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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