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

Question

I've tried to build my own Map to increase the performance for a special environment, and I realized something pretty interesting: Creating a new Hashmap<Integer,String>(2000) is faster than new Object[2000] - no matter in which order I execute these commands. That's pretty confusing to me, esp. because the Hashmap constructor contains a table = new Entry[capacity], according to this. Is there something wrong with my testbench?

public static void test(int amm){ //amm=1_000_000     Map<Integer,String> m1 = null;     Object[] arr = null;      long time = System.nanoTime();     for(int i = 0; i < amm; i++){         m1 = new HashMap<Integer, String>(2000);     }     System.out.println("m1: " + (System.nanoTime() - time)); //m1: 70_455_065      time = System.nanoTime();     for(int i = 0; i < amm; i++){         arr = new Object[2000];     }     System.out.println("arr: " + (System.nanoTime() - time)); //arr: 1_322_473_803 } 

I'd love to see the results of testing on another computer. I've got no clue why creating a HashMap is 10 times faster than creating a Object[].

Answer 1

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.