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Java 8 Stream reduce() Example

By Arvind Rai, August 02, 2018
On this page we will provide Java 8 Stream reduce() example. Stream reduce() performs a reduction on the elements of the stream. It uses identity and accumulator function for reduction. In parallel processing we can pass combiner function as additional parameter to this method. Stream reduce() can be used to get the sum of numbers stored in collection. It can also concatenate the string data stored in collection with given separator. Stream reduce() method can perform many more reducing task as required. We are providing here some examples.

Stream.reduce() with Accumulator

Here we will pass BinaryOperator as accumulator. In case of numeric BinaryOperator, the start value will be 0. In case of string, the start value will be a blank string.

reduce(BinaryOperator accumulator)

The method will return Optional instance. Find the example.
ReduceDemo1.java
package com.concretepage;
import java.util.Arrays;
public class ReduceDemo1 {
    public static void main(String[] args) {
    	  int[] array = {23,43,56,97,32};
    	  Arrays.stream(array).reduce((x,y) -> x+y).ifPresent(s -> System.out.println(s));
    	  Arrays.stream(array).reduce(Integer::sum).ifPresent(s -> System.out.println(s));
    	  Arrays.stream(array).reduce(StatisticsUtility::addIntData).ifPresent(s -> System.out.println(s));
    }
}  
StatisticsUtility.java
package com.concretepage;
public class StatisticsUtility {
	public static int addIntData(int num1, int num2) {
		return num1 + num2;
	}
} 
Output
251
251
251 

Stream.reduce() with Identity and Accumulator

Here we will use an identity and accumulator. We will pass the identity as start value.

reduce(T identity, BinaryOperator<T> accumulator)

Find the example.
ReduceDemo2.java
package com.concretepage;
import java.util.Arrays;
public class ReduceDemo2 {
    public static void main(String[] args) {
    	  int[] array = {23,43,56,97,32};
    	  //Set start value. Result will be start value + sum of array. 
    	  int startValue = 100;
    	  int sum = Arrays.stream(array).reduce(startValue, (x,y) -> x+y);
    	  System.out.println(sum);
    	  sum = Arrays.stream(array).reduce(startValue, Integer::sum);
    	  System.out.println(sum);
    	  sum = Arrays.stream(array).reduce(startValue, StatisticsUtility::addIntData);
    	  System.out.println(sum);
    }
}  
Output
351
351
351 

Stream.reduce() with Identity, Accumulator and Combiner

Here we will pass three arguments identity, accumulator and combiner in reduce() method. The identity value must be an identity for the combiner function. This method with three arguments is used in parallel processing. Combiner works with parallel stream only, otherwise there is nothing to combine.

reduce(U identity, BiFunction<U,? super T,U> accumulator, BinaryOperator<U> combiner)

Find the example.
ReduceDemo3.java
package com.concretepage;
import java.util.Arrays;
import java.util.List;
public class ReduceDemo3 {
    public static void main(String[] args) {
      List<Integer> list2 = Arrays.asList(5, 6, 7);
      int res = list2.parallelStream().reduce(1, (s1, s2) -> s1 * s2, (p, q) -> p * q);
      System.out.println(res);
    }
} 
Output
210 

Reduce List and Array into a String


ReduceToString.java
package com.concretepage;
import java.util.Arrays;
import java.util.List;
public class ReduceToString {
	public static void main(String[] args) {
		//Reduce Array to String.
		String[] array = {"Mohan", "Sohan", "Mahesh"};
		Arrays.stream(array).reduce((x, y) -> x +"," + y)
		       .ifPresent(s -> System.out.println("Array to String: "+ s));
		//Reduce List to String.
		List<String> list = Arrays.asList("Mohan", "Sohan", "Mahesh");
		list.stream().reduce((x, y) -> x +"," + y)
			       .ifPresent(s -> System.out.println("List to String: "+ s));
	}
} 
Output
Array to String: Mohan,Sohan,Mahesh
List to String: Mohan,Sohan,Mahesh 

Reduce List and Array into Sum


ReduceToSum.java
package com.concretepage;
import java.util.Arrays;
import java.util.List;
public class ReduceToSum {
	public static void main(String[] args) {
		//Reduce Array to sum.
		int[] array = {30, 10, 20, 40};
		int sum = Arrays.stream(array).reduce(0, (x, y) -> x + y);
		System.out.println("Sum of Array: "+ sum);
		//Reduce List to sum.
		List<Integer> list = Arrays.asList(30, 10, 20, 40);
		sum = list.stream().reduce(0, (x, y) -> x + y);
		System.out.println("Sum of List: "+ sum);
	}
} 
Output
Sum of Array: 100
Sum of List: 100 
POSTED BY
ARVIND RAI
ARVIND RAI
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