ArrayList
ArrayList内部使用数组存放元素，因此访问元素效率高，插入元素效率低。
官方文档（Java9）
java.util public class ArrayList
extends AbstractList
implements List, RandomAccess, Cloneable, java.io.Serializable
Resizable-array implementation of the List interface. Implements all optional list operations, and permits all elements, including null. In addition to implementing the List interface, this class provides methods to manipulate the size of the array that is used internally to store the list. (This class is roughly equivalent to Vector, except that it is unsynchronized.)
The size, isEmpty, get, set, iterator, and listIterator operations run in constant time. The add operation runs in amortized constant time, that is, adding n elements requires O(n) time. All of the other operations run in linear time (roughly speaking). The constant factor is low compared to that for the LinkedList implementation.
Each ArrayList instance has a capacity. The capacity is the size of the array used to store the elements in the list. It is always at least as large as the list size. As elements are added to an ArrayList, its capacity grows automatically. The details of the growth policy are not specified beyond the fact that adding an element has constant amortized time cost.
An application can increase the capacity of an ArrayList instance before adding a large number of elements using the ensureCapacity operation. This may reduce the amount of incremental reallocation.
Note that this implementation is not synchronized. If multiple threads access an ArrayList instance concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more elements, or explicitly resizes the backing array; merely setting the value of an element is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the list should be “wrapped” using the Collections.synchronizedList method. This is best done at creation time, to prevent accidental unsynchronized access to the list:
List list = Collections.synchronizedList(new ArrayList(…));
The iterators returned by this class’s iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.
This class is a member of the Java Collections Framework.
Since:
1.2
See Also:
Collection, List, LinkedList, Vector
Type parameters:
– the type of elements in this list
< 9 >
我的理解（如有错误欢迎指正）
- ArrayList是List接口的实现，是可变长的数组，允许为任意数据类型（基本数据类型、引用数据类型），包括null
- 提供了操作数组大小的方法
- 大致相当于Vector，但ArrayList是不同步的
- size, isEmpty, get, set, iterator 和 listIterator 时间复杂度为O(1)
- 添加n个元素所需要的时间为O(n),其他操作的时间耗费粗略的说是线性的
- 每个ArrayList实例都有一个容量，容量是list中用来存储元素的数组的大小，容量至少和list的大小一样，每向ArrayList中添加一个元素，容量的值就会自动增长
- 向ArrayList的实例添加大量数据（赋值）之前，可以先为其指明容量大小，以避免jvm为其多次分配
- 再次强调，ArrayList是不同步的，在多线程访问ArrayList情况下，如果某个线程对其进行结构上的修改（添加、删除、改变大小），必须从外部对其进行同步，同步通常由list封装的对象完成，若不存在这样的对象，应该用Collections.synchronizedList 方法对其“打包”，“打包”最好在创建ArrayList时完成，示例:
List list = Collections.synchronizedList(new ArrayList(…)); - 当ArrayList的迭代器iterator创建之后，如果ArrayList发生了结构上的修改（除由iterator本身引发的删除或添加操作外），iterator将会抛出异常：ConcurrentModificationException，fail-fast（快速失败），需要注意的是，不能编写一个依赖于此异常的程序，iterator的fail-fast行为只应该用于检查错误
