ArrayList的源码分析

/**

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• Default initial capacity.
• 初始化容量为10
  */
  private static final int DEFAULT_CAPACITY = 10;
  private static final Object[] EMPTY_ELEMENTDATA = {};
  /**
• Shared empty array instance used for default sized empty instances. We
• distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
• first element is added.

• */
  private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
  /**

• The array buffer into which the elements of the ArrayList are stored.
• The capacity of the ArrayList is the length of this array buffer. Any
• empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
• will be expanded to DEFAULT_CAPACITY when the first element is added.
  */
  transient Object[] elementData; // non-private to simplify nested class access
  /**
• The size of the ArrayList (the number of elements it contains).

• @serial
  */
  private int size;
  //最大数组大小
  private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
  //默认实例为一个空数组
  public ArrayList() {
  this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
  }
  //添加元素
  public boolean add(E e) {
  //确定容量
  ensureCapacityInternal(size + 1); // Increments modCount!!
  elementData[size++] = e;
  return true;
  }

  private void ensureCapacityInternal(int minCapacity) {
  ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
  }

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

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

  }
  private void grow(int minCapacity) {
  // overflow-conscious code
  int oldCapacity = elementData.length;
  //扩容方式 旧容量+旧容量右移一位
  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);
  }
  //循环清除
  public void clear() {
  modCount++;

  // clear to let GC do its work
  for (int i = 0; i < size; i++)
      elementData[i] = null;
  
  size = 0;

  }
  //删除 --很巧妙的删除
  public E remove(int index) {
  rangeCheck(index);

  modCount++;
  E oldValue = elementData(index);
  //获得需要移动的位数
  int numMoved = size - index - 1;
  if (numMoved > 0)
      //进行复制，最后一位会保留下来
      System.arraycopy(elementData, index+1, elementData, index,
                       numMoved);
  //数组大小减少一位，最后一位赋值为空                  
  elementData[--size] = null; // clear to let GC do its work
  
  return oldValue;

  }

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