apache/hadoop · error · HadoopIllegalArgumentException

!(element instanceof LinkedElement), element.getClass()=" +

Error message

!(element instanceof LinkedElement), element.getClass()=" + element.getClass()

What it means

Elements stored in LightWeightGSet must implement LinkedElement - the set chains hash collisions through the elements themselves instead of allocating wrapper nodes. put() catches the ClassCastException from the cast and rethrows HadoopIllegalArgumentException naming the element's actual class, telling you the type violates the GSet contract.

Source

Thrown at hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/LightWeightGSet.java:159

    return null;
  }

  @Override
  public boolean contains(final K key) {
    return get(key) != null;
  }

  @Override
  public E put(final E element) {
    // validate element
    if (element == null) {
      throw new NullPointerException("Null element is not supported.");
    }
    LinkedElement e = null;
    try {
      e = (LinkedElement)element;
    } catch (ClassCastException ex) {
      throw new HadoopIllegalArgumentException(
          "!(element instanceof LinkedElement), element.getClass()="
          + element.getClass());
    }

    // find index
    final int index = getIndex(element);

    // remove if it already exists
    final E existing = remove(index, element);

    // insert the element to the head of the linked list
    modification++;
    size++;
    e.setNext(entries[index]);
    entries[index] = e;

    return existing;
  }

View on GitHub (pinned to 2add963021)

Solutions

1. Make the element implement LinkedElement (getNext/setNext chaining), following existing element classes in the codebase.
2. Type the reference as GSet<K, E extends LinkedElement> so misuse becomes a compile error.
3. If the class cannot be modified, use a wrapper-based structure (java.util.HashSet) instead of GSet.

Example fix

// before
class MyEntry { // plain POJO
  ...
}
gset.put(myEntry); // HadoopIllegalArgumentException

// after
class MyEntry implements LightWeightGSet.LinkedElement {
  private LinkedElement next;
  @Override public LinkedElement getNext() { return next; }
  @Override public void setNext(LinkedElement n) { next = n; }
}
gset.put(myEntry);

static <E> boolean isStorableInGSet(E element) {
  return element != null && (element instanceof LinkedElement);
}

When it happens

Trigger: put(element) where the element class does not implement LinkedElement: passing a plain POJO, or a subclass hierarchy from which the interface was dropped.

Common situations: Porting a custom cache to GSet; modifying inode-like classes in HDFS forks; test code inserting stand-in objects that implement only equals/hashCode.

Related errors

• FileSystem '{}'is not a ViewFileSystem.
• Input files cannot be merged as they have different Key and
• Input files cannot be merged as they have different Key clas
• key == null
• Null element is not supported.

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/ac1085374ba42087. Report an issue: GitHub.