Core threadpool implementation examples (#588)

core-java-8/src/main/java/com/baeldung/threadpool/CountingTask.java

@@ -0,0 +1,22 @@
+package com.baeldung.threadpool;
+
+import java.util.concurrent.ForkJoinTask;
+import java.util.concurrent.RecursiveTask;
+import java.util.stream.Collectors;
+
+public class CountingTask extends RecursiveTask<Integer> {
+
+    private final TreeNode node;
+
+    public CountingTask(TreeNode node) {
+        this.node = node;
+    }
+
+    @Override
+    protected Integer compute() {
+        return node.value + node.children.stream()
+                .map(childNode -> new CountingTask(childNode).fork())
+                .collect(Collectors.summingInt(ForkJoinTask::join));
+    }
+
+}

core-java-8/src/main/java/com/baeldung/threadpool/TreeNode.java

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+package com.baeldung.threadpool;
+
+import java.util.Arrays;
+import java.util.HashSet;
+import java.util.Set;
+
+public class TreeNode {
+
+    int value;
+
+    Set<TreeNode> children;
+
+    public TreeNode(int value, TreeNode... children) {
+        this.value = value;
+        this.children = new HashSet<>();
+        this.children.addAll(Arrays.asList(children));
+    }
+
+}