java - 为什么创建 HashMap 比创建 Object[] 更快？

Question

我尝试构建自己的 map 来提高特殊环境的性能，我发现了一些非常有趣的事情:创建 new Hashmap<Integer,String>(2000)比 new Object[2000] 快- 无论我按什么顺序执行这些命令。这让我很困惑，尤其是。因为 Hashmap 构造函数包含 table = new Entry[capacity] , 根据 this .我的测试台有问题吗？

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
}

我很想看看在另一台计算机上的测试结果。我不知道为什么要创建 HashMap比创建 Object[] 快 10 倍.

Answer 1

如果你看一下 HashMap 的实现，构造函数看起来像:

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();
}

而 init() 看起来像:

/**
 * 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() {
}

所以 initialCapacity 实际上并不习惯于创建数组。它在哪里使用？查看 put() 方法。

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

在执行 put 时，实际上创建了数组。我没有显示 inflateTable() 但它做了一些数学运算并初始化了数组。