The example code for the article "A Guide to the Fork/Join Framework"

core-java-8/src/main/java/com/baeldung/forkjoin/CustomRecursiveAction.java

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+package com.baeldung.forkjoin;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
+import java.util.concurrent.ForkJoinTask;
+import java.util.concurrent.RecursiveAction;
+
+public class CustomRecursiveAction extends RecursiveAction {
+
+    private String workLoad = "";
+
+    private static final int THRESHOLD = 4;
+
+    public CustomRecursiveAction(String workLoad) {
+        this.workLoad = workLoad;
+    }
+
+    @Override
+    protected void compute() {
+
+        if(workLoad.length() > THRESHOLD) {
+            ForkJoinTask.invokeAll(createSubtasks());
+        } else {
+           processing(workLoad);
+        }
+    }
+
+    private Collection<CustomRecursiveAction> createSubtasks() {
+
+        List<CustomRecursiveAction> subtasks =
+                 new ArrayList<>();
+
+        String partOne = workLoad.substring(0, workLoad.length()/2);
+        String partTwo = workLoad.substring(workLoad.length()/2, workLoad.length());
+
+        subtasks.add(new CustomRecursiveAction(partOne));
+        subtasks.add(new CustomRecursiveAction(partTwo));
+
+        return subtasks;
+    }
+
+    private void processing(String work) {
+        work.toUpperCase();
+    }
+}

core-java-8/src/main/java/com/baeldung/forkjoin/CustomRecursiveTask.java

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+package com.baeldung.forkjoin;
+
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.List;
+import java.util.concurrent.ForkJoinTask;
+import java.util.concurrent.RecursiveTask;
+
+public class CustomRecursiveTask extends RecursiveTask<Integer> {
+
+    private int[] arr;
+
+    private static final int THRESHOLD = 20;
+
+    public CustomRecursiveTask(int[] arr) {
+        this.arr = arr;
+    }
+
+    @Override
+    protected Integer compute() {
+
+        if (arr.length > THRESHOLD) {
+
+            return  ForkJoinTask.invokeAll(createSubtasks())
+                    .stream()
+                    .mapToInt(ForkJoinTask::join)
+                    .sum();
+
+        } else {
+            return processing(arr);
+        }
+    }
+
+    private Collection<CustomRecursiveTask> createSubtasks() {
+        List<CustomRecursiveTask> dividedTasks = new ArrayList<>();
+        dividedTasks.add(new CustomRecursiveTask(
+                Arrays.copyOfRange(arr, 0, arr.length / 2)));
+        dividedTasks.add(new CustomRecursiveTask(
+                Arrays.copyOfRange(arr, arr.length / 2, arr.length)));
+        return dividedTasks;
+    }
+
+    private Integer processing(int[] arr) {
+        return Arrays.stream(arr)
+                .filter(a -> a > 10 && a < 27)
+                .map(a -> a * 10)
+                .sum();
+    }
+}

core-java-8/src/main/java/com/baeldung/forkjoin/util/PoolUtil.java

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+package com.baeldung.forkjoin.util;
+
+
+import java.util.concurrent.ForkJoinPool;
+
+public class PoolUtil {
+
+    public static ForkJoinPool forkJoinPool = new ForkJoinPool(2);
+
+}