apache/hadoop · error · IllegalArgumentException

null argument passed in equal().

Error message

null argument passed in equal().

What it means

EnumSetWritable.equals() deliberately violates the standard equals contract by throwing IllegalArgumentException on a null argument instead of returning false. Any caller that follows the normal Java convention of comparing possibly-null references (HashMap lookups, Objects.equals expectations) will get an exception instead of false.

Source

Thrown at hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/io/EnumSetWritable.java:163

        if (this.elementType == null)
          throw new UnsupportedOperationException(
              "Unable to serialize empty EnumSet with no element type provided.");
        WritableUtils.writeString(out, this.elementType.getName());
      }
      for (int i = 0; i < length; i++) {
        ObjectWritable.writeObject(out, array[i], array[i].getClass(), conf);
      }
    }
  }

  /**
   * Returns true if <code>o</code> is an EnumSetWritable with the same value,
   * or both are null.
   */
  @Override
  public boolean equals(Object o) {
    if (o == null) {
      throw new IllegalArgumentException("null argument passed in equal().");
    }

    if (!(o instanceof EnumSetWritable))
      return false;

    EnumSetWritable<?> other = (EnumSetWritable<?>) o;

    if (this == o || (this.value == other.value))
      return true;
    if (this.value == null) // other.value must not be null if we reach here
      return false;

    return this.value.equals(other.value);
  }

  /**
   * Returns the class of all the elements of the underlying EnumSetWriable. It
   * may return null.

View on GitHub (pinned to 2add963021)

Solutions

1. Null-check before comparing: if (o != null && writable.equals(o)).
2. Reverse the comparison when possible: pass the writable as the argument to a null-safe equals.
3. Use java.util.Objects.equals carefully — it calls a.equals(b), so guard the EnumSetWritable side being non-null and the argument being non-null.

Example fix

// before
boolean same = esw.equals(maybeNull); // throws when maybeNull == null

// after
boolean same = maybeNull != null && esw.equals(maybeNull);

boolean same = (maybeNull instanceof EnumSetWritable) && esw.equals(maybeNull);

static boolean safeEquals(EnumSetWritable<?> a, Object b) {
  return b != null && a.equals(b);
}

When it happens

Trigger: writable.equals(null); collections APIs that probe with null (e.g. list.contains(null), map.get-like scans); assertion libraries comparing against an expected null.

Common situations: Putting EnumSetWritable into ArrayList/HashSet and calling contains(null); comparing a decoded value to a null expected value in tests; generic pipeline code doing value.equals(current) where current may be null.

Related errors

• source map cannot be null
• null component type not allowed
• null value not allowed
• null valueClass
• The EnumSet argument is null, or is an empty set but with no

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/320289abd98ed1bb. Report an issue: GitHub.