ArrayList分析

 1 // 序列化id
 2 private static final long serialVersionUID = 8683452581122892189L;
 3 //ArrayList的初始容量大小
 4 private static final int DEFAULT_CAPACITY = 10;
 5 //空对象数组
 6 private static final Object[] EMPTY_ELEMENTDATA = {};
 7 //空对象数组，如果使用默认构造函数创建，则默认对象内容默认是该值 
 8 private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
 9 //存放当前数据，不参与序列化
10 transient Object[] elementData;
11 //list大小
12 private int size;
13 //list最大长度
14 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

//默认构造方法，将elementData初始化为DEFAULTCAPACITY_EMPTY_ELEMENTD（及空数组）
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
    //指定容量的构造方法，传入参数小于0抛出异常，否则创建指定容量的
    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);
      }
    }
    /**
    *传入参数为Collection对象，先调用toArray()方法将Collection对象转换为Object[]
    *更新size的值，同时判断size的大小，如果是size等于0，直接将空对象EMPTY_ELEMENTDATA的地址赋给elementData
    *如果size的值大于0，则执行Arrays.copy方法，把collection对象的内容（可以理解为深拷贝）copy到elementData中，代码注释toArray可能不是返回Object[]，所以进行深拷贝
    **/
    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;
        }
    }

//minCapacity 最小即为初始容量大小10
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        ensureExplicitCapacity(minCapacity);
    }
 
    //需求长度大于现在长度，则扩充数组长度
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    /**
    *首先将oldCapacity 右移一位缩小1/2，newCapacity 扩大为oldCapacity 的1.5倍
    *如果newCapacity不能满足需求，那就直接扩大为minCapacity大小
    *如果newCapacity超过list最大容量，则返回Interger类型最大值
    *最后更新elementData
    **/
    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);
    }