ArrayList源码（局部）

public class ArrayList extends AbstractList  implements List, RandomAccess, Cloneable, java.io.Serializable
{
 //默认的数组大小
 private static final int DEFAULT_CAPACITY = 10;

 private static final Object[] EMPTY_ELEMENTDATA = {};
 
 private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
 //
 private int size;
 //这个就是ArrayList中的数组，所有对ArrayList的操作实际上是对该数组的操作
 transient Object[] elementData;

 public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
 }
 
 public boolean add(E e) {
  ensureCapacityInternal(size + 1);
  elementData[size++] = e;
  return true;
  }

 private void ensureCapacityInternal(int minCapacity) {
    //如果当前数组为空,则取默认容量值和minCapacity的较大值
  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);
 }

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