Why is creating a HashMap faster than creating an Object[]?

If you look at the implementation of HashMap, the constructor looks like:

public HashMap(int initialCapacity, float loadFactor) {
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal initial capacity: " +
                                           initialCapacity);
    if (initialCapacity > MAXIMUM_CAPACITY)
        initialCapacity = MAXIMUM_CAPACITY;
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        throw new IllegalArgumentException("Illegal load factor: " +
                                           loadFactor);

    this.loadFactor = loadFactor;
    threshold = initialCapacity;
    init();
}

And init() looks like:

/**
 * Initialization hook for subclasses. This method is called
 * in all constructors and pseudo-constructors (clone, readObject)
 * after HashMap has been initialized but before any entries have
 * been inserted.  (In the absence of this method, readObject would
 * require explicit knowledge of subclasses.)
 */
void init() {
}

So initialCapacity doesn’t actually get used to create an array. Where does it get used? Look at the put() method.

public V put(K key, V value) {
    if (table == EMPTY_TABLE) {
        inflateTable(threshold);
    }
    // hidden
} 

When doing a put, the array is actually created. I didn’t show inflateTable() but it does some math and initializes the array.