Java Streams API provides a powerful functional programming paradigm for working with collections in a more concise and expressive way. While most stream operations are stateless and operate on individual elements, there are cases where we need to perform stateful operations. In this blog post, we will explore how to implement stateful operations using Java Streams API.

What are stateful operations?

Stateful operations are those operations where the output of one element depends on the state accumulated from previous elements. These operations require maintaining some state information across stream elements. Examples of stateful operations include distinct(), sorted(), and limit().

Using Stream#collect() method

Java Streams API provides the collect() method, which allows us to accumulate the elements into a mutable container. This mutable container can hold the intermediate or final result of stateful operations.

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

List<Integer> distinctNumbers = numbers.stream()
                                       .collect(Collectors.toList());

System.out.println(distinctNumbers);

In the above example, we generate a list of numbers and then use the stream() method to create a stream. We then perform the distinct() operation, which returns a stream of distinct elements. Finally, we collect the distinct numbers into a list using the collect() method.

Implementing custom stateful operations

In some cases, the built-in stateful operations might not fulfill our requirements. In such scenarios, we can implement custom stateful operations by extending the java.util.stream.AbstractPipeline class.

import java.util.stream.Stream;

public class RunningTotal {

    public static void main(String[] args) {
        Stream<Integer> numbers = Stream.of(1, 2, 3, 4, 5);

        int[] runningTotal = {0};

        numbers.map(e -> {
            runningTotal[0] += e;
            return runningTotal[0];
        }).forEach(System.out::println);
    }

}

In the above example, we calculate the running total of a stream of numbers. We declare an array runningTotal outside the stream operation and use it to maintain the state of the running total. Inside the map() operation, we update the runningTotal array and return the updated value. Finally, we print each calculated running total using the forEach() terminal operation.

Conclusion

Stateful operations in Java Streams API allow us to perform operations that depend on the state accumulated from previous elements. We can use the built-in collect() method for simple stateful operations or extend the AbstractPipeline class to implement custom stateful operations. By leveraging stateful operations, we can write more expressive and concise code when working with collections in Java.

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