Java 8 Stream.reduce() examples

By mkyong | Updated: February 24, 2020

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Tags: bigdecimal java 8 map reduce stream sum

In Java 8, the Stream.reduce() combine elements of a stream and produces a single value.

A simple sum operation using a for loop.

  int[] numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
  int sum = 0;
  for (int i : numbers) {
      sum += i;
  }

  System.out.println("sum : " + sum); // 55

The equivalent in Stream.reduce()

  int[] numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

  // 1st argument, init value = 0
  int sum = Arrays.stream(numbers).reduce(0, (a, b) -> a + b);

  System.out.println("sum : " + sum); // 55

or method reference with Integer::sum

int sum = Arrays.stream(numbers).reduce(0, Integer::sum); // 55

    /**
     * Adds two integers together as per the + operator.
     *
     * @param a the first operand
     * @param b the second operand
     * @return the sum of {@code a} and {@code b}
     * @see java.util.function.BinaryOperator
     * @since 1.8
     */
    public static int sum(int a, int b) {
        return a + b;
    }

1. Method Signature

1.1 Review the Stream.reduce() method signature:

T reduce(T identity, BinaryOperator<T> accumulator);

int reduce(int identity, IntBinaryOperator op);

long reduce(int identity, LongBinaryOperator op);

• identity = default or initial value.
• BinaryOperator = functional interface, take two values and produces a new value.

1.2 if the identity argument is missing, there is no default or initial value, and it returns an optional.

Optional<T> reduce(BinaryOperator<T> accumulator);

2. More Examples

2.1 Math operations.

int[] numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

int sum = Arrays.stream(numbers).reduce(0, (a, b) -> a + b);    // 55
int sum2 = Arrays.stream(numbers).reduce(0, Integer::sum);      // 55

int sum3 = Arrays.stream(numbers).reduce(0, (a, b) -> a - b);   // -55
int sum4 = Arrays.stream(numbers).reduce(0, (a, b) -> a * b);   // 0, initial is 0, 0 * whatever = 0
int sum5 = Arrays.stream(numbers).reduce(0, (a, b) -> a / b);   // 0

2.2 Max and Min.

int[] numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

int max = Arrays.stream(numbers).reduce(0, (a, b) -> a > b ? a : b);  // 10
int max1 = Arrays.stream(numbers).reduce(0, Integer::max);            // 10

int min = Arrays.stream(numbers).reduce(0, (a, b) -> a < b ? a : b);  // 0
int min1 = Arrays.stream(numbers).reduce(0, Integer::min);            // 0

2.3 Join String.

  String[] strings = {"a", "b", "c", "d", "e"};

  // |a|b|c|d|e , the initial | join is not what we want
  String reduce = Arrays.stream(strings).reduce("", (a, b) -> a + "|" + b);

  // a|b|c|d|e, filter the initial "" empty string
  String reduce2 = Arrays.stream(strings).reduce("", (a, b) -> {
      if (!"".equals(a)) {
          return a + "|" + b;
      } else {
          return b;
      }
  });

  // a|b|c|d|e , better uses the Java 8 String.join :)
  String join = String.join("|", strings);

3. Map & Reduce

A simple map and reduce example to sum BigDecimal from a list of invoices.

package com.mkyong;

import java.math.BigDecimal;
import java.math.RoundingMode;
import java.util.Arrays;
import java.util.List;

public class JavaReduce {

    public static void main(String[] args) {

        List<Invoice> invoices = Arrays.asList(
                new Invoice("A01", BigDecimal.valueOf(9.99), BigDecimal.valueOf(1)),
                new Invoice("A02", BigDecimal.valueOf(19.99), BigDecimal.valueOf(1.5)),
                new Invoice("A03", BigDecimal.valueOf(4.99), BigDecimal.valueOf(2))
        );

        BigDecimal sum = invoices.stream()
                .map(x -> x.getQty().multiply(x.getPrice()))    // map
                .reduce(BigDecimal.ZERO, BigDecimal::add);      // reduce

        System.out.println(sum);    // 49.955
        System.out.println(sum.setScale(2, RoundingMode.HALF_UP));  // 49.96

    }

}

class Invoice {

    String invoiceNo;
    BigDecimal price;
    BigDecimal qty;

    // getters, stters n constructor
}

Output

49.955
49.96

References

• Reduction JavaDoc
• BinaryOperator JavaDoc
• Java – How to join List String with commas
• Java 8 – StringJoiner example