package java.util;
/**
* 繼承自 AbstractList ，實現List<E>（集合）, RandomAccess（標記接口，支持快速隨機訪問）,
* Cloneable（實現對象的淺拷貝）, java.io.Serializable（序列化）接口 （非線程安全）
*/
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
private static final long serialVersionUID = 8683452581122892189L;
/**
* 定義默認容量爲 10
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* 定義一個空集合
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* 定義一個空的數組
*/
private transient Object[] elementData;
/**
* 定義一個值 用於表示集合的元素數量 （元素數量不一定等於集合容量）
*/
private int size;
/**
* 構造具有指定初始容量的空集合
*/
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: " + initialCapacity);
this.elementData = new Object[initialCapacity];
}
/**
* 構造初始容量爲 10 的空集合.
*/
public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}
/**
* 按照集合迭代器返回的順序構造包含指定集合元素的集合
*/
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);
}
/**
* 去掉集合中的多餘位置，優化存儲（以實際元素個數爲集合容量大小）
*/
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = Arrays.copyOf(elementData, size);
}
}
/**
* 增加ArrayList實例的容量，如果有必要，確保它至少能容納的最小容量爲參數指定元素個數。
*/
public void ensureCapacity(int minCapacity) {
int minExpand = (elementData != EMPTY_ELEMENTDATA) ? 0 : DEFAULT_CAPACITY;
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}
// 判斷集合默認容量與指定最小容量的大小（取大）
private void ensureCapacityInternal(int minCapacity) {
if (elementData == EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
// 判斷指定容量與元素多少，如容量不足，則擴容
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
/**
* 集合的最大空間大小
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* 增加容量(每次1.5倍)，確保集合的最小容量能容納指定元素數量。
*/
private void grow(int minCapacity) {
int oldCapacity = elementData.length;
// 注意此處擴充capacity的方式是將其向右一位再加上原來的數，實際上是擴充了1.5倍(原始容量x3)/2 + 1
int newCapacity = oldCapacity + (oldCapacity >> 1);
//如果還不夠，則直接將minCapacity設置爲當前容量
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
//如果還不夠，則直接將Interger的容量作爲空間
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
elementData = Arrays.copyOf(elementData, newCapacity);
}
/**
* 返回指定集合空間的最大值（Integer的最大值）
*/
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0)
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;
}
/**
* 返回集合空間的實際大小
*/
public int size() {
return size;
}
/**
* 判斷 集體是否爲空
*/
public boolean isEmpty() {
return size == 0;
}
/**
* 判斷集合是否包含指定元素
*/
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
/**
* 返回指定元素在集合中首次出現的索引位置（正向匹配）如無，則返回 -1 指定元素爲空，數組某個位置元素也爲空，則返回該索引
*/
public int indexOf(Object o) {
if (o == null) {
for (int i = 0; i < size; i++)
if (elementData[i] == null)
return i;
} else {
for (int i = 0; i < size; i++)
if (o.equals(elementData[i]))
return i;
}
return -1;
}
/**
* 返回指定元素最後出現的索引位置（反向匹配）如無，則返回 -1
*/
public int lastIndexOf(Object o) {
if (o == null) {
for (int i = size - 1; i >= 0; i--)
if (elementData[i] == null)
return i;
} else {
for (int i = size - 1; i >= 0; i--)
if (o.equals(elementData[i]))
return i;
}
return -1;
}
/**
* 返回該arraylis實例淺拷貝 (元素本身不被複制)Object的 clone()方法
*/
public Object clone() {
try {
@SuppressWarnings("unchecked")
ArrayList<E> v = (ArrayList<E>) super.clone();
v.elementData = Arrays.copyOf(elementData, size);
v.modCount = 0;
return v;
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
}
/**
* 返回包含此集合中所有元素按適當順序（從第一個到最後一個元素）組成的Object數組
*/
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
/**
* 返回包含此集合中所有元素（從第一個到最後一個元素）組成的數組；
* 如果傳入數組的長度小於集合size，返回一個新的數組，大小爲size，類型與傳入數組相同。所傳入數組長度與size相等，
* 則將elementData複製到傳入數組中並返回傳入的數組。若傳入數組長度大於size，除了複製elementData外，
* 還將把返回數組的第size個元素置爲空
*/
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
/**
* 返回指定位置集合元素值
*/
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
/**
* 返回集合中指定位置的元素
*/
public E get(int index) {
// 判斷index是否越界
rangeCheck(index);
return elementData(index);
}
/**
* 設置集合中指定位置的元素
*/
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
/**
* 在集合末尾添加元素（同時增加集合容量）
*/
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
/**
* 向集合指定位置添加元素（同時增加集合容量）
*/
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1);
// 實現數組間的複製(源數組，源數組複製的起始位置，目標數據，目標數組放置的起始位置，複製的長度)
// System.arraycopy只是把index位置及其後面的元素，拷貝到數組的index+1及其後面的位置中，
// 也就是把index及其後面的元素全部後移一位。 最後，把新的元素放到數組的index位置
System.arraycopy(elementData, index, elementData, index + 1, size - index);
elementData[index] = element;
size++;
}
/**
* 移除指定位置的元素（同時減少集合容量：將最後一位賦值爲Null,以便垃圾回收）
*/
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;
}
/**
* 刪除集合中某元素（如爲空，則刪除數組中值爲空的值）返回true,刪除成功
*/
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;
}
/*
* 快速刪除指定位置的元素
*/
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
//從 index+1 開始，後面的元素依次替換前面的元素
if (numMoved > 0)
System.arraycopy(elementData, index + 1, elementData, index, numMoved);
//將最後一個元素設置爲Null,以便垃圾回收
elementData[--size] = null; // clear to let GC do its work
}
/**
* 刪除集合中所有的元素（設置所有值爲null，以便垃圾回收）
*/
public void clear() {
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
/**
* 按照指定集合中的元素順序將所有元素添加到現有集合的末尾（如果操作過程中修改指定集合，操作爲空）
*/
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}
/**
* 按照指定集合中的元素順序將所有元素添加到現有集合的指定位置（如果操作過程中修改指定集合，操作爲空）
*/
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
// 將集合轉換爲數組
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
int numMoved = size - index;
// 擴大集合容量
if (numMoved > 0)
System.arraycopy(elementData, index, elementData, index + numNew, numMoved);
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}
/**
* 刪除集合中fromIndex到toIndex之間的所有元素。將剩下的元素左移並減少它們的索引
*/
protected void removeRange(int fromIndex, int toIndex) {
modCount++;
int numMoved = size - toIndex;
System.arraycopy(elementData, toIndex, elementData, fromIndex, numMoved);
// clear to let GC do its work
int newSize = size - (toIndex - fromIndex);
for (int i = newSize; i < size; i++) {
elementData[i] = null;
}
size = newSize;
}
/**
* 檢查指定index是否越界
*/
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
/**
* 判斷指定index是否合理
*/
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
/**
* 錯誤信息的詳解
*/
private String outOfBoundsMsg(int index) {
return "Index: " + index + ", Size: " + size;
}
/**
* 刪除集合中等於指定集合中的元素的元素
*/
public boolean removeAll(Collection<?> c) {
return batchRemove(c, false);
}
/**
* 移除集合中所有未包含在指定集合中的元素
*/
public boolean retainAll(Collection<?> c) {
return batchRemove(c, true);
}
/**
* 移除此集合中包含着指定集合裏的元素（如果存在）
*/
private boolean batchRemove(Collection<?> c, boolean complement) {
final Object[] elementData = this.elementData;
int r = 0, w = 0;
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;
}
/**
* 將集合保存爲一個流，即序列化，將 容量和元素值信息寫入流中
*/
private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {
// Write out element count, and any hidden stuff
int expectedModCount = modCount;
s.defaultWriteObject();
// 寫入容量
s.writeInt(size);
//寫入第一個元素
for (int i = 0; i < size; i++) {
s.writeObject(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
/**
* 反序列化，將一個流轉換爲一個ArrayList實例.
*/
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 > 0) {
// 分配集合空間根據集合長度，而不是容量
ensureCapacityInternal(size);
Object[] a = elementData;
// 按適當順序讀入所有元素
for (int i = 0; i < size; i++) {
a[i] = s.readObject();
}
}
}
/**
* 返回一個從index開始迭代的實例（建立在AbstractList的遍歷基礎上）
*/
public ListIterator<E> listIterator(int index) {
if (index < 0 || index > size)
throw new IndexOutOfBoundsException("Index: " + index);
return new ListItr(index);
}
/**
* 返回一個從從0開始的迭代實例
*/
public ListIterator<E> listIterator() {
return new ListItr(0);
}
/**
* 返回一個迭代器實例（包含hasNext,next,remove等操作）
*/
public Iterator<E> iterator() {
return new Itr();
}
/**
* 迭代類，對集合進行迭代處理(內部私有類)
*/
private class Itr implements Iterator<E> {
int cursor; // index of next element to return
int lastRet = -1; // index of last element returned; -1 if no such
int expectedModCount = modCount;
/**
* 根據下一個索引值與空間大小判斷是否有下一個元素
*/
public boolean hasNext() {
return cursor != size;
}
/**
* 返回下一個索引對應的元素
*/
@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
}
/**
*
*/
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
ArrayList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
/**
* 判斷是否改變數組結構
*/
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
/**
* 內部迭代類，繼承Itr，實現ListIterator
*/
private class ListItr extends Itr implements ListIterator<E> {
ListItr(int index) {
super();
cursor = index;
}
public boolean hasPrevious() {
return cursor != 0;
}
public int nextIndex() {
return cursor;
}
public int previousIndex() {
return cursor - 1;
}
@SuppressWarnings("unchecked")
public E previous() {
checkForComodification();
int i = cursor - 1;
if (i < 0)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i;
return (E) elementData[lastRet = i];
}
public void set(E e) {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
ArrayList.this.set(lastRet, e);
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
public void add(E e) {
checkForComodification();
try {
int i = cursor;
ArrayList.this.add(i, e);
cursor = i + 1;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
}
/**
* 返回集合中從fromIndex到toIndex的元素構成一個新的集合
*/
public List<E> subList(int fromIndex, int toIndex) {
//判斷參數是否越界
subListRangeCheck(fromIndex, toIndex, size);
return new SubList(this, 0, fromIndex, toIndex);
}
static void subListRangeCheck(int fromIndex, int toIndex, int size) {
if (fromIndex < 0)
throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
if (toIndex > size)
throw new IndexOutOfBoundsException("toIndex = " + toIndex);
if (fromIndex > toIndex)
throw new IllegalArgumentException("fromIndex(" + fromIndex + ") > toIndex(" + toIndex + ")");
}
private class SubList extends AbstractList<E> implements RandomAccess {
private final AbstractList<E> parent;
private final int parentOffset;
private final int offset;
int size;
SubList(AbstractList<E> parent, int offset, int fromIndex, int toIndex) {
this.parent = parent;
this.parentOffset = fromIndex;
this.offset = offset + fromIndex;
this.size = toIndex - fromIndex;
this.modCount = ArrayList.this.modCount;
}
public E set(int index, E e) {
rangeCheck(index);
checkForComodification();
E oldValue = ArrayList.this.elementData(offset + index);
ArrayList.this.elementData[offset + index] = e;
return oldValue;
}
public E get(int index) {
rangeCheck(index);
checkForComodification();
return ArrayList.this.elementData(offset + index);
}
public int size() {
checkForComodification();
return this.size;
}
public void add(int index, E e) {
rangeCheckForAdd(index);
checkForComodification();
parent.add(parentOffset + index, e);
this.modCount = parent.modCount;
this.size++;
}
public E remove(int index) {
rangeCheck(index);
checkForComodification();
E result = parent.remove(parentOffset + index);
this.modCount = parent.modCount;
this.size--;
return result;
}
protected void removeRange(int fromIndex, int toIndex) {
checkForComodification();
parent.removeRange(parentOffset + fromIndex, parentOffset + toIndex);
this.modCount = parent.modCount;
this.size -= toIndex - fromIndex;
}
public boolean addAll(Collection<? extends E> c) {
return addAll(this.size, c);
}
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
int cSize = c.size();
if (cSize == 0)
return false;
checkForComodification();
parent.addAll(parentOffset + index, c);
this.modCount = parent.modCount;
this.size += cSize;
return true;
}
public Iterator<E> iterator() {
return listIterator();
}
public ListIterator<E> listIterator(final int index) {
checkForComodification();
rangeCheckForAdd(index);
final int offset = this.offset;
return new ListIterator<E>() {
int cursor = index;
int lastRet = -1;
int expectedModCount = ArrayList.this.modCount;
public boolean hasNext() {
return cursor != SubList.this.size;
}
@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
if (i >= SubList.this.size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (offset + i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[offset + (lastRet = i)];
}
public boolean hasPrevious() {
return cursor != 0;
}
@SuppressWarnings("unchecked")
public E previous() {
checkForComodification();
int i = cursor - 1;
if (i < 0)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (offset + i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i;
return (E) elementData[offset + (lastRet = i)];
}
public int nextIndex() {
return cursor;
}
public int previousIndex() {
return cursor - 1;
}
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
SubList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = ArrayList.this.modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
public void set(E e) {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
ArrayList.this.set(offset + lastRet, e);
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
public void add(E e) {
checkForComodification();
try {
int i = cursor;
SubList.this.add(i, e);
cursor = i + 1;
lastRet = -1;
expectedModCount = ArrayList.this.modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
final void checkForComodification() {
if (expectedModCount != ArrayList.this.modCount)
throw new ConcurrentModificationException();
}
};
}
public List<E> subList(int fromIndex, int toIndex) {
subListRangeCheck(fromIndex, toIndex, size);
return new SubList(this, offset, fromIndex, toIndex);
}
private void rangeCheck(int index) {
if (index < 0 || index >= this.size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private void rangeCheckForAdd(int index) {
if (index < 0 || index > this.size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private String outOfBoundsMsg(int index) {
return "Index: " + index + ", Size: " + this.size;
}
private void checkForComodification() {
if (ArrayList.this.modCount != this.modCount)
throw new ConcurrentModificationException();
}
}
}總結：
1.ArrayList 是基於動態數組的 可以自動增長空間容量的集合.
2.對於隨機訪問集合中的元素，使用arrayList 效率比較高.
3.arrayList 插入或者刪除元素都將對後面的剩餘元素進行移動，操作量比較大（末尾插入不需要）.
4.通過索引進行遍歷速度最快（for循環list.size()），通過迭代器遍歷速度最慢（Itreator）.
5.非線性安全，無法自動同步.如需線性安全，可使用vector.
6.直接刪除元素（remove(Object o)）需要遍歷數組，如果刪除索引（remove(int index)），不需要遍歷數組.