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

*** 1080,1089 ****
--- 1080,1096 ----
              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();
*** 1113,1123 ****
--- 1120,1132 ----
              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);
*** 1128,1138 ****
          else {
              tab[i] = newNode(hash, key, v, first);
              if (binCount >= TREEIFY_THRESHOLD - 1)
                  treeifyBin(tab, hash);
          }
!         ++modCount;
          ++size;
          afterNodeInsertion(true);
          return v;
      }
  
--- 1137,1147 ----
          else {
              tab[i] = newNode(hash, key, v, first);
              if (binCount >= TREEIFY_THRESHOLD - 1)
                  treeifyBin(tab, hash);
          }
!         modCount = mc + 1;
          ++size;
          afterNodeInsertion(true);
          return v;
      }