What causes "Collection can not be null" in apache/hadoop?

Pass Collections.emptyList() when there is nothing to record

apache/hadoop · error · NullPointerException

Collection<Key> can not be null

Error message

Collection<Key> can not be null

What it means

The Collection overload of addFalsePositive first checks the collection reference itself; a null collection throws NullPointerException before any element is read. A non-null collection is then forwarded element-by-element to addFalsePositive(Key), which rejects null elements individually.

Source

Thrown at hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/bloom/RetouchedBloomFilter.java:158

    if (key == null) {
      throw new NullPointerException("key can not be null");
    }

    int[] h = hash.hash(key);
    hash.clear();

    for (int i = 0; i < nbHash; i++) {
      fpVector[h[i]].add(key);
    }
  }

  /**
   * Adds a collection of false positive information to <i>this</i> retouched Bloom filter.
   * @param coll The collection of false positive.
   */
  public void addFalsePositive(Collection<Key> coll) {
    if (coll == null) {
      throw new NullPointerException("Collection<Key> can not be null");
    }
    
    for (Key k : coll) {
      addFalsePositive(k);
    }
  }

  /**
   * Adds a list of false positive information to <i>this</i> retouched Bloom filter.
   * @param keys The list of false positive.
   */
  public void addFalsePositive(List<Key> keys) {
    if (keys == null) {
      throw new NullPointerException("ArrayList<Key> can not be null");
    }

    for (Key k : keys) {
      addFalsePositive(k);

View on GitHub (pinned to 2add963021)

Solutions

Pass Collections.emptyList() when there is nothing to record
Null-check the collection before the call
Initialize collection fields at declaration so they are never null

Example fix

// before
rbf.addFalsePositive(coll); // NPE when coll == null

// after
rbf.addFalsePositive(coll != null ? coll : Collections.<Key>emptyList());

Defensive patterns

Strategy: validation

Validate before calling

rbf.addFalsePositive(coll != null ? coll : Collections.<Key>emptyList());

Prevention

Make your helpers never return null collections (return empty ones)
Initialize collection fields at declaration
Where 'absent' is meaningful, decide the empty-vs-null policy once at the API edge

When it happens

Trigger: rbf.addFalsePositive((Collection<Key>) null); passing a never-initialized field; a method parameter that an upstream branch left null.

Common situations: Optional feedback stages that skip building the collection and pass null; fields initialized only inside a conditional; glue code mapping 'absent' to null.

Related errors

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/8b8c4e0c42d5161d. Report an issue: GitHub.