apache/hadoop · error · HadoopIllegalArgumentException

Illegal initial capacity: " + initCapacity

Error message

Illegal initial capacity: " + initCapacity

What it means

LightWeightResizableGSet is a chained, resizable hash set used by NameNode components. Its constructor follows the java.util.HashMap contract: the first argument, initCapacity, is the initial table-size hint, and a negative value throws HadoopIllegalArgumentException("Illegal initial capacity: N") before any LinkedElement[] array is allocated.

Source

Thrown at hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/LightWeightResizableGSet.java:68

  static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;

  /**
   * The load factor used when none specified in constructor.
   */
  static final float DEFAULT_LOAD_FACTOR = 0.75f;

  /** Size of the entry table. */
  private int capacity;

  /**
   * The load factor for the hash set.
   */
  private final float loadFactor;
  private int threshold;

  public LightWeightResizableGSet(int initCapacity, float loadFactor) {
    if (initCapacity < 0) {
      throw new HadoopIllegalArgumentException("Illegal initial capacity: " +
          initCapacity);
    }
    if (loadFactor <= 0 || loadFactor > 1.0f) {
      throw new HadoopIllegalArgumentException("Illegal load factor: " +
          loadFactor);
    }
    this.capacity = actualArrayLength(initCapacity);
    this.hash_mask = capacity - 1;
    this.loadFactor = loadFactor;
    this.threshold = (int) (capacity * loadFactor);

    entries = new LinkedElement[capacity];
  }

  public LightWeightResizableGSet() {
    this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
  }

View on GitHub (pinned to 2add963021)

Solutions

1. Pass initCapacity >= 0 (0 is legal and means the minimum table)
2. Clamp computed values: Math.max(0, expectedSize - slack)
3. Use the no-arg constructor (DEFAULT_INITIAL_CAPACITY / DEFAULT_LOAD_FACTOR) when the size is unknown

Example fix

// before
new LightWeightResizableGSet(estimated - RESERVED, 0.75f);

// after
new LightWeightResizableGSet(Math.max(0, estimated - RESERVED), 0.75f);

int cap = Math.max(0, estimatedSize - RESERVED);
new LightWeightResizableGSet<>(cap, 0.75f);

try { set = new LightWeightResizableGSet<>(cap, 0.75f); } catch (HadoopIllegalArgumentException e) { throw new IllegalArgumentException("Bad GSet init capacity " + cap + ": " + e.getMessage(), e); }

When it happens

Trigger: new LightWeightResizableGSet(-1, 0.75f); an initCapacity computed from arithmetic that underflows below zero (e.g. expectedSize - slack); tests enumerating invalid constructor arguments.

Common situations: Deriving the initial table size from a configuration value or size estimate and passing it straight through; copying HashMap-style code where the identical validation exists; a 'not set' -1 sentinel from config reaching the constructor.

Related errors

• Memory " + maxMemory + " must be greater than or equal to 0
• Illegal load factor: " + loadFactor
• {} parameters are required but {} arguments are provided
• Illegal option {}
• Not enough arguments: expected {} but got {}

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/5adfd14940c4d45a. Report an issue: GitHub.