1.简介
继承AbstractList，实现List接口，继承添加，修改，遍历，包含等父类方法。实现RandmoAccess接口，标记性接口，即ArrayList是支持快速随机访问的。实现Cloneable接口，可以被克隆。实现java.io.Serializable接口，支持序列化。
2.成员属性
//默认的初始化容量
private static final int DEFAULT_CAPACITY = 10;
//当用户指定ArrayList容量为0时返回该实例
private static final Object[] EMPTY_ELEMENTDATA = {};
//当用户调用无参构造器时使用该实例返回
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
//数据存放数组，不参与序列化
transient Object[] elementData; // non-private to simplify nested class access
//实际存储的数据容量
private int size;
//修改次数
protected transient int modCount = 0;
3.构造函数
/**
* 指定初始化容量构建
*/
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);
}
}
/**
* 无参构造器，默认构造一个空数组的。
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* 通过其他集合构造ArrayList
*/
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;
}
}
4.方法分析
boolean add(E e)方法
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
//无参构造，设置为默认容量10
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
//当前的最小容量比实际容量大，就扩容
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
//每次扩容为当前容量的1.5倍，首次小于默认10时，按照10扩容
private void grow(int minCapacity) {
// overflow-conscious code
//当前容量
int oldCapacity = elementData.length;
//扩容1.5倍后的容量
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)//小于默认容量时使用默认容量10
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)//大于最大容量时，使用大容量分配
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);
}
/**
* 大容量分配，最大为int值的上限
*/
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
void add(int index, E element)
public void add(int index, E element) {
rangeCheckForAdd(index);
//扩容
ensureCapacityInternal(size + 1); // Increments modCount!!
//复制数组：相当于从index的后一位开始，整体后移一位
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
/**
* 判断索引是否越界，可以放到最后面故无等于判断
*/
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
E get(int index)
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
//越界检查
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
E set(int index, E element)
public E set(int index, E element) {
//越界检查
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;//替换
return oldValue;
}
E remove(int index)
public E remove(int index) {
//越界检查
rangeCheck(index);
modCount++;
E oldValue = elementData(index);
//将被删除元素的移除，后面的补上来。
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // 垃圾回收
return oldValue;
}
boolean remove(Object o)
public boolean remove(Object o) {
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
} else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}
//和上面一样
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
}
5.总结
ArrayList底层是数组。存储的元素是有序的。存储元素可以重复。随机访问效率高，增删效率低。通过索引可以很快的查找到对应元素，而增删元素许多元素的位置都要改变。线程不安全，并没有实现同步。
