对Java ArrayList的自动扩容机制示例讲解

 public boolean add(E e) {
  ensureCapacityInternal(size + 1); // Increments modCount!!
  elementData[size++] = e;
  return true;
 }

private void ensureCapacityInternal(int minCapacity) {
  if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
   minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
  }
  ensureExplicitCapacity(minCapacity);
 }

 private void ensureExplicitCapacity(int minCapacity) {
  modCount++;

  // overflow-conscious code
  if (minCapacity - elementData.length > 0)
   grow(minCapacity);
 }

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);
 }

 int newCapacity = oldCapacity + (oldCapacity >> 1);

private static int hugeCapacity(int minCapacity) {
  if (minCapacity < 0) // overflow
   throw new OutOfMemoryError();
  return (minCapacity > MAX_ARRAY_SIZE) ?
   Integer.MAX_VALUE :
   MAX_ARRAY_SIZE;
 }

elementData = Arrays.copyOf(elementData, newCapacity); 

 public static native void arraycopy
 (Object src, int srcPos,
 Object dest, int destPos,
 int length);

 public ArrayList(int initialCapacity) {
  if (initialCapacity > 0) {
   this.elementData = new Object[initialCapacity];
  } else if (initialCapacity == 0) {
   this.elementData = EMPTY_ELEMENTDATA;
  } else {
   throw new IllegalArgumentException(
   "Illegal Capacity: "+initialCapacity);            
  }
 }