ArrayList的源碼分析

/**

• Default initial capacity.
• 初始化容量為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;
  //最大數(shù)組大小
  private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
  //默認(rèn)實例為一個空數(shù)組
  public ArrayList() {
  this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
  }
  //添加元素
  public boolean add(E e) {
  //確定容量
  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
  //超過擴(kuò)容
  if (minCapacity - elementData.length > 0)
      grow(minCapacity);

  }
  private void grow(int minCapacity) {
  // overflow-conscious code
  int oldCapacity = elementData.length;
  //擴(kuò)容方式 舊容量+舊容量右移一位
  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);
  }
  //循環(huán)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;

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

  modCount++;
  E oldValue = elementData(index);
  //獲得需要移動的位數(shù)
  int numMoved = size - index - 1;
  if (numMoved > 0)
      //進(jìn)行復(fù)制，最后一位會保留下來
      System.arraycopy(elementData, index+1, elementData, index,
                       numMoved);
  //數(shù)組大小減少一位，最后一位賦值為空                  
  elementData[--size] = null; // clear to let GC do its work
  
  return oldValue;

  }