src/java.base/share/classes/java/util/HashMap.java

@@ -1080,10 +1080,17 @@
             return oldValue;
         }
         return null;
     }
 
+    /**
+     * {@inheritDoc}
+     * 
+     * If the function itself causes a structural modification to the map, a
+     * ConcurrentModificationException will be thrown.  As with iterators, this
+     * exception is thrown on a best-effort basis.
+     */
     @Override
     public V computeIfAbsent(K key,
                              Function<? super K, ? extends V> mappingFunction) {
         if (mappingFunction == null)
             throw new NullPointerException();

@@ -1113,11 +1120,13 @@
             if (old != null && (oldValue = old.value) != null) {
                 afterNodeAccess(old);
                 return oldValue;
             }
         }
+        int mc = modCount;
         V v = mappingFunction.apply(key);
+        if (mc != modCount) { throw new ConcurrentModificationException(); }
         if (v == null) {
             return null;
         } else if (old != null) {
             old.value = v;
             afterNodeAccess(old);

@@ -1128,11 +1137,11 @@
         else {
             tab[i] = newNode(hash, key, v, first);
             if (binCount >= TREEIFY_THRESHOLD - 1)
                 treeifyBin(tab, hash);
         }
-        ++modCount;
+        modCount = mc + 1;
         ++size;
         afterNodeInsertion(true);
         return v;
     }