Reduce Stream Examples

The Java 8 Stream API contains a set of terminal operations (such as average, sum, min, max, and count) which return one value by combining the elements of a stream. Such terminal operations are called reduction operations. In addition to those terminal operations, the JDK also provides the general-purpose reduction method – reduce(), which this post describes.

1. A Simple Example of Stream.reduce()

Assuming we have a word stream like the following:

List<String> list = new ArrayList<String>();
list.add("java");
list.add("php");
list.add("python");
list.add("perl");
list.add("c");
list.add("lisp");
list.add("c#");
Stream<String> wordStream = list.stream();

We can use use the following code to get the sum of the length of strings in the list:

int s = wordStream.map(s -> s.length())
		.mapToInt(Integer::new)
		.sum();
 
System.out.println(s);

We can also use the Stream.reduce() method which is a more general method:

Stream<Integer> lengthStream = wordStream.map(s -> s.length());
Optional<Integer> sum = lengthStream.reduce((x, y) -> x + y);
sum.ifPresent(System.out::println);

Stream.reduce() take input as an accumulator function which takes two parameters: a partial result of the reduction (in this case, the sum of all processed integers so far) and the next element of the stream (in this case, an integer). It returns a new partial result. In this case, the accumulator function is a lambda expression that adds two Integer values and returns an Integer value.

Instead of using lambda expression, you can also use:

Optional<Integer> sum = lengthStream.reduce(Integer::sum);

2. Provide Identify Value

In the previous example, the reduce() method returns an Optional. We can avoid optional type, by giving it an initial value 0:

Stream<Integer> lengthStream = wordStream.map(s -> s.length());
int sum = lengthStream.reduce(0, (x, y) -> x + y);
System.out.println(sum);

The initial value must be a value for the reduce function. For all t, reduce(identity, t) and reduce(t) should return the same value, but not in the same type (One is a value, the other is an Optional).

In addition, we can also write reduce() method like the following:

int s = wordStream.reduce(0, (x, y) -> x + y.length(), (x, y) -> x + y);
System.out.println(s);

The three parameters are identify, reducer, and combiner.
– identity – identity value for the combiner function
– reducer – function for combining two results
– combiner – function for adding an additional element into a result.

Reference:
1. Stream.reduce()