java数据结构之ArrayList源码分析
ArrayList的源码对它的定义为Resizable-array implementation of the List interface，ArrayList是一种动态数组，它可以动态的增加和减少元素，灵活的设置数组的大小。我们知道，数组定义后其大小就无法变化了，那么ArrayList如是如何实现动态数组这个功能的呢？
下面我们就从ArrayList的源码来分析：
## １、ArrayList的初始化
public class ArrayList extends AbstractList
implements List, RandomAccess, Cloneable, java.io.Serializable
{
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* Shared empty array instance used for default sized empty instances. We
* distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
* first element is added.
*/
private static final Object[] DEFAULTCAPACITY_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 == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
* will be expanded to DEFAULT_CAPACITY when the first element is added.
*/
// Android-note: Also accessed from java.util.Collections
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
private int size;
｝
我们再来看下ArrayList的构造函数：
/**
* 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) {
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* 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 c) {
elementData = c.toArray();
if ((size = elementData.length) != 0) {
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// replace with empty array.
this.elementData = EMPTY_ELEMENTDATA;
}
}
可以看到，当我们调用ArrayList的无参构造函数时，elementData等于默认容量下的空数组DEFAULTCAPACITY_EMPTY_ELEMENTDATA;而当我们调用ArrayList(int capacity)的构造函数时，capacity如果合法的话将会覆盖默认容量并给elementData赋予对应的初始值;如果是调用ArrayList(Collection c)的构造方法的话，ArrayList直接取出Collection中的数组并拷贝给elementData。
到这里ArrayList的初始化过程我们已经知道了，其初始化主要就是数组elementData的初始化。
## ２、ArrayList的数据添加
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return true (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) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
ensureCapacityInternal(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
ArrayList的数据添加有两个重载的方法，add(E e)是通过 elementData[size++] = e将数据添加到数组末端，add(int index, E element)是将通过System.arraycopy(elementData, index, elementData, index + 1,size - index)先将elementData[index]及后面的所有数据整体向后移一位，然后通过elementData[index] = element;实现将数据添加到指定位置。而在添加数据之前都会调用ensureCapacityInternal来确保ArrayList的容量能够容得下即将添加的数据。
下面我们来看下ensureCapacityInternal是如何做到这点的。
/**
* Increases the capacity of this ArrayList 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 ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_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.
*
* @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;
}
可以看到，ensureCapacityInternal首先判断ArrayList是否时走的无参构造函数－若是的话则此时对应的默认容量是10，而elementData应该等于DEFAULTCAPACITY_EMPTY_ELEMENTDATA，因此minCapacity就等于10和minCapacity中的最大值了，然后调用ensureExplicitCapacity来判断ArrayList是否需要拓容，若要求的容量minCapacity大于当前容量则调用grow来进行拓容。
grow是这样进行拓容的：先将新容量newCapacity设定为当前容量的1.5倍，然后再看newCapacity是否能满足需求容量minCapacity，如果不满足则直接将newCapacity设为minCapacity，最后还要判断当前要求的容量是否大于MAX_ARRAY_SIZE，若大于的话执行newCapacity = hugeCapacity(minCapacity);在hugeCapacity中首先判断minCapacity是否过大——即是否大于Integer.MAX_VALUE（一个大于Integer.MAX_VALUE的值——比如由long强制转换为int赋给一个int类型的minCapacity时会截断从而产生负值-如Byte.MAX_VALUE(127) +1赋值给一个byte变量会得到-128），如若minCapacity未超过Integer.MAX_VALUE的话则根据其是否大于MAX_ARRAY_SIZE来取值。至此，grow通过将新容量设置为当前容量的1.5倍并经过一系列判断得到最终newCapacity，然后调用 elementData = Arrays.copyOf(elementData, newCapacity)将数组大小拓展为newCapacity。
上面介绍的是ArrayList添加一个单一的元素，下面我们看下ArrayList直接添加Collection:
/**
* 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 true if this list changed as a result of the call
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(Collection 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 true 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 c) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(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;
}
其实addAll(Collection c) 和addAll(int index, Collection c)对应add(E e)和add(int index, E e)，只不过这两个方法添加的是collection中的数组，在添加之前同样会调用ensureCapacityInternal来确保后面的添加不会超出容量，addAll(Collection c)是直接通过 System.arraycopy(a, 0, elementData, size, numNew);将collection中的数组添加到elementData的尾部；addAll(int index, Collection c)是先通过System.arraycopy(elementData, index, elementData, index + numNew,numMoved);将elementData[index]及后面所有数据整体后移numNew位以给新插入的数组预留位置，然后调用System.arraycopy(a, 0, elementData, index, numNew);将collection中的数组a拷贝到预留出来的位置上，就这样实现了collection的添加。
从上面ArryList的数据添加过程我们已经知道了原理，下面我们来看看ArrayList的数据移除。
## ３、ArrayList的数据移除
/**
* 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) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
modCount++;
E oldValue = (E) 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;
}
/**
* 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
* i such that
* (o==null ? get(i)==null : o.equals(get(i)))
* (if such an element exists). Returns true 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 true 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; // clear to 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++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
移除ArrayList的某一个元素调用remove(Object o)，该方法会找出object的位置index并通过fastRemove(int index)来真正移除该元素，移除的核心代码是System.arraycopy(elementData, index+1, elementData, index,numMoved);通过将elementData[index+1]以及其后的所有数据整体向前移一位并将elementData[size-1]置为null，这样先前在index位置的object被覆盖从而不存在了而且数组elementData[size-1]位置也空出来一个空位置可供添加新元素。调用clear()会清除掉elementData中所有的元素。
再来看看ArrayList的数据更新：
## ４、ArrayList的数据更新
/**
* 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) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
E oldValue = (E) elementData[index];
elementData[index] = element;
return oldValue;
}
数据需要更新的话调用set(int index, E element)将指定位置index处的元素替换为element，该接口很简单，这里就不多介绍了。
最后我们看下ArrayList的数据查询。
## ５、ArrayList的数据获取
/**
* Returns an array containing all of the elements in this list
* in proper sequence (from first to last element).
*
* 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.
*
*
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.
*
*
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
* null. (This is useful in determining the length of the
* list only 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[] 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;
}
/**
* 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) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
return (E) elementData[index];
}
......
/**
* 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 i such that
* (o==null ? get(i)==null : o.equals(get(i))),
* 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 i such that
* (o==null ? get(i)==null : o.equals(get(i))),
* 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;
}
......
首先toArray()和toArray(T[] a) 都是返回ArrayList的数组elementData的一份拷贝，因此对返回的数组进行操作不行影响到elementData本身；get(int index)中如何index合法将返回ArrayList的数组elementData中索引为index的元素－即elementData[index]。indexOf(Object o) 和lastIndexOf(Object o)是检索Object在ArrayList中的位置，只是indexOf是顺序检索数组，而lastIndexOf是逆序检索。
## 6、ArrayList的fail-fast机制
因为ArrayList是非线程安全的，因此采用fail-fast机制来控制对Iterator的使用，从而避免导致程序更严重的错误。ArrayList使用modCount来表示修改的次数，当我们使用iterator()来获取Iterator时，Iterator开始的expectedModCount等于modCount，此时我们进行Iterator的遍历或删除等操作是可以正常运行的，但是倘若此时有另一个线程将要修改ArrayList的数据结构-如添加或者删除操作时modCount会进行+1，在此之后Iterator再进行操作时将会检测到modCount != expectedModCount，进而抛出ConcurrentModificationException异常并退出。
/**
* Returns an iterator over the elements in this list in proper sequence.
*
* The returned iterator is fail-fast.
*
* @return an iterator over the elements in this list in proper sequence
*/
public Iterator iterator() {
return new Itr();
}
/**
* An optimized version of AbstractList.Itr
*/
private class Itr implements Iterator {
// Android-changed: Add "limit" field to detect end of iteration.
// The "limit" of this iterator. This is the size of the list at the time the
// iterator was created. Adding & removing elements will invalidate the iteration
// anyway (and cause next() to throw) so saving this value will guarantee that the
// value of hasNext() remains stable and won't flap between true and false when elements
// are added and removed from the list.
protected int limit = ArrayList.this.size;
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 < limit;
}
@SuppressWarnings("unchecked")
public E next() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
int i = cursor;
if (i >= limit)
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();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
try {
ArrayList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount;
limit--;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
@Override
@SuppressWarnings("unchecked")
public void forEachRemaining(Consumer consumer) {
Objects.requireNonNull(consumer);
final int size = ArrayList.this.size;
int i = cursor;
if (i >= size) {
return;
}
final Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length) {
throw new ConcurrentModificationException();
}
while (i != size && modCount == expectedModCount) {
consumer.accept((E) elementData[i++]);
}
// update once at end of iteration to reduce heap write traffic
cursor = i;
lastRet = i - 1;
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
至此，ArrayList的数据处理原理我们已经分析完了，
总的来说，ArrayList的动态数组功能主要是通过ensureCapacityInternal和System.arraycopy来实现的。首先通过ensureCapacityInternal来确保下一步的数据操作不会超出ArrayList的容量，然后通过System.arraycopy来进行数组元素的变迁。