ArrayList
概述
ArrayList是使用頻率最高的集合之一了，在需要使用List的情況下，往往都是優(yōu)先考慮ArrayList。首先我們來看一下聲明：
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
ArrayList實(shí)現(xiàn)的幾個(gè)接口中，RandomAccess、Cloneable、Serializable都是標(biāo)記接口，所以ArrayList是很純粹的List接口的實(shí)現(xiàn)，不像它兄弟LinkedList還實(shí)現(xiàn)了Deque接口，還要作為雙向隊(duì)列使用。
結(jié)構(gòu)
transient Object[] elementData;
// 這個(gè)繼承自父類AbstractList
protected transient int modCount = 0;
ArrayList的名稱中我們就可以看出來，這是一個(gè)用數(shù)組實(shí)現(xiàn)的List，或者說是可變數(shù)組，數(shù)據(jù)就是存儲(chǔ)在elementData這個(gè)對(duì)象數(shù)組里。除了elementData我們還需要關(guān)注一個(gè)重要的成員變量modCount，modCount成員變量是繼承自父類AbstractList。modCount表示這個(gè)List被結(jié)構(gòu)化修改的次數(shù)，結(jié)構(gòu)化修改就是那些會(huì)改變List的大小的操作。modCount主要被用在迭代器上，如果一個(gè)List在迭代的過程中發(fā)生了結(jié)構(gòu)化修改，就會(huì)導(dǎo)致結(jié)果出錯(cuò)。在List迭代過程中，如果因?yàn)槠渌€程對(duì)List的操作，導(dǎo)致結(jié)構(gòu)發(fā)生變化，那么迭代器就拋出ConcurrentModificationException，這就是迭代器的fail-fast機(jī)制。
添加元素
/**
* 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++;
}
/**
* 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;
}
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
有多個(gè)方法來給ArrayList添加元素，add(E e)是添加到數(shù)組末尾，add(int index, E element)是添加到指定位置，addAll(Collection<? extends E> c)批量添加元素到數(shù)組末尾，addAll(int index, Collection<? extends E> c)批量添加元素到指定位置。
本質(zhì)上這幾個(gè)方法都是相同的，首先通過rangeCheck或者rangeCheckForAdd方法判斷index是否合法。然后通過ensureCapacityInternal方法來確保數(shù)組的容量足夠，該方法會(huì)先判斷當(dāng)前數(shù)組容量是否足夠，如果不夠就進(jìn)行擴(kuò)容，待會(huì)會(huì)進(jìn)行介紹。不過需要注意的是，添加元素是會(huì)造成ArrayList結(jié)構(gòu)化改變的，所以modCount的值要增加。而源碼中將modCount自增操作放在了ensureCapacityInternal方法里，感覺有點(diǎn)怪怪的，從方法的命名中可以看出這個(gè)方法是用來確保數(shù)組容量的，但是卻在這個(gè)方法里修改了與方法容量無關(guān)的成員變量，所以我覺得設(shè)計(jì)得不是很合理。寫代碼的人也覺得自己這樣搞不是很合理，所以才通過注釋來說明。
ensureCapacityInternal(size + 1); // Increments modCount!!
接著剛才的話題，當(dāng)確保數(shù)組的容量足夠之后，再通過靜態(tài)方法System.arraycopy()將元素拷貝到合適的位置，對(duì)原數(shù)組進(jìn)行重新排序就可以了。當(dāng)然，添加到末尾就不用考慮到數(shù)組重排序的問題了，直接將待添加元素放到末尾就可以了。最后修改size到相應(yīng)的數(shù)值，添加元素的操作就完成了。
擴(kuò)容
ArrayList是基于可變數(shù)組的，當(dāng)?shù)讓訑?shù)組容量不足時(shí)會(huì)進(jìn)行擴(kuò)容，以改變數(shù)組的容量。代碼如下：
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);
}
/**
* 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;
}
前面那些ensure開頭的方法是用來檢測(cè)當(dāng)前數(shù)組容量是否足夠容納minCapacity的，如果容量不足才會(huì)進(jìn)行擴(kuò)容，即調(diào)用grow(int capacity)方法，我們直接來看grow()方法。
grow()方法首先將數(shù)組容量擴(kuò)張為原來的1.5倍，即int newCapacity = oldCapacity + (oldCapacity >> 1)這條語句。然后再判斷新容量是否滿足最小所需容量minCapacity，如果還是不能滿足，就將newCapacity設(shè)置為minCapacity。接下來要判斷newCapacity是否超過了最大允許的數(shù)組大小MAX_ARRAY_SIZE，如果超過了就調(diào)整為最大的int值。最后就是將原數(shù)組的值拷貝到新的數(shù)組上。
移除元素
/**
* 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;
}
/**
* 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
*/
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;
}
/**
* 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;
}
其實(shí)移除元素的原理很簡(jiǎn)單，就是通過System.arraycopy方法將需要保留的元素復(fù)制到正確的位置上，然后調(diào)整size的大小。最后為了防止內(nèi)存泄露，需要顯式將不再使用的位置中存放的元素置為null。雖然原理簡(jiǎn)單，但是需要注意的細(xì)節(jié)很多，大多是索引值方面的小細(xì)節(jié)。
接下來看一下批量刪除或者保留元素的方法。
/**
* Removes from this list all of its elements that are contained in the
* specified collection.
*/
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(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) {
Objects.requireNonNull(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++)
//1) 移除c中元素，complement == false
// 若elementData[r]不在c中，則保留
//2）保留c中元素，complement == true
// 若elementData[r]在c中，則保留
if (c.contains(elementData[r]) == complement)
elementData[w++] = elementData[r];
} finally {
// Preserve behavioral compatibility with AbstractCollection,
// even if c.contains() throws.
// 1）r == size, 則操作成功了
// 2）r != size, c.contains拋出了異常，
// 可能是因?yàn)樵睾蚦中元素類型不兼容，或者c不支持null元素
// 則將后面尚未檢查的元素向前復(fù)制
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;
}
其中，無論是批量移除removeAll()方法還是批量保留retainAll()方法，都是使用了batchRemove方法，我們直接來看這個(gè)方法。
先來說一下原理，首先通過便利整個(gè)數(shù)組，找出需要保留的元素，從索引0開始依次保存到elementData數(shù)組中。如果便利過程沒有異常出現(xiàn)(也就是r==size)，則顯式將不再使用的位置中存放的元素置為null，讓GC回收。當(dāng)然如果便利過程出現(xiàn)異常(r!=size)，則要將未被便利的值拷貝到w索引及之后的位置。暫時(shí)不清楚對(duì)異常的處理是否合理。
查找與更新
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 the element at the specified position in this list.
*/
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
/**
* 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;
}
因?yàn)槭腔跀?shù)組實(shí)現(xiàn)的，所以查找元素和更新元素比較簡(jiǎn)單。這幾個(gè)方法都沒有改變List的結(jié)構(gòu)，所以不會(huì)修改modCount的值。
迭代
列表的迭代也是開發(fā)中經(jīng)常使用到了，特別是使用for each語句進(jìn)行迭代。因?yàn)?code>Collection接口繼承了Iterable接口，ArrayList間接實(shí)現(xiàn)了Collection，所以需要實(shí)現(xiàn)Iterable接口的iterator()方法，下面我們來看一下。
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();
}
}
迭代器中通過cursor來標(biāo)注下一個(gè)待返回元素的索引值，還有一個(gè)lastRet來標(biāo)注上一個(gè)被返回元素的索引值。ArrayList的實(shí)現(xiàn)不是線程安全的，其fail-fast機(jī)制的實(shí)現(xiàn)是通過modCount變量來實(shí)現(xiàn)的。在next和remove里都有checkForComodification()的方法，在該方法中，會(huì)比較Iterator創(chuàng)建時(shí)的modCount(expectedModCount)和當(dāng)前的modCount的值是否相等。不過不相，證明在迭代器創(chuàng)建之后ArrayList的結(jié)構(gòu)有被修改過，此時(shí)拋出ConcurrentModificationException異常。
需要注意的一點(diǎn)在于，remove()方法調(diào)用時(shí)，會(huì)判斷lastRet < 0，如果小于0，就會(huì)拋出異常。出現(xiàn)lastRet<0只有兩種情況，一種是剛創(chuàng)建迭代器，還未調(diào)用next()方法的時(shí)候，一種是調(diào)用過一次remove()方法后會(huì)把lastRet設(shè)置為-1。所以連續(xù)兩次調(diào)用remove()方法是會(huì)拋出異常的。
List接口還支持另一種迭代器ListIterator，它不僅可以使用next()向前迭代，還可以使用previous()向后迭代；不僅可以使用remove()在迭代中移除元素，還可以使用add()方法在迭代中添加元素。
小結(jié)
ArrayList內(nèi)部使用數(shù)組實(shí)現(xiàn)，具有高效的隨機(jī)訪問的特性。但是插入和刪除元素時(shí)往往需要復(fù)制數(shù)組，開銷較大。在容器創(chuàng)建之后需要進(jìn)行大量訪問，但插入和刪除操作使用較少的情況下比較適合使用ArrayList。
