public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
private static final long serialVersionUID = 8683452581122892189L;
/**
* 默认容量为10
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* 空的数组
*/
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 = {};
/**
* 用于存储数据的数组，创建ArrayList的时候被初始化
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
*元素的数量（不是elementData的长度）
*/
private int size;
/**
*指定容量的构造方法
** 如果确定列表长度，可以使用此方法来创建列表，省去了扩容的消耗
** 长度小于10的情况，也可以使用此方法来创建列表 节省空间
*/
public ArrayList(int initialCapacity) {
/**
* initialCapacity大于0时，创建initialCapacity长度的数组
*/
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity];
}
//initialCapacity为0时，就使用空数组EMPTY_ELEMENTDATA
else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA;
}
//initialCapacity小于0时抛出异常
else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
/**
* 默认构造器：创建一个空数组
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* 初始化一个ArrayList，包含c集合的所有元素
*/
public ArrayList(Collection<? extends E> c) {
//调用Collection的toArray返回一个数组
elementData = c.toArray();
if ((size = elementData.length) != 0) {
//toArray返回的可能不是Object[]类型
/**以下情况返回的是String类型
* String[] ss = new String[10];
* List l = Arrays.asList(ss);
* System.out.println(l.toArray());
*/
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// 如果size为0，elementData为空数组
this.elementData = EMPTY_ELEMENTDATA;
}
}
/**
* 清除elementData中空余的空间（elementData的长度可能大于size）
*/
public void trimToSize() {
modCount++;
/**
* 如果size小于elementData的长度，就清除空闲空间
*/
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
//重新生成一个size长度的数组，并将elementData中的元素拷贝进来
: Arrays.copyOf(elementData, size);
}
}
/**
* 确保容量大小（共外部调用）
*/
public void ensureCapacity(int minCapacity) {
int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
/**
* 如果elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA说明最小容量为minCapacity
*/
? 0
/**
* 如果elementData= DEFAULTCAPACITY_EMPTY_ELEMENTDATA需要扩展，默认初始化长度为10
*/
: DEFAULT_CAPACITY;
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}
/**
* 确保容量大小（内部调用）
*/
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
//最小容量为DEFAULT_CAPACITY 10
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// minCapacity大于elementData.length就扩展
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) {
// 获得当前数组长度
int oldCapacity = elementData.length;
//新的数组长度=当前数组长度+0.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
//如果新的数组长度小于最小容量minCapacity，则新的数组长度为minCapacity
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
/**
* 新的数组长度大于MAX_ARRAY_SIZE（Integer.MAX_VALUE - 8），调用hugeCapacity
* 要么抛出OOM，要么扩展到Integer.MAX_VALUE长度
*/
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
//创建一个newCapacity长度的新数组，拷贝之前的数据
elementData = Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity) {
/**
* 如果当前数组长度已经达到Integer.MAX_VALUE，此时再继续添加数据就会得到负值
* Integer.MAX_VALUE+1 <0
*/
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE ://最多扩展到Integer.MAX_VALUE长度
MAX_ARRAY_SIZE;
}
/**
*返回当前列表内元素的个数
*/
public int size() {
return size;
}
/**
* 返回当前列表内是否有元素
*/
public boolean isEmpty() {
return size == 0;
}
/**
* 判断o是否在列表里
*/
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;
}
/**
* 返回o在列表中的位置
*/
public int lastIndexOf(Object o) {
/**
* o为null时，遍历数组（最大长度为size）通过==来判断
*/
if (o == null) {
for (int i = size-1; i >= 0; i--)
if (elementData[i]==null)
return i;
} else {
/**
* o不为null时，遍历数组（最大长度为size）通过equals来判断
* 如果某个对象重写equals方法，两个不同的对象可能相等
*/
for (int i = size-1; i >= 0; i--)
if (o.equals(elementData[i]))
return i;
}
return -1;
}
/**
* 拷贝一份列表，只是拷贝了元素的引用
*/
public Object clone() {
try {
ArrayList<?> v = (ArrayList<?>) 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(e);
}
}
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
// a的长度小于列表元素个数时，就只创建一个size长度的新数组
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;//将第size值设为null
return a;
}
// Positional Access Operations
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1); // Increments modCount!!
//将从index开始的元素都往后移一位
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
public E remove(int index) {
rangeCheck(index);
modCount++;
E oldValue = elementData(index);
/**
* 计算需要移动的元素个数
*/
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
//列表最后一个元素设为null（不是数组最后一位设为null）
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}
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 void fastRemove(int index) {
//不需要检查边界
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++;
//只是将元素全部置为null，elementData数组长度没变
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;
}
/**
* 实现原理：将toIndex之后的元素往前移动到fromIndex之后
*/
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;
}
/**
* 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) {
Objects.requireNonNull(c);
return batchRemove(c, false);
}
/**
* 保留c中的元素，其他的元素都删除
* @param c
* @return
*/
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, true);
}
/**
* 批量删除
* @param c
* @param complement true表示删除c以外的元素 false表示删除c中所含的元素
* @return
*/
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;
}
}
相关的面试题：
1、ArrayList的大小是如何自动增加的？你能分享一下你的代码吗？
arrayList每次新增元素的时候回确保容量是否够用，如果不够用会扩大容量，重新创建一个更大容量的数组，并将原有元素拷贝进去，具体代码如下
private void grow(int minCapacity) {
// 获得当前数组长度
int oldCapacity = elementData.length;
//新的数组长度=当前数组长度+0.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
//如果新的数组长度小于最小容量minCapacity，则新的数组长度为minCapacity
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
/**
* 新的数组长度大于MAX_ARRAY_SIZE（Integer.MAX_VALUE - 8），调用hugeCapacity
* 要么抛出OOM，要么扩展到Integer.MAX_VALUE长度
*/
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
//创建一个newCapacity长度的新数组，拷贝之前的数据
elementData = Arrays.copyOf(elementData, newCapacity);
}
2、什么情况下你会使用ArrayList？什么时候你会选择LinkedList？
这又是一个大多数面试者都会困惑的问题。多数情况下，当你遇到访问元素比插入或者是删除元素更加频繁的时候，你应该使用ArrayList。另外一方面，当你在某个特别的索引中，插入或者是删除元素更加频繁，或者你压根就不需要访问元素的时候，你会选择LinkedList。这里的主要原因是，在ArrayList中访问元素的最糟糕的时间复杂度是”1″，而在LinkedList中可能就是”n”了。在ArrayList中增加或者删除某个元素，通常会调用System.arraycopy方法，这是一种极为消耗资源的操作，因此，在频繁的插入或者是删除元素的情况下，LinkedList的性能会更加好一点。
3、当传递ArrayList到某个方法中，或者某个方法返回ArrayList，什么时候要考虑安全隐患？如何修复安全违规这个问题呢？
当array被当做参数传递到某个方法中，如果array在没有被复制的情况下直接被分配给了成员变量，那么就可能发生这种情况，即当原始的数组被调用的方法改变的时候，传递到这个方法中的数组也会改变。下面的这段代码展示的就是安全违规以及如何修复这个问题。
ArrayList被直接赋给成员变量——安全隐患：
修复这个安全隐患：
4、如何复制某个ArrayList到另一个ArrayList中去？写出你的代码？
下面就是把某个ArrayList复制到另一个ArrayList中去的几种技术：
- 使用clone()方法，比如ArrayList newArray = oldArray.clone();
- 使用ArrayList构造方法，比如：ArrayList myObject = new ArrayList(myTempObject);
- 使用Collection的copy方法。
注意1和2是浅拷贝(shallow copy)。
5、在索引中ArrayList的增加或者删除某个对象的运行过程？效率很低吗？解释一下为什么？
在ArrayList中增加或者是删除元素，要调用System.arraycopy这种效率很低的操作，如果遇到了需要频繁插入或者是删除的时候，你可以选择其他的Java集合，比如LinkedList。看一下下面的代码：
在ArrayList的某个索引i处添加元素：
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1); // Increments modCount!!
//将从index开始的元素都往后移一位
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
删除ArrayList的某个索引i处的元素：
public E remove(int index) {
rangeCheck(index);
modCount++;
E oldValue = elementData(index);
/**
* 计算需要移动的元素个数
*/
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
//列表最后一个元素设为null（不是数组最后一位设为null）
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}