ArrayList及HashMap的扩容规则讲解

1、ArrayList

默认大小为10

/**
 * Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;

最大容量为2^30 - 8

 /**
 * The maximum size of array to allocate.
 * Some VMs reserve some header words in an array.
 * Attempts to allocate larger arrays may result in
 * OutOfMemoryError: Requested array size exceeds VM limit
 */
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
 * A constant holding the maximum value an {@code int} can
 * have, 2<sup>31</sup>-1.
*/
public static final int  MAX_VALUE = 0x7fffffff;

扩容规则为：oldCapacity*1.5

/**
 * Increases the capacity to ensure that it can hold at least the
 * number of elements specified by the minimum capacity argument.
 * @param minCapacity the desired minimum capacity
 */
private void grow(int minCapacity) {
  // overflow-conscious code
  int oldCapacity = elementData.length;
  int newCapacity = oldCapacity + (oldCapacity >> 1);
  if (newCapacity - minCapacity < 0)
    newCapacity = minCapacity;
  if (newCapacity - MAX_ARRAY_SIZE > 0)
    newCapacity = hugeCapacity(minCapacity);
  // minCapacity is usually close to size, so this is a win:
  elementData = Arrays.copyOf(elementData, newCapacity);
}

2、HashMap

默认大小： 16

/**
 * The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

最大容量为：2^30

/**
 * The maximum capacity, used if a higher value is implicitly specified
 * by either of the constructors with arguments.
 * MUST be a power of two <= 1<<30.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;

扩容规则为：大于oldCapacity的最小的2的n次方整数

/**
 * Adds a new entry with the specified key, value and hash code to
 * the specified bucket. It is the responsibility of this
 * method to resize the table if appropriate.
 * Subclass overrides this to alter the behavior of put method.
 */
void addEntry(int hash, K key, V value, int bucketIndex) {
  if ((size >= threshold) && (null != table[bucketIndex])) {
    resize(2 * table.length);
    hash = (null != key) ? hash(key) : 0;
    bucketIndex = indexFor(hash, table.length);
  }
  createEntry(hash, key, value, bucketIndex);
}

总结

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