1. ArrayList概要： public ArrayList() { this(10);
}
- 底层采用数组存储：private transient Object[] elementData;
transient表示数组elementData不需要通过serialization序列化传输
- 线程不安全，在多线程场景会出现问题，可以考虑使用Vector或者Collections.synchronizedList同步该容器
- 查询效率比较高，比较适用于查询
- 扩容的长度为初始长度的一半，建议初始化的时候设置已知的长度，免得容器自己去扩容，浪费空间以及效率
2. ArrayList代码详解： public boolean add(E e) { ensureCapacity(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
首先检查数组是否已满，如果满了就开始扩容，扩容后的长度为原长度的1.5倍。
/** * Increases the capacity of this <tt>ArrayList</tt> instance, if
* necessary, to ensure that it can hold at least the number of elements
* specified by the minimum capacity argument.
*
* @param minCapacity the desired minimum capacity
*/
public void ensureCapacity(int minCapacity) {
modCount++;//modCount表示数组的操作计数，用于iterator的时候与Count比较，防止迭代操作对add,remove等操作影响迭代操作
int oldCapacity = elementData.length;
if (minCapacity > oldCapacity) { //新插入元素后的长度大于老的长度，数组开始扩容
Object oldData[] = elementData;
int newCapacity = (oldCapacity * 3)/2 + 1;//新空间为原长度的1.5倍，等于是扩容了50%
if (newCapacity < minCapacity)
newCapacity = minCapacity;
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);//把之前的元素拷贝到新的数组 }
}
/**
* Removes the element at the specified position in this list.
* Shifts any subsequent elements to the left (subtracts one from their
* indices).
*
* @param index the index of the element to be removed
* @return the element that was removed from the list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E remove(int index) {
RangeCheck(index); //检查索引是否溢出
modCount++; //操作计数
E oldValue = (E) elementData[index];
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,//复制原数组制定index+1到length-1的元素到elementData的index的索引位
numMoved);
elementData[--size] = null; // Let gc do its work//最后一位设置为null
return oldValue;
}
/**
* Checks if the given index is in range. If not, throws an appropriate
* runtime exception. This method does *not* check if the index is
* negative: It is always used immediately prior to an array access,
* which throws an ArrayIndexOutOfBoundsException if index is negative.
*/
private void RangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(
"Index: "+index+", Size: "+size);
}
/**
* Returns the element at the specified position in this list.
*
* @param index index of the element to return
* @return the element at the specified position in this list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
RangeCheck(index);
return (E) elementData[index]; //直接获取数组的索引位
}