ArrayList是基於數組實現的，是一個動態數組，其容量能自動增長。
ArrayList不是線程安全的，只能用在單線程環境下。
實現了RandomAccess接口，支持快速隨機訪問，實際上就是通過下標序號進行快速訪問；
實現了Cloneable接口，能被克隆。
實現了Serializable接口，因此它支持序列化，能夠通過序列化傳輸；說明：以下代碼解釋來源於jdk8源碼，不同版本可能有所不同。
一、源碼指明ArrayList的本質還是Array數組，其實從命名我們也可以窺探到。
/**
* The array buffer into which the elements of the ArrayList are stored.
* The capacity of the ArrayList is the length of this array buffer. Any
* empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
* will be expanded to DEFAULT_CAPACITY when the first element is added.
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
private int size;
二、ArrayList初始在不指定容量的情況下初始化爲一個長度爲0的空數組。初始化未指定長度，第一次add數據時ArrayList默認的初始化長度爲10。
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* Shared empty array instance used for default sized empty instances. We
* distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
* first element is added.
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
三、擴容 肯定是在add方法中實現
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return <tt>true</tt> (as specified by {@link Collection#add})
*/
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
擴容核心代碼：
/**
* Increases the capacity to ensure that it can hold at least the
* number of elements specified by the minimum capacity argument.
*
* @param minCapacity the desired minimum capacity
*/
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);
}
由此正常可見情況下新的容量等於 現有數組長度 + 現有數組長度右移1位（除以2取整）。
四、再看看remove操作（此處只展示指定座標元素移除，傳入元素移除無非就是遍歷一次數組）
* 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 = elementData(index);
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
return oldValue;
}
int numMoved = size - index - 1;計算出數組需要移動的次數。通過System.arraycopy進行位移並將數組的長度Size減一，最後一位置爲null由GC進行回收。
五、關於序列化問題：
聲明爲transient，爲什麼還可以序列化成功呢？
回答是ArrayList重寫了writeObject方法。
/**
* Save the state of the <tt>ArrayList</tt> instance to a stream (that
* is, serialize it).
*
* @serialData The length of the array backing the <tt>ArrayList</tt>
* instance is emitted (int), followed by all of its elements
* (each an <tt>Object</tt>) in the proper order.
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException{
// Write out element count, and any hidden stuff
int expectedModCount = modCount;
s.defaultWriteObject();
// Write out size as capacity for behavioural compatibility with clone()
s.writeInt(size);
// Write out all elements in the proper order.
for (int i=0; i<size; i++) {
s.writeObject(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
原因可能如下：ArrayList是會開闢多餘空間來保存數據的，而系列化和反序列化這些沒有存放數據的空間是要消耗更多資源的，
所以ArrayList的數組就聲明爲transient，告訴虛擬機這個你別管，我自己來處理，然後就自己實現write/readObject方法，僅僅系列化已經存放的數據。
先這樣吧，後面會繼續整理其他集合類。