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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"first implementation - reducing oneToThree Integer stream by addition: " + oneToThree.reduce((acc, item) -> acc+item ) );"second implementation - reducing oneToTwo Integer stream by addition starting " + "with an initial accumulator of 10: " + oneToTwo.reduce(10, (acc, item) -> acc+item ) );"third implementation - reducing stringStream to a list of Integer containing the count of each string in the stream :" +


                        //the initial output
                        new ArrayList<Integer>(),  

                        //accumulating into the initial output
                        (acc, item) -> {              
                            return acc;

                        // combining the output 
                        // (needed only for parallel stream)
                        (acc1, acc2) -> acc1         


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