标签：允许 his media immediate check initial 构造 throws 实现
转载:http://wiki.jikexueyuan.com/project/java-collection/arraylist.html
ArrayList 可以理解为动态数组，用 MSDN 中的说法，就是 Array 的复杂版本。与 Java 中的数组相比，它的容量能动态增长。ArrayList 是 List 接口的可变数组的实现。实现了所有可选列表操作，并允许包括 null 在内的所有元素。除了实现 List 接口外，此类还提供一些方法来操作内部用来存储列表的数组的大小。（此类大致上等同于 Vector 类，除了此类是不同步的。）
每个 ArrayList 实例都有一个容量，该容量是指用来存储列表元素的数组的大小。它总是至少等于列表的大小。随着向 ArrayList 中不断添加元素，其容量也自动增长。自动增长会带来数据向新数组的重新拷贝，因此，如果可预知数据量的多少，可在构造 ArrayList 时指定其容量。在添加大量元素前，应用程序也可以使用 ensureCapacity 操作来增加 ArrayList 实例的容量，这可以减少递增式再分配的数量。
注意，此实现不是同步的。如果多个线程同时访问一个 ArrayList 实例，而其中至少一个线程从结构上修改了列表，那么它必须保持外部同步。（结构上的修改是指任何添加或删除一个或多个元素的操作，或者显式调整底层数组的大小；仅仅设置元素的值不是结构上的修改。）
我们先学习了解其内部的实现原理，才能更好的理解其应用。
对于 ArrayList 而言，它实现 List 接口、底层使用数组保存所有元素。其操作基本上是对数组的操作。下面我们来分析 ArrayList 的源代码：
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
}ArrayList 继承了 AbstractList，实现了 List。它是一个数组队列，提供了相关的添加、删除、修改、遍历等功能。
ArrayList 实现了 RandmoAccess 接口，即提供了随机访问功能。RandmoAccess 是 java 中用来被 List 实现，为 List 提供快速访问功能的。在 ArrayList 中，我们即可以通过元素的序号快速获取元素对象；这就是快速随机访问。
ArrayList 实现了 Cloneable 接口，即覆盖了函数 clone()，能被克隆。 ArrayList 实现 java.io.Serializable 接口，这意味着 ArrayList 支持序列化，能通过序列化去传输。
/**
* 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;
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this(10);
}
/**
* 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 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);
}ArrayList 提供了三种方式的构造器：
public ArrayList()可以构造一个默认初始容量为10的空列表；public ArrayList(int initialCapacity)构造一个指定初始容量的空列表；public ArrayList(Collection<? extends E> c)构造一个包含指定 collection 的元素的列表，这些元素按照该collection的迭代器返回它们的顺序排列的。ArrayList 中提供了多种添加元素的方法，下面将一一进行讲解：
1.set(int index, E element)：该方法首先调用rangeCheck(index)来校验 index 变量是否超出数组范围，超出则抛出异常。而后，取出原 index 位置的值，并且将新的 element 放入 Index 位置，返回 oldValue。
/**
* 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;
}
/**
* 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));
}2.add(E e)：该方法是将指定的元素添加到列表的尾部。当容量不足时，会调用 grow 方法增长容量。
/**
* 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;
}
private void ensureCapacityInternal(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
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);
}3.add(int index, E element)：在 index 位置插入 element。
/**
* 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++;
}4.addAll(Collection<? extends E> c) 和 addAll(int index, Collection<? extends E> c)：将特定 Collection 中的元素添加到 Arraylist 末尾。
/**
* 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;
}在 ArrayList 的存储方法，其核心本质是在数组的某个位置将元素添加进入。但其中又会涉及到关于数组容量不够而增长等因素。
这个方法就比较简单了，ArrayList 能够支持随机访问的原因也是很显然的，因为它内部的数据结构是数组，而数组本身就是支持随机访问。该方法首先会判断输入的index值是否越界，然后将数组的 index 位置的元素返回即可。
/**
* 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 (E) elementData[index];
}
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}ArrayList 提供了根据下标或者指定对象两种方式的删除功能。需要注意的是该方法的返回值并不相同，如下：
/**
* 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;
}注意：从数组中移除元素的操作，也会导致被移除的元素以后的所有元素的向左移动一个位置。
从上面介绍的向 ArrayList 中存储元素的代码中，我们看到，每当向数组中添加元素时，都要去检查添加后元素的个数是否会超出当前数组的长度，如果超出，数组将会进行扩容，以满足添加数据的需求。数组扩容有两个方法，其中开发者可以通过一个 public 的方法ensureCapacity(int minCapacity)来增加 ArrayList 的容量，而在存储元素等操作过程中，如果遇到容量不足，会调用priavte方法private void ensureCapacityInternal(int minCapacity)实现。
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);
}
/**
* 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);
}从上述代码中可以看出，数组进行扩容时，会将老数组中的元素重新拷贝一份到新的数组中，每次数组容量的增长大约是其原容量的 1.5 倍（从int newCapacity = oldCapacity + (oldCapacity >> 1)这行代码得出）。这种操作的代价是很高的，因此在实际使用时，我们应该尽量避免数组容量的扩张。当我们可预知要保存的元素的多少时，要在构造 ArrayList 实例时，就指定其容量，以避免数组扩容的发生。或者根据实际需求，通过调用ensureCapacity 方法来手动增加 ArrayList 实例的容量。
ArrayList 也采用了快速失败的机制，通过记录 modCount 参数来实现。在面对并发的修改时，迭代器很快就会完全失败，而不是冒着在将来某个不确定时间发生任意不确定行为的风险。 关于 Fail-Fast 的更详细的介绍，我在之前将 HashMap 中已经提到。
标签：允许 his media immediate check initial 构造 throws 实现
原文地址：http://www.cnblogs.com/NewMan13/p/6034350.html
