Java ArrayList扩容问题实例详解

transient Object[] elementData; 

ArrayList al = new ArrayList();

transient Object[] elementData; 
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

public ArrayList() {
  this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

ArrayList al = new ArrayList(5);

transient Object[] elementData; 
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

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);
  }
}

ArrayList al = new ArrayList<Integer>(Arrays.asList(1, 2, 3, 4, 5));

private int size;
transient Object[] elementData; 
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

public ArrayList(Collection<? extends E> c) {
  elementData = c.toArray();
  if ((size = elementData.length) != 0) {
    // c.toArray might (incorrectly) not return Object[] (see 6260652)
    if (elementData.getClass() != Object[].class)
      elementData = Arrays.copyOf(elementData, size, Object[].class);
  } else {
    // replace with empty array.
    this.elementData = EMPTY_ELEMENTDATA;
  }
}

ArrayList<Integer> collection = new ArrayList<Integer>(Arrays.asList(1, 2, 3, 4, 5));
Integer[] moreInts = { 6, 7, 8, 9, 10 };
collection.addAll(Arrays.asList(moreInts));

public Boolean addAll(Collection<? extends E> c) {
	// 得到插入数组
	Object[] a = c.toArray();
	// 得到插入内容长度
	int numNew = a.length;
	ensureCapacityInternal(size + numNew);
	// Increments modCount
	System.arraycopy(a, 0, elementData, size, numNew);
	size += numNew;
	return numNew != 0;
}
private void ensureCapacityInternal(int minCapacity) {
	//如果ArrayList里面的内容为空
	if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
		minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
	}
	ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
	modCount++;
	// 进一步计算扩充后的大小minCapacity
	if (minCapacity - elementData.length > 0)
	    grow(minCapacity);
}
private void grow(int minCapacity) {
	// ArrayList的原始大小
	int oldCapacity = elementData.length;
	// 在原始大小的基础上计算扩充后的大小，扩充后的大小是元素大小的1.5倍
	int newCapacity = oldCapacity + (oldCapacity >> 1);
	//跟前面计算的扩充后长度minCapacity比较，取较大的那个为扩充后长度
	if (newCapacity - minCapacity < 0)
	    newCapacity = minCapacity;
	// 如果扩充后长度大于最大长度
	if (newCapacity - MAX_ARRAY_SIZE > 0)
	    newCapacity = hugeCapacity(minCapacity);
	// 扩充
	elementData = Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity) {
	// minCapacity小于0，说明溢出，否则将最大整数作为最终扩充长度
	if (minCapacity < 0) // overflow
	throw new OutOfMemoryError();
	return (minCapacity > MAX_ARRAY_SIZE) ?
	    Integer.MAX_VALUE :
	    MAX_ARRAY_SIZE;
}