From ff4f753f257ed67b4fdb50b644213cba527b0ed7 Mon Sep 17 00:00:00 2001
From: Maiklins <kruemel@quantentunnel.de>
Date: Sat, 9 May 2020 14:00:25 +0200
Subject: [PATCH] Bael-3857 introduction to lock free data structures (#9238)

* BAEL-3857 Introduction to Lock-Free Data Structures

* BAEL-3857 Declare variables as final / volatile

* BAEL-3857 Declare node value as final

Co-authored-by: mikr <michael.krimgen@ximedes.com>
---
 .../baeldung/lockfree/NonBlockingQueue.java   | 81 +++++++++++++++++++
 1 file changed, 81 insertions(+)
 create mode 100644 core-java-modules/core-java-concurrency-advanced-3/src/main/java/com/baeldung/lockfree/NonBlockingQueue.java

diff --git a/core-java-modules/core-java-concurrency-advanced-3/src/main/java/com/baeldung/lockfree/NonBlockingQueue.java b/core-java-modules/core-java-concurrency-advanced-3/src/main/java/com/baeldung/lockfree/NonBlockingQueue.java
new file mode 100644
index 0000000000..9140dc287d
--- /dev/null
+++ b/core-java-modules/core-java-concurrency-advanced-3/src/main/java/com/baeldung/lockfree/NonBlockingQueue.java
@@ -0,0 +1,81 @@
+package com.baeldung.lockfree;
+
+import java.util.NoSuchElementException;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.AtomicReference;
+
+public class NonBlockingQueue<T> {
+
+    private final AtomicReference<Node<T>> head, tail;
+    private final AtomicInteger size;
+
+    public NonBlockingQueue() {
+        head = new AtomicReference<>(null);
+        tail = new AtomicReference<>(null);
+        size = new AtomicInteger();
+        size.set(0);
+    }
+
+    public void add(T element) {
+        if (element == null) {
+            throw new NullPointerException();
+        }
+
+        Node<T> node = new Node<>(element);
+        Node<T> currentTail;
+        do {
+            currentTail = tail.get();
+            node.setPrevious(currentTail);
+        } while(!tail.compareAndSet(currentTail, node));
+
+        if(node.previous != null) {
+            node.previous.next = node;
+        }
+
+        head.compareAndSet(null, node); //if we are inserting the first element
+        size.incrementAndGet();
+    }
+
+    public T get() {
+        if(head.get() == null) {
+            throw new NoSuchElementException();
+        }
+
+        Node<T> currentHead;
+        Node<T> nextNode;
+        do {
+            currentHead = head.get();
+            nextNode = currentHead.getNext();
+        } while(!head.compareAndSet(currentHead, nextNode));
+
+        size.decrementAndGet();
+        return currentHead.getValue();
+    }
+
+    public int size() {
+        return this.size.get();
+    }
+
+    private class Node<T> {
+        private final T value;
+        private volatile Node<T> next;
+        private volatile Node<T> previous;
+
+        public Node(T value) {
+            this.value = value;
+            this.next = null;
+        }
+
+        public T getValue() {
+            return value;
+        }
+
+        public Node<T> getNext() {
+            return next;
+        }
+
+        public void setPrevious(Node<T> previous) {
+            this.previous = previous;
+        }
+    }
+}