Java Streams - Collectors reducing(BinaryOperator op) example

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Collectors reducing(BinaryOperator<T> op) returns a Collector to reduce its input elements under a specified BinaryOperator.

Syntax

reducing has the following syntax.

public static <T> Collector<T,?,Optional<T>> reducing(BinaryOperator<T> op)

Example

The following example shows how to use reducing.

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.stream.Collectors;
/* ww  w .j a  v a2 s .  c o m*/
public class Main {
  public static void main(String...args){
    Map<Type, Optional<Food>> o = 
    Food.menu.stream().collect(
        Collectors.groupingBy(Food::getType,
            Collectors.reducing((Food d1, Food d2) -> d1.getCalories() > d2.getCalories() ? d1 : d2)));
    
    System.out.println(o);
    
  }
}
enum Type { MEAT, FISH, OTHER }
class Food {

  private final String name;
  private final boolean vegetarian;
  private final int calories;
  private final Type type;

  public Food(String name, boolean vegetarian, int calories, Type type) {
      this.name = name;
      this.vegetarian = vegetarian;
      this.calories = calories;
      this.type = type;
  }

  public String getName() {
      return name;
  }

  public boolean isVegetarian() {
      return vegetarian;
  }

  public int getCalories() {
      return calories;
  }

  public Type getType() {
      return type;
  }
  @Override
  public String toString() {
      return name;
  }

  public static final List<Food> menu =
          Arrays.asList( new Food("pork", false, 1800, Type.MEAT),
                         new Food("beef", false, 7100, Type.MEAT),
                         new Food("chicken", false, 1400, Type.MEAT),
                         new Food("french fries", true, 1530, Type.OTHER),
                         new Food("rice", true, 3510, Type.OTHER),
                         new Food("season fruit", true, 1120, Type.OTHER),
                         new Food("pizza", true, 5150, Type.OTHER),
                         new Food("prawns", false, 1400, Type.FISH),
                         new Food("salmon", false, 4150, Type.FISH));
}

The code above generates the following result.

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