGE log.info("first implementation - reducing oneToThree Integer stream by addition: " + oneToThree.reduce((acc, ite

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