feat: Add Fork/Join design pattern (#3227) (#3550)

* Add fork-join pattern implementation * Add input validation for start > end in SumTask Signed-off-by: SandhyaDevadiga <sandhyadevadiga8197@gmail.com> * Add fork-join module to parent pom.xml * Add missing assertThrows import in SumTaskTest * Format fork-join code with Spotless * Add AppTest to satisfy coverage requirements Signed-off-by: SandhyaDevadiga <sandhyadevadiga8197@gmail.com> --------- Signed-off-by: SandhyaDevadiga <sandhyadevadiga8197@gmail.com>
2026-09-12 16:18:37 +00:00 · 2026-08-25 07:39:07 +03:00
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+---
+title: "Fork/Join Pattern in Java: Parallel Divide-and-Conquer Processing"
+shortTitle: Fork/Join
+description: "Learn the Fork/Join design pattern in Java with real-world examples, class diagrams, and code samples. Understand how to split large tasks into parallel subtasks for improved performance."
+category: Concurrency
+language: en
+tag:
+  - Performance
+  - Scalability
+  - Concurrency
+---
+
+## Also known as
+
+* Divide and Conquer Parallelism
+* Work-Stealing Parallelism
+
+## Intent of Fork/Join Design Pattern
+
+The Fork/Join pattern recursively splits a large task into independent subtasks (fork),
+processes them in parallel across multiple threads, and combines their results (join) to
+produce a final outcome. It maximizes CPU utilization for computationally intensive problems.
+
+## Detailed Explanation of Fork/Join Pattern with Real-World Examples
+
+Real-world example
+
+> Imagine a large warehouse that needs to count all its inventory items across 100 aisles.
+> Instead of one person counting every aisle sequentially, the manager divides the warehouse
+> into sections and assigns a team of workers to count each section simultaneously. Once
+> every section is counted, the manager collects all partial counts and sums them into the
+> total inventory. This is the Fork/Join pattern: split the work, do it in parallel, merge
+> the results.
+
+In plain words
+
+> Fork/Join splits a big problem into smaller pieces, solves each piece in parallel on
+> separate threads, then combines all results back together.
+
+## Programmatic Example of Fork/Join Pattern in Java
+
+We demonstrate the pattern by computing the sum of a large array in parallel using Java's
+built-in `ForkJoinPool` and `RecursiveTask`.
+
+The `SumTask` is a recursive task that splits the array when it's too large:
+
+```java
+public class SumTask extends RecursiveTask<Long> {
+
+  private static final int THRESHOLD = 1000;
+  private final long[] numbers;
+  private final int start;
+  private final int end;
+
+  @Override
+  protected Long compute() {
+    int length = end - start;
+
+    if (length <= THRESHOLD) {
+      // Base case: sum directly
+      long sum = 0;
+      for (int i = start; i < end; i++) {
+        sum += numbers[i];
+      }
+      return sum;
+    }
+
+    // Fork: split into two halves
+    int mid = start + length / 2;
+    SumTask leftTask = new SumTask(numbers, start, mid);
+    SumTask rightTask = new SumTask(numbers, mid, end);
+
+    leftTask.fork();                    // run left half asynchronously
+    long rightResult = rightTask.compute();  // compute right half here
+    long leftResult = leftTask.join();       // wait for left half
+
+    // Join: combine results
+    return leftResult + rightResult;
+  }
+}
+```
+
+The `ForkJoinSumCalculator` provides a clean API:
+
+```java
+public class ForkJoinSumCalculator {
+
+  private final ForkJoinPool pool;
+
+  public ForkJoinSumCalculator() {
+    this.pool = ForkJoinPool.commonPool();
+  }
+
+  public long calculateSum(long[] numbers) {
+    SumTask task = new SumTask(numbers, 0, numbers.length);
+    return pool.invoke(task);
+  }
+}
+```
+
+Running the example in `App`:
+
+```java
+long[] numbers = LongStream.rangeClosed(1, 10_000_000).toArray();
+ForkJoinSumCalculator calculator = new ForkJoinSumCalculator();
+long result = calculator.calculateSum(numbers);
+System.out.println("Fork/Join sum: " + result);
+```
+
+Program output:
+
+```
+Fork/Join sum: 50000005000000
+Expected sum:  50000005000000
+Correct: true
+Time taken: 45 ms
+Available processors: 8
+```
+
+## When to Use the Fork/Join Pattern in Java
+
+* When you have a large, CPU-intensive task that can be divided into independent subtasks.
+* When the subtasks are roughly the same size and don't depend on each other.
+* When you want to utilize multiple CPU cores without manually managing threads.
+* When the problem naturally fits a divide-and-conquer strategy (e.g., sorting, searching,
+  numerical computation).
+
+## When NOT to Use Fork/Join
+
+* For I/O-bound tasks (network calls, file reads) — use virtual threads or async I/O instead.
+* When subtasks are too small — the overhead of forking exceeds the benefit.
+* When tasks have dependencies on each other and cannot run independently.
+
+## Benefits and Trade-offs of Fork/Join Pattern
+
+Benefits:
+
+* Maximizes CPU utilization through work-stealing algorithm.
+* Scales automatically with the number of available processors.
+* Built into Java's standard library (`java.util.concurrent`) — no external dependencies.
+* Clean recursive decomposition makes the code readable and maintainable.
+
+Trade-offs:
+
+* Overhead from task creation and thread management for very small problems.
+* Requires tasks to be independent — shared mutable state introduces bugs.
+* Choosing an appropriate threshold requires tuning for optimal performance.
+* Debugging parallel code is inherently harder than sequential code.
+
+## Related Java Design Patterns
+
+* [Divide and Conquer](https://java-design-patterns.com/patterns/divide-and-conquer/):
+  Fork/Join is the parallel execution variant of the classic divide-and-conquer strategy.
+* [Thread Pool](https://java-design-patterns.com/patterns/thread-pool/): Fork/Join uses a
+  specialized pool with work-stealing semantics.
+
+## References
+
+* [Java Documentation for ForkJoinPool](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/concurrent/ForkJoinPool.html)
+* [Java Concurrency in Practice — Brian Goetz](https://amzn.to/4aRMruW)
+* [Java Documentation for RecursiveTask](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/concurrent/RecursiveTask.html)
@@ -0,0 +1,50 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    This project is licensed under the terms of the MIT license, see LICENSE.md in the root
+    of this project's GitHub repository for the full license text.
+
+    Copyright © 2014-2024 Ilkka Seppälä
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.26.0-SNAPSHOT</version>
+  </parent>
+
+  <artifactId>fork-join</artifactId>
+
+  <dependencies>
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter-engine</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-assembly-plugin</artifactId>
+        <executions>
+          <execution>
+            <configuration>
+              <archive>
+                <manifest>
+                  <mainClass>com.iluwatar.forkjoin.App</mainClass>
+                </manifest>
+              </archive>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
+  </build>
+
+</project>
@@ -0,0 +1,43 @@
+package com.iluwatar.forkjoin;
+
+import java.util.stream.LongStream;
+
+/**
+ * The Fork/Join pattern is a concurrency design pattern that splits a large task into smaller
+ * subtasks (fork), processes them in parallel, and then combines the results (join).
+ *
+ * <p>In Java, this pattern is implemented using {@link java.util.concurrent.ForkJoinPool} and
+ * {@link java.util.concurrent.RecursiveTask}. Worker threads in the pool use a work-stealing
+ * algorithm — idle threads take tasks from busy threads — maximizing CPU utilization.
+ *
+ * <p>In this example, we demonstrate the pattern by computing the sum of a large array in parallel.
+ * The array is recursively split in half until each piece is small enough to sum directly, then
+ * results are combined back up.
+ */
+public final class App {
+
+  private App() {}
+
+  /**
+   * @param args command line arguments, not used
+   */
+  public static void main(String[] args) {
+    // Create an array of 10 million numbers: [1, 2, 3, ..., 10_000_000]
+    long[] numbers = LongStream.rangeClosed(1, 10_000_000).toArray();
+
+    // Calculate sum using Fork/Join
+    ForkJoinSumCalculator calculator = new ForkJoinSumCalculator();
+    long startTime = System.currentTimeMillis();
+    long result = calculator.calculateSum(numbers);
+    long endTime = System.currentTimeMillis();
+
+    // The expected sum of 1 to N is N*(N+1)/2
+    long expected = 10_000_000L * 10_000_001L / 2;
+
+    System.out.println("Fork/Join sum: " + result);
+    System.out.println("Expected sum:  " + expected);
+    System.out.println("Correct: " + (result == expected));
+    System.out.println("Time taken: " + (endTime - startTime) + " ms");
+    System.out.println("Available processors: " + Runtime.getRuntime().availableProcessors());
+  }
+}
@@ -0,0 +1,44 @@
+package com.iluwatar.forkjoin;
+
+import java.util.concurrent.ForkJoinPool;
+
+/**
+ * ForkJoinSumCalculator provides a convenient API to sum an array of numbers using the Fork/Join
+ * framework. It creates a {@link ForkJoinPool}, submits a {@link SumTask}, and returns the computed
+ * sum.
+ *
+ * <p>The pool manages a set of worker threads that process subtasks in parallel. Idle threads can
+ * "steal" work from busy threads, maximizing CPU utilization.
+ */
+public class ForkJoinSumCalculator {
+
+  private final ForkJoinPool pool;
+
+  /** Creates a calculator using the common ForkJoinPool (uses all available CPU cores). */
+  public ForkJoinSumCalculator() {
+    this.pool = ForkJoinPool.commonPool();
+  }
+
+  /**
+   * Creates a calculator with a specific number of threads.
+   *
+   * @param parallelism the number of worker threads to use
+   */
+  public ForkJoinSumCalculator(int parallelism) {
+    this.pool = new ForkJoinPool(parallelism);
+  }
+
+  /**
+   * Calculates the sum of all elements in the array using Fork/Join parallelism.
+   *
+   * @param numbers the array of numbers to sum
+   * @return the total sum of all elements
+   */
+  public long calculateSum(long[] numbers) {
+    if (numbers == null || numbers.length == 0) {
+      return 0;
+    }
+    SumTask task = new SumTask(numbers, 0, numbers.length);
+    return pool.invoke(task);
+  }
+}
@@ -0,0 +1,93 @@
+package com.iluwatar.forkjoin;
+
+import java.util.concurrent.RecursiveTask;
+
+/**
+ * SumTask demonstrates the Fork/Join pattern by recursively splitting an array summation problem
+ * into smaller subtasks until each subtask is small enough to compute directly.
+ *
+ * <p>How it works: If the portion of the array is smaller than THRESHOLD, sum it in a simple loop.
+ * Otherwise, split the array in half, fork one half to run in parallel, compute the other half in
+ * the current thread, and then join the results. This approach utilizes multiple CPU cores to
+ * perform the summation significantly faster than a single-threaded loop for large arrays.
+ */
+public class SumTask extends RecursiveTask<Long> {
+
+  /**
+   * If the number of elements to process is at or below this threshold, the task computes the sum
+   * directly instead of splitting further.
+   */
+  private static final int THRESHOLD = 1000;
+
+  private final long[] numbers;
+  private final int start;
+  private final int end;
+
+  /**
+   * Creates a task to sum elements of the given array from index {@code start} (inclusive) to index
+   * {@code end} (exclusive).
+   *
+   * @param numbers the array of numbers to sum
+   * @param start the starting index (inclusive)
+   * @param end the ending index (exclusive)
+   */
+  public SumTask(long[] numbers, int start, int end) {
+    if (start > end) {
+      throw new IllegalArgumentException(
+          "start (" + start + ") must not be greater than end (" + end + ")");
+    }
+    this.numbers = numbers;
+    this.start = start;
+    this.end = end;
+  }
+
+  /**
+   * The main computation method. This is where the fork/join magic happens.
+   *
+   * @return the sum of elements from start to end
+   */
+  @Override
+  protected Long compute() {
+    int length = end - start;
+
+    // BASE CASE: if the chunk is small enough, just sum directly
+    if (length <= THRESHOLD) {
+      return computeDirectly();
+    }
+
+    // FORK: split the task into two halves
+    int mid = start + length / 2;
+
+    // Create subtask for the left half
+    SumTask leftTask = new SumTask(numbers, start, mid);
+
+    // Create subtask for the right half
+    SumTask rightTask = new SumTask(numbers, mid, end);
+
+    // Fork the left task — it will run in a separate thread
+    leftTask.fork();
+
+    // Compute the right task in the current thread (no need to fork both)
+    long rightResult = rightTask.compute();
+
+    // JOIN: wait for the left task to finish and get its result
+    long leftResult = leftTask.join();
+
+    // Combine the results from both halves
+    return leftResult + rightResult;
+  }
+
+  /**
+   * Computes the sum directly using a simple loop. This is used when the chunk size is at or below
+   * the threshold — no further splitting needed.
+   *
+   * @return the sum of elements in the range [start, end)
+   */
+  private long computeDirectly() {
+    long sum = 0;
+    for (int i = start; i < end; i++) {
+      sum += numbers[i];
+    }
+    return sum;
+  }
+}
@@ -0,0 +1,11 @@
+package com.iluwatar.forkjoin;
+
+import org.junit.jupiter.api.Test;
+
+class AppTest {
+
+  @Test
+  void shouldExecuteWithoutException() {
+    App.main(new String[] {});
+  }
+}
@@ -0,0 +1,56 @@
+package com.iluwatar.forkjoin;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.stream.LongStream;
+import org.junit.jupiter.api.Test;
+
+class ForkJoinSumCalculatorTest {
+
+  @Test
+  void shouldReturnZeroForNullArray() {
+    ForkJoinSumCalculator calculator = new ForkJoinSumCalculator();
+
+    assertEquals(0L, calculator.calculateSum(null));
+  }
+
+  @Test
+  void shouldReturnZeroForEmptyArray() {
+    ForkJoinSumCalculator calculator = new ForkJoinSumCalculator();
+
+    assertEquals(0L, calculator.calculateSum(new long[0]));
+  }
+
+  @Test
+  void shouldCalculateSumOfSmallArray() {
+    ForkJoinSumCalculator calculator = new ForkJoinSumCalculator();
+    long[] numbers = {10, 20, 30, 40, 50};
+
+    long result = calculator.calculateSum(numbers);
+
+    assertEquals(150L, result);
+  }
+
+  @Test
+  void shouldCalculateSumOfLargeArray() {
+    ForkJoinSumCalculator calculator = new ForkJoinSumCalculator();
+    long[] numbers = LongStream.rangeClosed(1, 100_000).toArray();
+
+    long result = calculator.calculateSum(numbers);
+
+    long expected = 100_000L * 100_001L / 2;
+    assertEquals(expected, result);
+  }
+
+  @Test
+  void shouldWorkWithCustomParallelism() {
+    // Use only 2 threads
+    ForkJoinSumCalculator calculator = new ForkJoinSumCalculator(2);
+    long[] numbers = LongStream.rangeClosed(1, 50_000).toArray();
+
+    long result = calculator.calculateSum(numbers);
+
+    long expected = 50_000L * 50_001L / 2;
+    assertEquals(expected, result);
+  }
+}
@@ -0,0 +1,85 @@
+package com.iluwatar.forkjoin;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.concurrent.ForkJoinPool;
+import java.util.stream.LongStream;
+import org.junit.jupiter.api.Test;
+
+class SumTaskTest {
+
+  @Test
+  void shouldSumSmallArrayDirectly() {
+    // Array smaller than threshold — should compute without forking
+    long[] numbers = {1, 2, 3, 4, 5};
+    SumTask task = new SumTask(numbers, 0, numbers.length);
+
+    long result = ForkJoinPool.commonPool().invoke(task);
+
+    assertEquals(15L, result);
+  }
+
+  @Test
+  void shouldSumLargeArrayUsingForkJoin() {
+    // Array larger than threshold — will fork into subtasks
+    long[] numbers = LongStream.rangeClosed(1, 10_000).toArray();
+    SumTask task = new SumTask(numbers, 0, numbers.length);
+
+    long result = ForkJoinPool.commonPool().invoke(task);
+
+    // Sum of 1 to N = N*(N+1)/2
+    long expected = 10_000L * 10_001L / 2;
+    assertEquals(expected, result);
+  }
+
+  @Test
+  void shouldSumPartialRange() {
+    // Sum only a portion of the array (indices 2 to 5)
+    long[] numbers = {10, 20, 30, 40, 50, 60};
+    SumTask task = new SumTask(numbers, 2, 5);
+
+    long result = ForkJoinPool.commonPool().invoke(task);
+
+    // 30 + 40 + 50 = 120
+    assertEquals(120L, result);
+  }
+
+  @Test
+  void shouldReturnZeroForEmptyRange() {
+    long[] numbers = {1, 2, 3};
+    SumTask task = new SumTask(numbers, 1, 1); // start == end, empty range
+
+    long result = ForkJoinPool.commonPool().invoke(task);
+
+    assertEquals(0L, result);
+  }
+
+  @Test
+  void shouldHandleSingleElement() {
+    long[] numbers = {42};
+    SumTask task = new SumTask(numbers, 0, 1);
+
+    long result = ForkJoinPool.commonPool().invoke(task);
+
+    assertEquals(42L, result);
+  }
+
+  @Test
+  void shouldProduceCorrectResultForMillionElements() {
+    long[] numbers = LongStream.rangeClosed(1, 1_000_000).toArray();
+    SumTask task = new SumTask(numbers, 0, numbers.length);
+
+    long result = ForkJoinPool.commonPool().invoke(task);
+
+    long expected = 1_000_000L * 1_000_001L / 2;
+    assertEquals(expected, result);
+  }
+
+  @Test
+  void shouldThrowExceptionWhenStartGreaterThanEnd() {
+    long[] numbers = {1, 2, 3, 4, 5};
+
+    assertThrows(IllegalArgumentException.class, () -> new SumTask(numbers, 4, 2));
+  }
+}
@@ -139,6 +139,7 @@
    <module>fluent-interface</module>
    <module>flux</module>
    <module>flyweight</module>
+    <module>fork-join</module>
    <module>front-controller</module>
    <module>function-composition</module>
    <module>game-loop</module>