Reduce operation on custom object in java

Question

How to use Reduce operation for performing sum on two fields of an object.

e.g.

class Pojo
{
    public Pojo(int a, int b) {
        super();
        this.a = a;
        this.b = b;
    }
    int a ;
    int b;
    public int getA() {
        return a;
    }
    public void setA(int a) {
        this.a = a;
    }
    public int getB() {
        return b;
    }
    public void setB(int b) {
        this.b = b;
    }

}

Pojo object1 = new Pojo(1, 1);
Pojo object2 = new Pojo(2, 2);
Pojo object3 = new Pojo(3, 3);
Pojo object4 = new Pojo(4, 4);

List<Pojo> pojoList = new ArrayList<>();

pojoList.add(object1);
pojoList.add(object2);
pojoList.add(object3);
pojoList.add(object4);

I can perform sum using IntStream like this:

int sum = pojoList.stream()
                  .mapToInt(ob -> (ob.getA() + ob.getB()))
                  .sum();

I want to perform the same operation using reduce, but somehow I am not getting the syntax correct:

pojoList.stream()
        .reduce(0, (myObject1, myObject2) -> (myObject1.getA() + myObject2.getB()));

Answer 1

Well, if you want to call reduce on the IntStream:

int sum = pojoList.stream()
                  .mapToInt(ob ->(ob.getA()+ob.getB()))
                  .reduce(0, (a,b)->a+b);

Of course, the same will work on a Stream<Integer>:

int sum = pojoList.stream()
                  .map(ob ->(ob.getA()+ob.getB()))
                  .reduce(0, (a,b)->a+b);

or with a method reference:

int sum = pojoList.stream()
                  .map(ob ->(ob.getA()+ob.getB()))
                  .reduce(0, Integer::sum);

or without the map():

int sum = pojoList.stream()
                  .reduce(0, (s,ob)->s+ob.getA()+ob.getB(),Integer::sum);

In this last example, I use the variant:

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

since the reduced value (an Integer) is different than the type of the Stream elements.

The first parameter is an identity value - 0.

The second parameter adds the getA() and getB() values of the current Pojo element to the intermediate sum.

The third parameter combines two partial sums.

Answer 2

sum() method implementation is as follows:

public final int sum() {
    return reduce(0, Integer::sum);
}

Replacing sum() with reduce():

int sum = pojoList.stream()
                  .mapToInt(ob -> (ob.getA() + ob.getB()))
                  .reduce(0, Integer::sum);

Or, without mapToInt():

int pojoSum = pojoList.stream()
                      .reduce(0, (sum, ob) -> sum + ob.getA() + ob.getB(), Integer::sum);

For more information, see Reduction operations paragraph.