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LongStream reduce(LongBinaryOperator op) in Java

  • Last Updated : 06 Dec, 2018

LongStream reduce(LongBinaryOperator op) performs a reduction on the elements of this stream, using an associative accumulation function, and returns an OptionalLong describing the reduced value, if any.

A reduction operation or fold takes a sequence of input elements and combines them into a single summary result, such as finding the sum or maximum of a set of numbers. An operator or function op is associative if the following holds :

(a op b) op c == a op (b op c)

This is a terminal operation i.e, it may traverse the stream to produce a result or a side-effect. After the terminal operation is performed, the stream pipeline is considered consumed, and can no longer be used.

Syntax :

OptionalLong reduce(LongBinaryOperator op)

Parameters :



  • OptionalLong : A container object which may or may not contain a long value. If a value is present, isPresent() will return true and getAsLong() will return the value.
  • LongBinaryOperator : An operation upon two long-valued operands and producing a long-valued result.
  • op : An associative, stateless function for combining two values.

Return Value : An OptionalLong describing the reduced value, if any.

Example 1 :




// Java code for LongStream reduce
// (LongBinaryOperator op)
import java.util.OptionalLong;
import java.util.stream.LongStream;
  
class GFG {
  
    // Driver code
    public static void main(String[] args)
    {
        // Creating a LongStream
        LongStream stream = LongStream.of(9L, 10L, 11L, 12L);
  
        // Using OptionalLong (a container object which
        // may or may not contain a non-null value)
        // Using LongStream reduce(LongBinaryOperator op)
        OptionalLong answer = stream.reduce(Long::sum);
  
        // if the stream is empty, an empty
        // OptionalLong is returned.
        if (answer.isPresent()) {
            System.out.println(answer.getAsLong());
        }
        else {
            System.out.println("no value");
        }
    }
}

Output :

42

Example 2 :




// Java code for LongStream reduce
// (LongBinaryOperator op)
import java.util.OptionalLong;
import java.util.stream.LongStream;
  
class GFG {
  
    // Driver code
    public static void main(String[] args)
    {
        // Creating a LongStream
        LongStream stream = LongStream.of(9L, 10L, 11L, 12L);
  
        // Using OptionalLong (a container object which
        // may or may not contain a non-null value)
        // Using LongStream reduce(LongBinaryOperator op)
        OptionalLong answer = stream.reduce((a, b) -> 2 * (a * b));
  
        // if the stream is empty, an empty
        // OptionalLong is returned.
        if (answer.isPresent()) {
            System.out.println(answer.getAsLong());
        }
        else {
            System.out.println("no value");
        }
    }
}

Output :

95040

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