ArrayList源码分析
源码版本为JDK1.7.0_79
ArrayList不是线程安全的，只能应用在单线程环境下。
ArrayList类定义
从ArrayList的类定义可以看出它是支持泛型的，继承自AbstractList，AbstractList 实现了List接口，提供了List接口的默认实现。
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E>
ArrayList自身也实现了List接口。同时，ArrayList实现了Serializable接口，因此它支持序列化，能够通过序列化传输。实现了RandomAccess接口，支持快速随机访问，实际上就是通过下标进行快速访问，RandomAccess是一个标记接口，接口内没有定义任何内容。实现了Cloneable接口，能被克隆。
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
ArrayList类属性
ArrayList是基于数组实现的，是一个动态数组，其容量能够自动增长。
/**
* 序列版本号
*/
private static final long serialVersionUID = 8683452581122892189L;
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* 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 == EMPTY_ELEMENTDATA will be expanded to
* DEFAULT_CAPACITY when the first element is added.
*/
private transient Object[] elementData;
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
private int size;
数组elementData为ArrayList存储元素的Buffer，ArrayList的容量即为该数组的长度。当ArrayList为空时，此时elementData==EMPTY_ELEMENTDATA，当往ArrayList添加一个元素时，elementData的长度就被初始化为10,也就是DEFAULT_CAPACITY的值。
size为elementData含有的元素的个数。
注意到elementData数组被transient修饰，因为ArrayList实现了Serializable接口，这意味着ArrayList可以被序列化，也就是说，ArrayList的属性可以通过网络传输，或者被存储到磁盘持久化。但是在很多情况下，我们并不希望有些属性被传输或者存储，比如一些敏感信息。这时使用trainsent标记属性，就可以使得ArrayList在序列化的过程中elementData不参与序列化。因此，elementData中的数据仅存在于调用者的内存中。
ArrayList类构造函数
ArrayList一共定义了三个构造函数。
/**
* Constructs an empty list with the specified initial capacity.
*/
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() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}
/**
* Constructs a list containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
*/
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);
}
构造函数public ArrayList(int initialCapacity){...}为构造给定大小的elementData数组。
构造函数public ArrayList() {...}为ArrayList的默认构造函数，此时elementData==EMPTY_ELEMENTDATA，数组长度为0。
构造函数 public ArrayList(Collection<? extends E> c) {...}使用一个集合来初始化ArrayList。
ArrayList类核心方法
/**
* 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++;
if (size < elementData.length) {
elementData = Arrays.copyOf(elementData, size);
}
}
modCount为ArrayList的父类AbstractList中定义的属性，其作用是 the number of times this list has been structurally modified。
trimToSize的作用是将ArrayList对象的capacity减小到size长度，这样可以减少ArrayList对象占用的内存。
/**
* 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.
*/
public void ensureCapacity(int minCapacity) {
int minExpand = (elementData != EMPTY_ELEMENTDATA)
// any size if real element table
? 0
// larger than default for empty table. It's already supposed to be
// at default size.
: DEFAULT_CAPACITY;
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
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.
*/
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;
}
oldCapacity + (oldCapacity >> 1)确保了一旦扩充容量，那么至少扩充到原来的1.5倍。
/**
* Returns the number of elements in this list.
*/
public int size() {
return size;
}
/**
* Returns <tt>true</tt> if this list contains no elements.
*/
public boolean isEmpty() {
return size == 0;
}
size方法和isEmpty方法都是根据属性size判断。
/**
* 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>.
*/
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;
}
contains方法中，通过indexOf方法遍历elementData数组。若待搜索的元素为null，那么就返回elementData中第一个为null的元素的下标。当然如果elementData的size==0，那么就返回-1。若待搜索的元素不为空，那么就返回对应的元素的下标，否则返回-1。注意当带搜索的元素不为null时，是使用equals方法来判断是否相等。
/**
* 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;
}
lastIndexOf方法与indexOf方法十分类似，只是它是从最后一个元素开始向前遍历。
/**
* Returns a shallow copy of this <tt>ArrayList</tt> instance. (The
* elements themselves are not copied.)
*/
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();
}
}
clone方法首先调用父类的clone方法返回一个对象的副本，将这个副本对象的elementData数组地址赋值为原来对象的elementData数组的地址，并将该副本对象的modCount重置为0。该方法返回的是ArrayList对象的浅副本，即不复制这些元素本身。
/**
* 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.
*
*/
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.)
*/
@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;
}
toArray方法返回的是elementData数组的副本，而不是ArrayList对象内的elementData数组本身。
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
elementData方法返回指定下标的元素，ArrayList的get方法，set方法，add方法，remove方法和clear方法都需要调用它。
/**
* Returns the element at the specified position in this list.
*/
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
get方法首先需要校验下标是否越界，若没有，则返回对应下标的元素。
/**
* Replaces the element at the specified position in this list with
* the specified element.
*/
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
set方法将ArrayList指定下标的元素设置为指定的值，并将原值作为返回值返回。
/**
* Appends the specified element to the end of this list.
*/
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).
*/
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++;
}
add(int index, E element)方法在指定的位置插入一个元素，首先将elementData中从index开始的 size - index 个元素整体向后移动一位，然后将element元素插入到index处。
/**
* Removes the element at the specified position in this list.
* Shifts any subsequent elements to the left (subtracts one from their
* indices).
*/
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;
}
如果要remove的元素为elementData的最后一个元素，那么numMoved等于0，因此不需要做数组拷贝的操作。若要删除的元素位于elementData之间，那么需要将index后的元素整体向前移动一位，并将elementData最后的元素置为null值让GC回收。
/**
* 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).
*/
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; // clear to let GC do its work
}
fastRemove和remove非常类似，仅仅是移除某个元素。remove(Object o)是将elementData中的某个对象移除(假设存在)，若传入的要移除的元素为null，那么移除elementData中首个为null的元素，返回true。若传入的要移除的元素不为null，遍历elementData，找到要移除的元素并移除。若要移除的元素并不存在于elementData，那么返回false。
/**
* 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.)
*
*/
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.
*
*/
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;
}
addAll(Collection<? extends E> c)是将集合c追加到elementData尾部，addAll(int index, Collection<? extends E> c)是将集合c插入到指定位置。若指定的index为elementData的size处，那么直接将c追加到elementData尾部。否则，需要先将elementData中以index下标开始的numMoved个元素后移一位，然后把c插入index处。
/**
* 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.)
*/
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;
}
removeRange将指定范围内的元素移除，首先将toIndex之后的元素拷贝到fromIndex之后，调整elementData的size为newSize，然后将newSize之后的元素全部置空供GC回收。
/**
* Removes from this list all of its elements that are contained in the
* specified collection.
*/
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.
*/
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) {
// 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;
}
removeAll的作用是删除elemetData中与指定集合c相同的元素。retainAll的作用是保留elementData中与指定集合c相同的元素，删除其他的元素。removeAll和retainAll传入一个boolean类型的complement，在batchRemove中通过if (c.contains(elementData[r]) == complement) 来决定是否保留元素。removeAll传递过来的complement为false，因此当elementData中的元素不包含在c中时，保留这个元素，达到了remove的效果。retainAll传递过来的complement为true，因此当elementData中的元素包含在c中，保留这个元素，达到了retain的效果。
总结
注意ArrayList每次在增加元素前都需要调用ensureCapacityInternal来确保elementData的容量足够，当容量不够时，调用ensureExplicitCapacity，ensureExplicitCapacity调用grow方法线将容量增加到原来容量的1.5倍，若果增加为原来的1.5倍之后容量还不够，那么就把新容量直接设置为传递过来的容量(minCapacity)。然后调用Arrays.copyOf将原来的元素拷贝到新数组中。可以看出，当容量不够时，增加元素变成了一个非常耗时的过程。因此建议在实现能够确定元素数量的情况下才使用ArrayList，否则更加建议使用LinkedList。
ArrayList的源码中多次调用了两个数组拷贝的方法，分别是Arrays.copyOf和System.arraycopy，查看这两个方法的源码。
public static <T> T[] copyOf(T[] original, int newLength) {
return (T[]) copyOf(original, newLength, original.getClass());
}
该copyOf方法调用了另一个copyOf方法：
public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength]
: (T[]) Array.newInstance(newType.getComponentType(), newLength);
System.arraycopy(original, 0, copy, 0,
Math.min(original.length, newLength));
return copy;
}
该方法创建了一个新的数组，然后调用System.arraycopy将元素拷贝到新数组中。
public static native void arraycopy(Object src, int srcPos,
Object dest, int destPos,
int length);
arraycopy是一个本地方法，通过调用系统的C/C++方法实现。该方法可以保证在同一个数组内的元素的复制和移动。
通过源码的分析，可以更加深刻的理解为什么说ArrayList的查找效率高而增加、删除元素的效率低。