java.util.ArrayList就是传说中的动态数组.
继承了关系，有此可看出ArrayList与list的collection的关系
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
分析期特性
具有实例化性质，从实现了serializable就可以看出来
private static final long serialVersionUID = 8683452581122892189L;
接下来定义了初始容量大小，和初始锥形。arralist里面本身存储的为一个Object类型的数组
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10; /**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
private transient Object[] elementData;
使用arraylist时需要实例化：有三种
/**
* 初始化一个给定大小容量的数组List，主要用于如果本事知道这个list的大小时。
* 能优化如果大小超过默认容量时导致arralist每次都要去增加并创建一个新的list
* @param initialCapacity the initial capacity of the list
* @throws IllegalArgumentException if the specified initial capacity
* is negative
*/
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
} /**
* 使用默认的大小10
*/
public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
} /**
* 根据一个collection来定义一个arralist
*
* @param c the collection whose elements are to be placed into this list
* @throws NullPointerException if the specified collection is null
*/
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}
Arraylist有提供期可能存在过多的空容量时清除处理
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = Arrays.copyOf(elementData, size);
}
}
另一个知识点。当arraylist新增时如果当前容量已满时的处理过程
public boolean add(E e) {
ensureCapacityInternal(size + 1); // 判断容量是否超标
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);// 容量超标情况
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)// 如果超标，则处理
grow(minCapacity);
}
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length; // 容量大小
int newCapacity = oldCapacity + (oldCapacity >> 1); //超标增加量计算法，x+x/2=50%左右
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
// 赋值原来的数组，并增加newCapacity个容量
elementData = Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}