headLock has been added! v2

pp.A4-HandOverHandLocking/src/main/java/pp/ThreadsafeSimplifiedList.java

@@ -2,6 +2,7 @@ package pp;
 
 public class ThreadsafeSimplifiedList<T> implements SimplifiedList<T> {
   private Node<T> first;
+  private final Object headLock = new Object();
 
   private static class Node<U> {
     U element;
@@ -22,36 +23,33 @@ public class ThreadsafeSimplifiedList<T> implements SimplifiedList<T> {
 
   @Override
   public boolean add(T element) {
-    synchronized (this) {
+    Node<T> curr;
+
+    synchronized (headLock) {
       if (first == null) {
         first = new Node<>(element, null, null);
         return true;
       }
+      curr = first;
     }
 
-    Node<T> curr;
-    synchronized (first.lock) {
-      curr = first;
-      while (true) {
-        Node<T> next;
-        synchronized (curr.lock) {
-          next = curr.next;
-          if (next == null) {
-            curr.next = new Node<>(element, curr, null);
-            return true;
-          }
-        }
-        synchronized (next.lock) {
-          curr = next;
+    while (true) {
+      Node<T> next;
+      synchronized (curr.lock) {
+        next = curr.next;
+        if (next == null) {
+          curr.next = new Node<>(element, curr, null);
+          return true;
         }
       }
+      curr = next;
     }
   }
 
   @Override
   public T get(int index) {
     Node<T> curr;
-    synchronized (this) {
+    synchronized (headLock) {
       if (first == null) throw new IndexOutOfBoundsException();
       curr = first;
     }
@@ -71,7 +69,7 @@ public class ThreadsafeSimplifiedList<T> implements SimplifiedList<T> {
   @Override
   public T set(int index, T element) {
     Node<T> curr;
-    synchronized (this) {
+    synchronized (headLock) {
       if (first == null) throw new IndexOutOfBoundsException();
       curr = first;
     }
@@ -92,7 +90,7 @@ public class ThreadsafeSimplifiedList<T> implements SimplifiedList<T> {
 
   @Override
   public boolean isEmpty() {
-    synchronized (this) {
+    synchronized (headLock) {
       return first == null;
     }
   }