一、總結
（1）ArrayList随機元素時間複雜度O(1)，插入删除操作需大量移動元素，效率較低
（2）為了節約記憶體，當建立容器為空時，會共享
Object[] EMPTY_ELEMENTDATA = {}
和
Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}
空數組
（3）容器底層采用數組存儲，每次擴容為1.5倍
（4）ArrayList的實作中大量地調用了
Arrays.copyof()
和
System.arraycopy()
方法，其實
Arrays.copyof()
内部也是調用
System.arraycopy()
。
System.arraycopy()
為Native方法
（5）兩個ToArray方法
-
Object[] toArray()
-
<T> T[] toArray(T[] a)
（6）ArrayList可以存儲null值
（7）ArrayList每次修改（增加、删除）容器時，都是修改自身的modCount；在生成疊代器時，疊代器會儲存該modCount值，疊代器每次擷取元素時，會比較自身的modCount與ArrayList的modCount是否相等，來判斷容器是否已經被修改，如果被修改了則抛出異常（fast-fail機制）。
二、源碼和注解
package java.util;
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
private static final long serialVersionUID = L;
// 預設初始大小
private static final int DEFAULT_CAPACITY = ;
// 指定大小的空的ArrayList共享此對象
private static final Object[] EMPTY_ELEMENTDATA = {};
// 預設大小的空的ArrayList共享
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
// 存儲容器，為數組
transient Object[] elementData;
// 存放容器大小
private int size;
// 傳入初始大小
public ArrayList(int initialCapacity) {
if (initialCapacity > ) {
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == ) {
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
// 無參構造
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
// 由Collection構造
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
if ((size = elementData.length) != ) {
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
this.elementData = EMPTY_ELEMENTDATA;
}
}
// 将目前容量設為實際元素個數
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = (size == )
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
// 確定容器容量，當容器非空時，不得小于預設大小
public void ensureCapacity(int minCapacity) {
int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
?
: DEFAULT_CAPACITY;
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
// 注意此方法在每次添加元素時調用，確定容量足夠，且更改modCount
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
if (minCapacity - elementData.length > )
grow(minCapacity);
}
// 數組最大元素個數，因為部分虛拟機會為數組預留了頭部，故這裡減8保證不會記憶體溢出
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - ;
// 擴容為minCapacity或者舊容器大小*1.5中較大的一個
private void grow(int minCapacity) {
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> );
if (newCapacity - minCapacity < )
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > )
newCapacity = hugeCapacity(minCapacity);
elementData = Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < ) // 溢出
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
public int size() {
return size;
}
public boolean isEmpty() {
return size == ;
}
// 周遊所有元素的方法，效率較低
public boolean contains(Object o) {
return indexOf(o) >= ;
}
public int indexOf(Object o) {
if (o == null) {
for (int i = ; i < size; i++)
if (elementData[i]==null)
return i;
} else {
for (int i = ; i < size; i++)
if (o.equals(elementData[i]))
return i;
}
return -;
}
public int lastIndexOf(Object o) {
if (o == null) {
for (int i = size-; i >= ; i--)
if (elementData[i]==null)
return i;
} else {
for (int i = size-; i >= ; i--)
if (o.equals(elementData[i]))
return i;
}
return -;
}
// 淺拷貝！！
public Object clone() {
try {
ArrayList<?> v = (ArrayList<?>) super.clone();
v.elementData = Arrays.copyOf(elementData, size);
v.modCount = ;
return v;
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError(e);
}
}
// 該方法有可能會抛出java.lang.ClassCastException異常，
// 如果直接用向下轉型的方法，将整個ArrayList集合轉變為指定類型的Array數組，便會抛出該異常
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
// 該方法可以直接将ArrayList轉換得到的Array進行整體向下轉型
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData, , a, , size);
if (a.length > size)
a[size] = null;
return a;
}
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
// 随機通路
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
// 随機存儲
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
// 末尾添加元素
public boolean add(E e) {
ensureCapacityInternal(size + ); // 改變modCount
elementData[size++] = e;
return true;
}
// 随機位置添加元素，需複制後續元素，效率較低
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + );
// 複制插入位置的後續元素
System.arraycopy(elementData, index, elementData, index + ,
size - index);
elementData[index] = element;
size++;
}
public E remove(int index) {
rangeCheck(index);
modCount++;
E oldValue = elementData(index);
int numMoved = size - index - ;
if (numMoved > )
System.arraycopy(elementData, index+, elementData, index,
numMoved);
// 設定不通路的元素為null，觸發gc
elementData[--size] = null;
return oldValue;
}
// 删除第一個出現的元素
public boolean remove(Object o) {
if (o == null) {
for (int index = ; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
} else {
for (int index = ; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}
// 快速删除，在確定下标正确的情況下，不檢查下标範圍
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - ;
if (numMoved > )
System.arraycopy(elementData, index+, elementData, index,
numMoved);
elementData[--size] = null;
}
// 清楚所有元素，設定為null，但存儲的elementData容量不變
public void clear() {
modCount++;
for (int i = ; i < size; i++)
elementData[i] = null;
size = ;
}
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
System.arraycopy(a, , elementData, size, numNew);
size += numNew;
return numNew != ;
}
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
int numMoved = size - index;
if (numMoved > )
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
System.arraycopy(a, , elementData, index, numNew);
size += numNew;
return numNew != ;
}
protected void removeRange(int fromIndex, int toIndex) {
modCount++;
int numMoved = size - toIndex;
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);
int newSize = size - (toIndex-fromIndex);
for (int i = newSize; i < size; i++) {
elementData[i] = null;
}
size = newSize;
}
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private void rangeCheckForAdd(int index) {
if (index > size || index < )
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, false);
}
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, true);
}
private boolean batchRemove(Collection<?> c, boolean complement) {
final Object[] elementData = this.elementData;
int r = , w = ;
boolean modified = false;
try {
for (; r < size; r++)
if (c.contains(elementData[r]) == complement)
elementData[w++] = elementData[r];
} finally {
// Preserve behavioral compatibility with AbstractCollection,
// even if c.contains() throws.
if (r != size) {
System.arraycopy(elementData, r,
elementData, w,
size - r);
w += size - r;
}
if (w != size) {
// clear to let GC do its work
for (int i = w; i < size; i++)
elementData[i] = null;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}
// java.io.Serializable的寫入函數
// 将ArrayList的“容量，所有的元素值”都寫入到輸出流中
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=; i<size; i++) {
s.writeObject(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
// java.io.Serializable的讀取函數：根據寫入方式讀出
// 先将ArrayList的“容量”讀出，然後将“所有的元素值”讀出
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
elementData = EMPTY_ELEMENTDATA;
// Read in size, and any hidden stuff
s.defaultReadObject();
// Read in capacity
s.readInt(); // ignored
if (size > ) {
// be like clone(), allocate array based upon size not capacity
ensureCapacityInternal(size);
Object[] a = elementData;
// Read in all elements in the proper order.
for (int i=; i<size; i++) {
a[i] = s.readObject();
}
}
}
}
三、參考
http://blog.csdn.net/ns_code/article/details/35568011
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