ArrayList 的底层实现是数组
1.构造函数
public ArrayList() {
this(10);
}
public ArrayList(int initialCapacity) {
super();//默认可以不写
if (initialCapacity < 0)//初始化的容量如果小于0就抛出异常
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
//创建一个大小为10的Object数据对象
this.elementData = new Object[initialCapacity];
}
2. add函数
/**
* Appends the specified element to the end of this list.//在列表的后面加上元素
*
* @param e element to be appended to this list
* @return <tt>true</tt> (as specified by {@link Collection#add})
*/
public boolean add(E e) {
ensureCapacity(size + 1); // Increments modCount!!
elementData[size++] = e;//将元素加到数组里面
return true;
}
//minCapacity
public void ensureCapacity(int minCapacity) {
modCount++;
int oldCapacity = elementData.length;
if (minCapacity > oldCapacity) {//如果超过范围
Object oldData[] = elementData;
int newCapacity = (oldCapacity * 3)/2 + 1;//增加容量为1.5倍加1
if (newCapacity < minCapacity)
newCapacity = minCapacity;
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);//把老的数组copy到新的数组中去
}
}
3 get函数的方法
/**
* 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);//判断数组下标是否越界
return (E) elementData[index];//通过下标得到元素然后返回
}
private void RangeCheck(int index) {
if (index >= size)//如果越界就抛出异常
throw new IndexOutOfBoundsException(
"Index: "+index+", Size: "+size);
}
4.remove的函数
/**
* 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);
modCount++;
E oldValue = (E) elementData[index];
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,//自己copy给自己，就是移位
numMoved);
elementData[--size] = null; // Let gc do its work
return oldValue;
}