Java集合类库 ArrayList 源码解析
集合类库是Java的一个重大突破，方便了我们对大数据的操作。其中 Arrays 和 Collections 工具类可以帮助我们快速操作集合类库。下面对Java集合类库的源码分析是基于jdk1.7的。今天我们来看看ArrayList的底层实现原理。
ArrayList的继承结构图
继承自 AbstractList 抽象类，在上层是 AbstractCollection 抽象类，直接去 AbstractCollection 类去看看。
AbstractCollection 类主要实现了 Collection 接口 和 Iterable 接口。Java 中的集合框架都是由 Collection 接口 和 Map 接口的实现类所构成的。
先看看 Collection 接口。
public interface Collection<E> extends Iterable<E>继承了 Iterable 接口，跟进去看看 Iterable 接口。
package java.lang;
import java.util.Iterator;
/** * Implementing this interface allows an object to be the target * of the "foreach" statement. * * @param <T> the type of elements returned by the iterator * * @since 1.5 */
public interface Iterable<T> {
/** * Returns an iterator over a set of elements of type T. * * @return an Iterator. */
Iterator<T> iterator();
}
代码很少，只有一个方法 iterator()，返回一个迭代器对象 Iterator。至于迭代器等会再看，我们先回到 Collection 接口，看看其中的方法。主要有以下方法：
1.int size() -- 集合的大小
2.boolean isEmpty() -- 集合是否为空
3.boolean contains(Object o) -- 集合是否包含指定的元素
4.boolean add(E e) -- 添加一个元素到集合
5.boolean remove(Object o) -- 把某个元素从集合中移除
6.boolean addAll(Collection<? extends E> c) -- 将一个集合中的所有元素加到此集合中
7.boolean removeAll(Collection<?> c) -- 从此集合中删除另一个集合中的元素
8.boolean retainAll(Collection<?> c) -- 保留集合中共有的元素（并集）
9.void clear() -- 清空集合
10.Object[] toArray() -- 将集合转化为数组
11.<T> T[] toArray(T[] a) -- 将集合转化为指定类型的数组，更进一步说，此方法允许对输出数组的运行时类型进行精确控制，并且在某些情况下，可以用来节省分配开销。
12.boolean containsAll(Collection<?> c) -- 判断此集合是否包含指定集合中的元素
13.Iterator<E> iterator() -- 返回一个迭代器对象，从Iterable接口继承过来的。
14.boolean equals(Object o)
15.int hashCode()这就是 Collection 接口中的方法，ArrayList 属于集合类库，肯定会实现 Collection 接口中的方法，只是有的方法可能有它的抽象父类 AbstractList 类或者 AbstractCollection 类实现，现在我们来看看 ArrayList 的具体实现。
首先，我们来看下ArrayList的构造方法：
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: " + initialCapacity);
this.elementData = new Object[initialCapacity];
}
public ArrayList() {
this(10);
}
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);
}有三个构造方法，我们通常用的是无参的构造方法来生成一个ArrayList对象。无参的构造器其实生成了一个长度为10的空的List列表，this(10)其实调用了 ArrayList(int initialCapacity) 这个构造方法，如果初始化的列表容量大小小于0，则抛出非法的参数异常。默认的初始化大小为10，不满足判断条件，往下走：
this.elementData = new Object[initialCapacity];
表明生成一个长度为initialCapacity大小的elementData数组，所以ArrayList的底层是以数组形式来实现的。其中elementData对象是Object类型的数组，所以ArrayList能存放任何类型的对象。那我们就可以得出平时我们通过 ArrayList list = new ArrayList() 代码，其实是生成了一个长度为10的Object类型的空数组。
在看下第三个构造方法 ArrayList(Collection < ? extends E > c)，传入一个 Collection 接口对象。用toArray()方法把集合转化为数组赋值给elementData数组。然后更改现在集合的大小，其中的size就是数组中元素的个数，也是集合的大小。再判断elementData的类型是不是Object[]类型，如果不是，则通过Arrays.copyOf()方法来复制数组，重新赋值给elementData对象。
private transient Object[] elementData;
private int size;这是ArrayList2个主要的全局变量，一个是维护的数组，一个是数组中元素的个数。
添加
数组的长度是有限制的，那ArrayList是怎么实现自动扩容的呢？对于集合，最重要的莫过如增删改查操作。Collection接口提供了一个add（E e）方法，ArrayList除了现在了这个方法，还重载了一个add（int index, E element）方法。
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}add(E e)的实现很简单，调用了ensureCapacityInternal（）方法。然后elementData[size++]中的元素指向e，size是一个全局变量，初始化为0，所以就是elementData数组的第一个元素为e，添加完之后，size大小变为1，就是数组中的元素个数为1了。添加成功返回true。
add（）中的ensureCapacityInternal（）方法是干嘛的呢？看名字是确保内部容量，可能跟扩大数组容量有关，来看看。
private void ensureCapacityInternal(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}modCount++ 是修改集合的次数，主要是给iterator迭代器哎用的，这个后面再分析。
if (minCapacity - elementData.length > 0)对于这个判断条件，我们刚进来传入的参数是size+1，初始size为0，那么minCapacity值就是1，elementData.length的值也就是数组的长度，初始化为10，判断条件不成立，所以不会走grow(minCapacity)这个方法。
当我们要添加第11个元素是，这时size是10，则minCapacity为11，判断条件成立，进入grow(minCapacity)方法。跟进去看看grow（）方法。
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);
}这个方法就是ArrayList的扩容方法，终于找到了，不容易~分析下这个方法
oldCapacity为现在的数组长度
int newCapacity = oldCapacity + (oldCapacity >> 1)。oldCapacity >> 1是一个移位操作，也就是把oldCapacity除以2的意思，然后再加上oldCapacity值赋值给newCapacity变量，这段代码的意思就是新的数组长度为原来的1.5倍。
if (newCapacity - minCapacity < 0)对于这个判断条件就很奇怪了，新的数组大小肯定会大于size+1的值？为什么会有这个判断呢？难道你忘了，我们可以指定生成的ArrayList的大小，如果我们初始化一个长度为0的数组，这时newCapacity 值为0，minCapacity值为1，判断条件就成立了，所以会把minCapacity赋值为新的数组大小，好精巧的代码~
if (newCapacity - MAX_ARRAY_SIZE > 0) 这个判断条件是针对大容量的集合了，只需记住ArrayList的最大的容量为2的31次方就行了。
最后通过Arrays.copyOf（）方法把老的数组复制到长度为newCapacity的新数组中，赋值给elementData对象。这就是ArrayList的扩容过程。顺便说一句，以前的扩容机制是int newCapacity = (oldCapacity * 3)/2 + 1，也就是1.5被+1。现在直接是1.5倍了。
再去看看重载的add(int index, E element)方法。
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,size - index);
elementData[index] = element;
size++;
}首先调用rangeCheckForAdd（）方法，范围检查，来跟进一下这个方法：
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}如果我们插入的位置大于存在的元素个数或者小于0，抛出一个数组越界的异常。
剩下的步骤就和add（E e）方法差不多了，主要有一个System.arraycopy（）方法，从指定数组中复制一个数组，复制从指定的位置开始，到目标数组的指定位置结束。
至于addAll()也是一样的，通过System.arraycopy（）来复制数组，就不细说了。
删除
ArrayList也重写了remove(Object o)方法，还重载了一个remove(int index)方法。用于我们来删除元素。
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;
}如果要删除的是一个空对象，先循环遍历这个数组，找到数组中为空的元素，调用fastRemove（）方法删除。
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,numMoved);
elementData[--size] = null; // Let gc do its work
}这是一个私有的方法，numMoved 变量表示数组要开始变化的位置，还是通过System.arraycopy（）来赋值数组，最后将数组中最后一个元素变为null，等待gc回收。
如果删除一个非空对象，循环遍历数组，通过对象的equal（）方法，找到相等的元素，调用fastRemove（）方法删除。
删除成功返回true，删除失败返回false。
所以最终删除元素的还是fastRemove（）方法，我们也知道了如果通过对象来删除元素的话，就必须要重写对象的equal（）方法，因为ArrarList的删除方法是通过equal（）方法来比较对象相等的。
再来看看重载的remove(int index)方法。
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; // Let gc do its work
return oldValue;
}首先，是一个范围检查rangeCheck（）方法。
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}如果删除的索引超出列表大小，则抛出数组越界异常。
通过索引，直接找到elementData数组中该位置的元素，然后跟fastRemove（）方法一样的操作来删除元素，最后返回被删除的元素。
修改
ArrayList中修改元素通过 set(int index, E element)方法，这是List特有的，不是从Collection接口得到的。
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}首先有个范围检查，然后就是elementData（）方法，来看看这个方法。
E elementData(int index) {
return (E) elementData[index];
}代码很少，就是返回指定位置的元素。set（）方法就是先得到指定位置的元素，在把该元素替换成药更改的元素，这是这样简单~
查询
要得到某个指定位置元素，可以通过get(int index) 方法。至于怎样实现的？肯定是调用上面的elementData（）方法，得到指定位置的元素，不过别忘了要注意范围检查的。
public E get(int index) {
rangeCheck(index);
return elementData(index);
}对于查询，还提供了2个赋值的方法indexOf(Object o)和lastIndexOf(Object o)，得到指定元素的位置。具体分析看源码很快就能明白，肯定是遍历数组，找到相等的元素（通过equal（）），返回下标，区别是一个正序，一个倒序。
常用方法的实现
size（）方法：返回数组中元素的个数，就是集合的大小
public int size() {
return size;
}clear（）方法：遍历数组，每个元素赋值为空，size大小更改为0。
public void clear() {
modCount++;
// Let gc do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}isEmpty（）方法：直接判断size是不是等于0。
public boolean isEmpty() {
return size == 0;
}toArray（）方法：直接通过Arrays.copyOf（）来实现数组的赋值。
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}至于 iterator（）方法和listIterator（）方法，将在分析迭代器的时候，对各种集合的迭代器一起分析。
当然，还有一些方法没有分析，大家如果去看下源码，都是通过差不多的原理来实现的，这里就不再追叙了。
为什么我们常说 ArrayList 的查询很快，而插入操作效率很低？LinedList的查询效率低，而插入操作很方便？为什么Set中没有重复的元素？他们是通过怎样的方法来迭代遍历元素的？底层到底是怎样实现的？下期继续分析。
下面是ArrayList的源码，不算太长，贴出来看看。
* Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
package java.util;
/** * Resizable-array implementation of the <tt>List</tt> interface. Implements * all optional list operations, and permits all elements, including * <tt>null</tt>. In addition to implementing the <tt>List</tt> interface, * this class provides methods to manipulate the size of the array that is * used internally to store the list. (This class is roughly equivalent to * <tt>Vector</tt>, except that it is unsynchronized.) * * <p>The <tt>size</tt>, <tt>isEmpty</tt>, <tt>get</tt>, <tt>set</tt>, * <tt>iterator</tt>, and <tt>listIterator</tt> operations run in constant * time. The <tt>add</tt> operation runs in <i>amortized constant time</i>, * that is, adding n elements requires O(n) time. All of the other operations * run in linear time (roughly speaking). The constant factor is low compared * to that for the <tt>LinkedList</tt> implementation. * * <p>Each <tt>ArrayList</tt> instance has a <i>capacity</i>. The capacity is * the size of the array used to store the elements in the list. It is always * at least as large as the list size. As elements are added to an ArrayList, * its capacity grows automatically. The details of the growth policy are not * specified beyond the fact that adding an element has constant amortized * time cost. * * <p>An application can increase the capacity of an <tt>ArrayList</tt> instance * before adding a large number of elements using the <tt>ensureCapacity</tt> * operation. This may reduce the amount of incremental reallocation. * * <p><strong>Note that this implementation is not synchronized.</strong> * If multiple threads access an <tt>ArrayList</tt> instance concurrently, * and at least one of the threads modifies the list structurally, it * <i>must</i> be synchronized externally. (A structural modification is * any operation that adds or deletes one or more elements, or explicitly * resizes the backing array; merely setting the value of an element is not * a structural modification.) This is typically accomplished by * synchronizing on some object that naturally encapsulates the list. * * If no such object exists, the list should be "wrapped" using the * {@link Collections#synchronizedList Collections.synchronizedList} * method. This is best done at creation time, to prevent accidental * unsynchronized access to the list:<pre> * List list = Collections.synchronizedList(new ArrayList(...));</pre> * * <p><a name="fail-fast"/> * The iterators returned by this class's {@link #iterator() iterator} and * {@link #listIterator(int) listIterator} methods are <em>fail-fast</em>: * if the list is structurally modified at any time after the iterator is * created, in any way except through the iterator's own * {@link ListIterator#remove() remove} or * {@link ListIterator#add(Object) add} methods, the iterator will throw a * {@link ConcurrentModificationException}. Thus, in the face of * concurrent modification, the iterator fails quickly and cleanly, rather * than risking arbitrary, non-deterministic behavior at an undetermined * time in the future. * * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed * as it is, generally speaking, impossible to make any hard guarantees in the * presence of unsynchronized concurrent modification. Fail-fast iterators * throw {@code ConcurrentModificationException} on a best-effort basis. * Therefore, it would be wrong to write a program that depended on this * exception for its correctness: <i>the fail-fast behavior of iterators * should be used only to detect bugs.</i> * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @author Josh Bloch * @author Neal Gafter * @see Collection * @see List * @see LinkedList * @see Vector * @since 1.2 */
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
private static final long serialVersionUID = 8683452581122892189L;
/** * The array buffer into which the elements of the ArrayList are stored. * The capacity of the ArrayList is the length of this array buffer. */
private transient Object[] elementData;
/** * The size of the ArrayList (the number of elements it contains). * * @serial */
private int size;
/** * 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 */
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
}
/** * Constructs an empty list with an initial capacity of ten. */
public ArrayList() {
this(10);
}
/** * 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) {
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);
}
/** * 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() {
modCount++;
int oldCapacity = elementData.length;
if (size < oldCapacity) {
elementData = Arrays.copyOf(elementData, size);
}
}
/** * 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) {
if (minCapacity > 0)
ensureCapacityInternal(minCapacity);
}
private void ensureCapacityInternal(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;
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);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
/** * Returns the number of elements in this list. * * @return the number of elements in this list */
public int size() {
return size;
}
/** * Returns <tt>true</tt> if this list contains no elements. * * @return <tt>true</tt> if this list contains no elements */
public boolean isEmpty() {
return size == 0;
}
/** * Returns <tt>true</tt> if this list contains the specified element. * More formally, returns <tt>true</tt> if and only if this list contains * at least one element <tt>e</tt> such that * <tt>(o==null ? e==null : o.equals(e))</tt>. * * @param o element whose presence in this list is to be tested * @return <tt>true</tt> if this list contains the specified element */
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
/** * 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) {
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;
}
/** * Returns the index of the last occurrence of the specified element * in this list, or -1 if this list does not contain the element. * More formally, returns the highest 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 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;
}
/** * 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 {
@SuppressWarnings("unchecked")
ArrayList<E> v = (ArrayList<E>) super.clone();
v.elementData = Arrays.copyOf(elementData, size);
v.modCount = 0;
return v;
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError();
}
}
/** * 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 */
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 */
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
// Make a new array of a's runtime type, but my contents:
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
// Positional Access Operations
@SuppressWarnings("unchecked")
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) {
rangeCheck(index);
return elementData(index);
}
/** * 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) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
/** * 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;
}
/** * 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) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
/** * 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; // Let gc do its work
return oldValue;
}
/** * Removes the first occurrence of the specified element from this list, * if it is present. If the list does not contain the element, it is * unchanged. More formally, removes the element with the lowest index * <tt>i</tt> such that * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt> * (if such an element exists). Returns <tt>true</tt> if this list * contained the specified element (or equivalently, if this list * changed as a result of the call). * * @param o element to be removed from this list, if present * @return <tt>true</tt> if this list contained the specified element */
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;
}
/* * Private remove method that skips bounds checking and does not * return the value removed. */
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // Let gc do its work
}
/** * Removes all of the elements from this list. The list will * be empty after this call returns. */
public void clear() {
modCount++;
// Let gc do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
/** * Appends all of the elements in the specified collection to the end of * this list, in the order that they are returned by the * specified collection's Iterator. The behavior of this operation is * undefined if the specified collection is modified while the operation * is in progress. (This implies that the behavior of this call is * undefined if the specified collection is this list, and this * list is nonempty.) * * @param c collection containing elements to be added to this list * @return <tt>true</tt> if this list changed as a result of the call * @throws NullPointerException if the specified collection is null */
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;
}
/** * Inserts all of the elements in the specified collection into this * list, starting at the specified position. Shifts the element * currently at that position (if any) and any subsequent elements to * the right (increases their indices). The new elements will appear * in the list in the order that they are returned by the * specified collection's iterator. * * @param index index at which to insert the first element from the * specified collection * @param c collection containing elements to be added to this list * @return <tt>true</tt> if this list changed as a result of the call * @throws IndexOutOfBoundsException {@inheritDoc} * @throws NullPointerException if the specified collection is null */
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 > 0)
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}
/** * Removes from this list all of the elements whose index is between * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive. * Shifts any succeeding elements to the left (reduces their index). * This call shortens the list by {@code (toIndex - fromIndex)} elements. * (If {@code toIndex==fromIndex}, this operation has no effect.) * * @throws IndexOutOfBoundsException if {@code fromIndex} or * {@code toIndex} is out of range * ({@code fromIndex < 0 || * fromIndex >= size() || * toIndex > size() || * toIndex < fromIndex}) */
protected void removeRange(int fromIndex, int toIndex) {
modCount++;
int numMoved = size - toIndex;
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);
// Let gc do its work
int newSize = size - (toIndex-fromIndex);
while (size != newSize)
elementData[--size] = null;
}
/** * 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));
}
/** * A version of rangeCheck used by add and addAll. */
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
/** * Constructs an IndexOutOfBoundsException detail message. * Of the many possible refactorings of the error handling code, * this "outlining" performs best with both server and client VMs. */
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}
/** * Removes from this list all of its elements that are contained in the * specified collection. * * @param c collection containing elements to be removed from this list * @return {@code true} if this list changed as a result of the call * @throws ClassCastException if the class of an element of this list * is incompatible with the specified collection * (<a href="Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if this list contains a null element and the * specified collection does not permit null elements * (<a href="Collection.html#optional-restrictions">optional</a>), * or if the specified collection is null * @see Collection#contains(Object) */
public boolean removeAll(Collection<?> c) {
return batchRemove(c, false);
}
/** * Retains only the elements in this list that are contained in the * specified collection. In other words, removes from this list all * of its elements that are not contained in the specified collection. * * @param c collection containing elements to be retained in this list * @return {@code true} if this list changed as a result of the call * @throws ClassCastException if the class of an element of this list * is incompatible with the specified collection * (<a href="Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if this list contains a null element and the * specified collection does not permit null elements * (<a href="Collection.html#optional-restrictions">optional</a>), * or if the specified collection is null * @see Collection#contains(Object) */
public boolean retainAll(Collection<?> c) {
return batchRemove(c, true);
}
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) {
for (int i = w; i < size; i++)
elementData[i] = null;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}
/** * 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 array length
s.writeInt(elementData.length);
// 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();
}
}
/** * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is, * deserialize it). */
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
// Read in size, and any hidden stuff
s.defaultReadObject();
// Read in array length and allocate array
int arrayLength = s.readInt();
Object[] a = elementData = new Object[arrayLength];
// Read in all elements in the proper order.
for (int i=0; i<size; i++)
a[i] = s.readObject();
}
/** * Returns a list iterator over the elements in this list (in proper * sequence), starting at the specified position in the list. * The specified index indicates the first element that would be * returned by an initial call to {@link ListIterator#next next}. * An initial call to {@link ListIterator#previous previous} would * return the element with the specified index minus one. * * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>. * * @throws IndexOutOfBoundsException {@inheritDoc} */
public ListIterator<E> listIterator(int index) {
if (index < 0 || index > size)
throw new IndexOutOfBoundsException("Index: "+index);
return new ListItr(index);
}
/** * Returns a list iterator over the elements in this list (in proper * sequence). * * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>. * * @see #listIterator(int) */
public ListIterator<E> listIterator() {
return new ListItr(0);
}
/** * Returns an iterator over the elements in this list in proper sequence. * * <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>. * * @return an iterator over the elements in this list in proper sequence */
public Iterator<E> iterator() {
return new Itr();
}
/** * An optimized version of AbstractList.Itr */
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;
public boolean hasNext() {
return cursor != size;
}
@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();
}
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
/** * An optimized version of AbstractList.ListItr */
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();
}
}
}
/** * Returns a view of the portion of this list between the specified * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive. (If * {@code fromIndex} and {@code toIndex} are equal, the returned list is * empty.) The returned list is backed by this list, so non-structural * changes in the returned list are reflected in this list, and vice-versa. * The returned list supports all of the optional list operations. * * <p>This method eliminates the need for explicit range operations (of * the sort that commonly exist for arrays). Any operation that expects * a list can be used as a range operation by passing a subList view * instead of a whole list. For example, the following idiom * removes a range of elements from a list: * <pre> * list.subList(from, to).clear(); * </pre> * Similar idioms may be constructed for {@link #indexOf(Object)} and * {@link #lastIndexOf(Object)}, and all of the algorithms in the * {@link Collections} class can be applied to a subList. * * <p>The semantics of the list returned by this method become undefined if * the backing list (i.e., this list) is <i>structurally modified</i> in * any way other than via the returned list. (Structural modifications are * those that change the size of this list, or otherwise perturb it in such * a fashion that iterations in progress may yield incorrect results.) * * @throws IndexOutOfBoundsException {@inheritDoc} * @throws IllegalArgumentException {@inheritDoc} */
public List<E> subList(int fromIndex, int toIndex) {
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();
}
}
}