The combiner in Java's Stream.collect() method is an important part of the method's behavior, especially when the stream is processed in parallel. Here's why your combiner might not have been called:

Understanding Stream.collect()

The collect method in Java Streams is used to transform elements of the stream into a different form, often a collection (like List, Set, or Map). It takes three arguments:

1. Supplier: A function that provides a new instance of the result container (e.g., a List or Set).
2. Accumulator: A function that folds a value into the result container.
3. Combiner: A function that merges two containers, which is particularly important in parallel processing.

When is the combiner used?

The combiner is primarily used in the following scenarios:

1. Parallel Streams: When a stream is processed in parallel, the stream is divided into multiple substreams that are processed independently. The combiner is then used to merge the results of these substreams.

2. Splitting Tasks: If a stream is large and split into multiple tasks, the combiner is used to combine the results from these tasks.

Why Wasn't Your combiner Called?

If you're testing this with a sequential stream (i.e., not using .parallelStream() or .parallel()), the combiner will not be called because the stream isn't split into substreams. In a sequential stream, all operations are performed on a single container, so there's no need to combine results.

Example:

Consider the following code:

import java.util.List;
import java.util.ArrayList;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class StreamExample {
    public static void main(String[] args) {
        List<Integer> result = Stream.of(1, 2, 3, 4, 5)
            .collect(
                ArrayList::new,                 // Supplier
                List::add,                      // Accumulator
                (list1, list2) -> {             // Combiner
                    System.out.println("Combiner called");
                    list1.addAll(list2);
                }
            );

        System.out.println(result);
    }
}

In this example:

• Supplier: Creates a new ArrayList.
• Accumulator: Adds elements to the list.
• Combiner: Merges two lists and prints a message when it's called.

If you run this code with a sequential stream (Stream.of(...)), the output will be:

[1, 2, 3, 4, 5]

The message "Combiner called" will not be printed because the combiner isn't needed.

Making the Combiner Called:

If you want to see the combiner in action, convert the stream to a parallel stream:

import java.util.List;
import java.util.ArrayList;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class StreamExample {
    public static void main(String[] args) {
        List<Integer> result = Stream.of(1, 2, 3, 4, 5)
            .parallel()
            .collect(
                ArrayList::new,                 // Supplier
                List::add,                      // Accumulator
                (list1, list2) -> {             // Combiner
                    System.out.println("Combiner called");
                    list1.addAll(list2);
                }
            );

        System.out.println(result);
    }
}

Now, since the stream is parallel, the combiner might be called, depending on how the stream is split:

Combiner called
Combiner called
[1, 2, 3, 4, 5]

Conclusion

The combiner is crucial for parallel stream operations but is not needed for sequential streams. If you're testing with a sequential stream, the combiner won't be called because there is no need to combine multiple partial results. To observe the combiner in action, use a parallel stream.