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The Java Stream API: Reduction Operations

Oct 2018

The sample code for this post can be found on GitHub.

Reduction operations are a way to consolidate collections into one simple result.

Given our SimplePair object:

public class SimplePair {

    private String name;
    private int id;

    public String getName() {
        return name;

    public void setName(String name) {
        this.name = name;

    public int getId() {
        return id;

    public void setId(int id) {
        this.id = id;

    public String toString() {
        return "name-" + name + ",id-" + id;

And a collection like:

public static List<SimplePair> generateSimplePairs(int numToGenerate) {
    List<SimplePair> pairs = new ArrayList<>();
    for (int i = 1; i <= numToGenerate; i++) {
        SimplePair pair = new SimplePair();

        pair.setName("pair" + i);

    return pairs;

Often, we would want to use this to sum up or multiply members in a particular way. Here, we will sum all of the ids of our simple pairs (the id's in this collection are 1, 2, 3, 4, and 5):

public void reduce_sumAllIds() {
    // sums like x_0 + x_1 + ... + x_n:
    Optional<Integer> idsSummed = pairs.stream()
            .reduce((firstId, secondId) -> firstId + secondId);

    assertEquals(15, idsSummed.orElseThrow(RuntimeException::new).intValue());

This above version of the reduce(..) method takes a BinaryOperator<T> object, which gets applied in sequence. In the above example, the result is a predictable addition of 1 + 2 + 3 + 4 + 5. It takes the resulting sum that has been accumulated so far and adds it to the next Integer in the sequence. If we wanted to multiply all of the ids together, we can do so by changing the addition operator to a multiplication operator, like so:

public void reduce_multiplyAllIds() {
    Optional<Integer> idsMultiplied = pairs.stream()
            .reduce((firstId, secondId) -> firstId * secondId);

    assertEquals(1 * 2 * 3 * 4 * 5, idsMultiplied.orElseThrow(RuntimeException::new).intValue());

Finally, we can seed an initial value in the reduce(..) method. Typically, this would be an identity operator, like zero for addition and one for multiplication, which allows you to drop the Optional<T> wrapper and just get a value, where the value returned would just be the seed value if there is no data in the Stream. Here, we will add up all the ids, starting with the number 10:

public void reduce_usingIdentityValue() {
    Integer idsSummedWithIdentity = pairs.stream()
            .reduce(10, (firstId, secondId) -> firstId + secondId);

    assertEquals(25, idsSummedWithIdentity.intValue());

Nick Fisher is a software engineer in the Pacific Northwest. He focuses on building highly scalable and maintainable backend systems.