ArrayList分析

1， ArrayList 无参构造 默认有开辟10空间

 /**     * Default initial capacity.     */    private static final int DEFAULT_CAPACITY = 10;

2，add添加元素的分析有两个接口

 ①尾巴添加

  public boolean add(E e) {        ensureCapacityInternal(size + 1);  // Increments modCount!!        elementData[size++] = e;        return true;    }

分析ensureCapacityInternal（）函数的接口

判断是否要内存扩大 如果扩大就调用grow()函数

 private void grow(int minCapacity) {        // overflow-conscious code        int oldCapacity = elementData.length;        //扩大一半            空间        int newCapacity = oldCapacity + (oldCapacity >> 1);        if (newCapacity - minCapacity < 0)            newCapacity = minCapacity;        //如果空间扩展最大值了 0x7fffffff  赋值大值        if (newCapacity - MAX_ARRAY_SIZE > 0)            newCapacity = hugeCapacity(minCapacity);        // minCapacity is usually close to size, so this is a win:        //拷贝数据        elementData = Arrays.copyOf(elementData, newCapacity);    }

System.arraycopy(original, 0, copy, 0,                         Math.min(original.length, newLength));在下面分析arraycopy拷贝的算法 ②指定位置插入

  public void add(int index, E element) {        if (index > size || index < 0)            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));        ensureCapacityInternal(size + 1);  // Increments modCount!!        //中间插入节点            System.arraycopy(elementData, index, elementData, index + 1,                         size - index);        elementData[index] = element;        size++;    }

拷贝数据判断是否32 的判

ARRAYCOPY_SHORT_CHAR_ARRAY_THRESHOLDif (length <= ARRAYCOPY_SHORT_CHAR_ARRAY_THRESHOLD) {            // Copy char by char for shorter arrays.     //运算符优先级的操作     + , < ,==, &&                    //判断是否是中间插入的算法            if (src == dst && srcPos < dstPos && dstPos < srcPos + length) {                // Copy backward (to avoid overwriting elements before                // they are copied in case of an overlap on the same                // array.)                //index 索引的后面拷贝                for (int i = length - 1; i >= 0; --i) {                    dst[dstPos + i] = src[srcPos + i];                }            } else {  //索引的前面拷贝                // Copy forward.                for (int i = 0; i < length; ++i) {                    dst[dstPos + i] = src[srcPos + i];                }            }        } else {            // Call the native version for longer arrays.            arraycopyCharUnchecked(src, srcPos, dst, dstPos, length);        }![这里写图片描述](http://img.blog.csdn.net/20171110232039221?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvUG9pc3g=/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast)###3，remove元素的操作 两个接口同上①，指定元素删除    public boolean remove(Object o) {        if (o == null) {            for (int index = 0; index < size; index++)                if (elementData[index] == null) {                    fastRemove(index);  //指定下标删除元素 gc                    return true;                }        } else {            for (int index = 0; index < size; index++)                if (o.equals(elementData[index])) {                    fastRemove(index);                    return true;                }        }        return false;    }fastRemove调用 System.arraycopy(elementData, index+1, elementData, index,                             numMoved);方法

②，指定下标删除
中间调用 System.arraycopy(elementData, index+1, elementData, index,
numMoved);方法

3，get和set方法都是先判断下标是否越界 ，直接操作数组的

4，clear方法数组置空的操作

5，addAll方法的操作

①，尾巴添加

 public boolean addAll(Collection<? extends E> c) {        Object[] a = c.toArray();        int numNew = a.length;        //添加数据的实际数据的空间        ensureCapacityInternal(size + numNew);  // Increments modCount        System.arraycopy(a, 0, elementData, size, numNew);        size += numNew;        return numNew != 0;    }

②，指定位置插入

    public boolean addAll(int index, Collection<? extends E> c) {            if (index > size || index < 0)                throw new IndexOutOfBoundsException(outOfBoundsMsg(index));            Object[] a = c.toArray();            int numNew = a.length;            ensureCapacityInternal(size + numNew);  // Increments modCount            int numMoved = size - index;            if (numMoved > 0)                System.arraycopy(elementData, index, elementData, index + numNew,                                 numMoved);            System.arraycopy(a, 0, elementData, index, numNew);            size += numNew;            return numNew != 0;        }    和上面一样的操作