本文基於JDK1.8
0x 00 總體描述
ArrayList是基於數組實現的，並且支持自動擴容，相對於普通數組而言，由於他自動擴容的的特性，在日常開發過程中，使用的十分多。
// ArrayList.java
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, java.io.Serializable
{
從類圖和源碼可知，ArrayList實現了四個接口：
java.util.List 接口，主要數組的接口類，提供數組的基本操作，如添加、刪除、修改、遍歷；
java.util.RandomAccess 接口，提供快速隨機訪問的能力。
java.io.Serializable 接口，表示ArrayList可序列化。
java.lang.Cloneable 表示其支持克隆
繼承了java.util.AbstractList 抽象類，是List接口的基本實現，減少了具體實現類中的大部分共性代碼的實現。
注：ArrayList重寫了大部分AbstractList的方法實現，所以其對ArrayList作用不大，主要是其他減少了其他子類的實現。
0x01 源碼閱讀
屬性
ArrayList的屬性很少，只有兩個，如下：
// ArrayList.java
/**
* 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.
* 元素數組，可自動擴容
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
* 已使用的數組大小
*/
private int size;
elementData 屬性：元素數組，存儲元素。
size屬性：數組的大小。size表示的是elementData數組存儲的元素數量，並且正好是元素添加到數組中的下標，ArrayList的真實大小是elementData的大小。
構造方法
ArrayList一共三個構造方法如下：
① #ArrayList(int initialCapacity)
/**
* Constructs an empty list with the specified initial capacity.
*
* @param initialCapacity the initial capacity of the list
* @throws IllegalArgumentException if the specified initial capacity
* is negative
* 帶構造方法，主要用於容量的初始化
*/
public ArrayList(int initialCapacity) {
// 初始容量大於0 創建Object數組
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity];
// 等於0 使用其自身的final空數組EMPTY_ELEMENTDATA
} else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA;
// 小於0 則拋出參數不合法異常
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
該構造方法可以根據傳入的初始化容量大小，創建對應的ArrayList數組。對於我們已經明確知道容量大小的場景，可以使用這個構造方法，可以減少數組的擴容，提高系統的性能，合理使用內存。’
② #ArrayList()
無參構造方法，是我們使用最多的構造方法。
// ArrayList.java
/**
* Constructs an empty list with an initial capacity of ten.
* 默認容量爲10
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
此處需要注意的是，創建的是爲空的數組，爲了節約內存，在初始化時爲空數組，在添加第一個元素時，會真正初始化爲10的數組。至於爲什麼要和EMPTY_ELEMENTDATA區分開來呢，是爲區分起點，空數組是從0開始按照1.5唄倍擴容，而不是10，而DEFAULTCAPACITY_EMPTY_ELEMENTDATA首次擴容爲10；
③ #ArrayList(Collection<? extends E> c)
從參數類型可以看出，傳的參數爲集合類型，作爲elementData。
//ArrayList.java
/**
* Constructs a list containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
*
* @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) {
// 轉換爲數組
elementData = c.toArray();
// 判斷數組長度
if ((size = elementData.length) != 0) {
// c.toArray might (incorrectly) not return Object[] (see 6260652)
// 轉化爲數組時，可能不是Object類型，則會創建新的Object數組，並賦值給elementData
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// replace with empty array.
// 若數組大小等於0則直接賦值空數組
this.elementData = EMPTY_ELEMENTDATA;
}
}
普通方法
① 擴容與縮容方法
#trimToSize()
數組的手動縮容方法，實現如下：
ArrayList.java
**
* Trims the capacity of this <tt>ArrayList</tt> instance to be the
* list's current size. An application can use this operation to minimize
* the storage of an <tt>ArrayList</tt> instance.
*/
public void trimToSize() {
// 記錄修改次數
modCount++;
if (size < elementData.length) {
elementData = (size == 0)
// 大小爲0則使用空數組
? EMPTY_ELEMENTDATA
// 大於0則創建大小爲size的新數組，將原數組複製進去
: Arrays.copyOf(elementData, size);
}
}
#grow()
擴容方法，在擴容過程中，首先創建一個新的更大的數組，一般時1.5倍大小，將原數組複製到新數組中，最後返回新數組。代碼如下：
ArrayList.java
/**
* 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) {
int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
// any size if not default element table
? 0
// larger than default for default empty table. It's already
// supposed to be at default size.
: DEFAULT_CAPACITY;
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
/**
* The maximum size of array to allocate.
* Some VMs reserve some header words in an array.
* Attempts to allocate larger arrays may result in
* OutOfMemoryError: Requested array size exceeds VM limit
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* 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;
// 1.5的大小 擴容
int newCapacity = oldCapacity + (oldCapacity >> 1);
// 擴容後的大小比所需要的容量小
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
// 擴容後比最大的數組大小要大
if (newCapacity - MAX_ARRAY_SIZE > 0)
// 則進行容量的相關判斷
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
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;
}
② 查找指定元素對應的下標
#indexOf(Object o)
ArrayList.java
/**
* Returns the index of the first occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
* More formally, returns the lowest index <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
* or -1 if there is no such index
*/
public int indexOf(Object o) {
// 判斷元素是否爲null
if (o == null) {
for (int i = 0; i < size; i++)
if (elementData[i]==null)
return i;
} else {
// 非null使用equals判斷全等,比較的是對象是否相等
for (int i = 0; i < size; i++)
if (o.equals(elementData[i]))
return i;
}
return -1;
}
③ 克隆方法
#clone()
// ArrayList.java
/**
* Returns a shallow copy of this <tt>ArrayList</tt> instance. (The
* elements themselves are not copied.)
*
* @return a clone of this <tt>ArrayList</tt> instance
*/
public Object clone() {
try {
// 調用父類的克隆方法
ArrayList<?> v = (ArrayList<?>) super.clone();
// 將原數組拷貝進新數組
// 此處的數組是新拷貝出來的,避免和原數組共享引用
v.elementData = Arrays.copyOf(elementData, size);
// 設置修改次數爲0
v.modCount = 0;
return v;
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError(e);
}
}
④ 轉換爲數組
#toArray() and #toArray(T[] a)
// ArrayList.java
/**
* Returns an array containing all of the elements in this list
* in proper sequence (from first to last element).
*
* <p>The returned array will be "safe" in that no references to it are
* maintained by this list. (In other words, this method must allocate
* a new array). The caller is thus free to modify the returned array.
*
* <p>This method acts as bridge between array-based and collection-based
* APIs.
*
* @return an array containing all of the elements in this list in
* proper sequence
*/
// 此方法返回是Object[]
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
/**
* Returns an array containing all of the elements in this list in proper
* sequence (from first to last element); the runtime type of the returned
* array is that of the specified array. If the list fits in the
* specified array, it is returned therein. Otherwise, a new array is
* allocated with the runtime type of the specified array and the size of
* this list.
*
* <p>If the list fits in the specified array with room to spare
* (i.e., the array has more elements than the list), the element in
* the array immediately following the end of the collection is set to
* <tt>null</tt>. (This is useful in determining the length of the
* list <i>only</i> if the caller knows that the list does not contain
* any null elements.)
*
* @param a the array into which the elements of the list are to
* be stored, if it is big enough; otherwise, a new array of the
* same runtime type is allocated for this purpose.
* @return an array containing the elements of the list
* @throws ArrayStoreException if the runtime type of the specified array
* is not a supertype of the runtime type of every element in
* this list
* @throws NullPointerException if the specified array is null
*/
//
public <T> T[] toArray(T[] a) {
// 若傳入的數組大小小於size的大小
if (a.length < size)
// Make a new array of a's runtime type, but my contents:
// 複製返回一個新數組
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
// 將elementData複製到a
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
// 返回a
return a;
}
由於可能返回新的數組,所以這裏建議使用時還是這麼寫:a = list.toArray(a)
⑤ 獲取指定位置元素
#get(int index)
// ArrayList.java
E elementData(int index) {
return (E) elementData[index];
}
/**
* 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) {
// 下標檢查 index 不能超過size
rangeCheck(index);
return elementData(index);
}
/**
* 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(outOfBoundsMsg(index));
}
⑥ 設置指定位置元素
#set(int index, E element)
// ArrayList.java
/**
* Replaces the element at the specified position in this list with
* the specified element.
*
* @param index index of the element to replace
* @param element element to be stored at the specified position
* @return the element previously at the specified position
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E set(int index, E element) {
// 下標檢驗
rangeCheck(index);
// 獲取下標的原數據
E oldValue = elementData(index);
// 對應下標設置新元素
elementData[index] = element;
// 返回老值
return oldValue;
}
⑦ 添加單個元素
#add(E e)
順序添加某個元素
// ArrayList.java
/**
* 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) {
// 擴容
ensureCapacityInternal(size + 1); // Increments modCount!!
// 將新建的元素添加到末尾
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private void ensureExplicitCapacity(int minCapacity) {
// 增加數組的修改次數
modCount++;
// overflow-conscious code
// 數組擴容
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
private static int calculateCapacity(Object[] elementData, int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
return Math.max(DEFAULT_CAPACITY, minCapacity);
}
return minCapacity;
}
#add(int index, E element)
在某個確定位置添加元素
// ArrayList.java
/**
* Inserts the specified element at the specified position in this
* list. Shifts the element currently at that position (if any) and
* any subsequent elements to the right (adds one to their indices).
*
* @param index index at which the specified element is to be inserted
* @param element element to be inserted
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public void add(int index, E element) {
// 範圍檢查
rangeCheckForAdd(index);
// 數組擴容
ensureCapacityInternal(size + 1); // Increments modCount!!
// 數組元素拷貝 空出index位置
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
// 對應位置賦值
elementData[index] = element;
// size增加1
size++;
}
⑧ 移除元素
#remove(int index)
// ArrayList.java
/**
* 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);
// clear to let GC do its work
elementData[--size] = null;
return oldValue;
}