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定义
在Java API中，官方给出的前两段话如下：
An ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list.
Unlike sets, lists typically allow duplicate elements. More formally, lists typically allow pairs of elements e1 and e2 such that e1.equals(e2), and they typically allow multiple null elements if they allow null elements at all. It is not inconceivable that someone might wish to implement a list that prohibits duplicates, by throwing runtime exceptions when the user attempts to insert them, but we expect this usage to be rare.
大概的意思是：一个有序的Collection（或者叫做序列）。使用这个接口可以精确掌控元素的插入，还可以根据index获取相应位置的元素。用户可以通过int型索引（list的位置）访问元素，并搜索这个list的元素。
与Set集合不同，List允许重复元素的插入。更正式地讲，列表通常允许元素e1和e2可以想e1.equals（e2）进行比较，并且如果它们通常允许多个空元素。有人可能为了禁止插入相同的值，希望自己去实现一个List，并且在重复元素插入的时候抛出异常，但是不建议这么做。
新接口listIterator
然后我们再看下List接口新增的接口，会发现add，get这些都多了index参数，说明在原来Collection的基础上，List是一个可以指定索引，有序的容器。在这注意以下添加的2个新Iteractor方法。
//Returns a list iterator over the elements in this list (in proper sequence).
ListIterator<E> listIterator();
//Returns a list iterator over the elements in this list (in proper sequence),
//starting at the specified position in the list.
ListIterator<E> listIterator(int index);
我们再看ListIterator的代码，去掉不太重要的注释
/**
* An iterator for lists that allows the programmer
* to traverse the list in either direction, modify
* the list during iteration, and obtain the iterator's
* current position in the list.
* 列表的迭代器，它允许程序员在任意方向遍历列表，
* 在迭代期间修改列表，并获得迭代器在列表中的当前位置。
*/
public interface ListIterator<E> extends Iterator<E> {
/**
* Returns {@code true} if this list iterator has more elements when
* traversing the list in the forward direction. (In other words,
* returns {@code true} if {@link #next} would return an element rather
* than throwing an exception.)
* 如果在往前遍历过程中有更多元素，并且没有抛出异常。则返回true
*/
boolean hasNext();
/**
* Returns the next element in the list and advances the cursor position.
* This method may be called repeatedly to iterate through the list,
* or intermixed with calls to {@link #previous} to go back and forth.
* (Note that alternating calls to {@code next} and {@code previous}
* will return the same element repeatedly.)
* 返回列表中的下一个元素并前进光标位置。可重复调用此方法以遍历列表
*/
E next();
/**
* Returns {@code true} if this list iterator has more elements when
* traversing the list in the reverse direction. (In other words,
* returns {@code true} if {@link #previous} would return an element
* rather than throwing an exception.)
* 如果这个列表在反向遍历列表时有更多的元素。
*/
boolean hasPrevious();
/**
* Returns the previous element in the list and moves the cursor
* position backwards. This method may be called repeatedly to
* iterate through the list backwards, or intermixed with calls to
* {@link #next} to go back and forth. (Note that alternating calls
* to {@code next} and {@code previous} will return the same
* element repeatedly.)
*返回列表中的前一个元素并向后移动光标位置
*/
E previous();
/**
* Returns the index of the element that would be returned by a
* subsequent call to {@link #next}. (Returns list size if the list
* iterator is at the end of the list.)
* 返回后续调用将返回的元素的索引
*/
int nextIndex();
/**
* Returns the index of the element that would be returned by a
* subsequent call to {@link #previous}. (Returns -1 if the list
* iterator is at the beginning of the list.)
*/
int previousIndex();
void remove();
/**
* Replaces the last element returned by {@link #next} or
* {@link #previous} with the specified element (optional operation).
* This call can be made only if neither {@link #remove} nor {@link
* #add} have been called after the last call to {@code next} or
* {@code previous}.
*/
void set(E e);
/**
* Inserts the specified element into the list (optional operation).
* The element is inserted immediately before the element that
* would be returned by {@link #next}, if any, and after the element
* that would be returned by {@link #previous}, if any. (If the
* list contains no elements, the new element becomes the sole element
* on the list.) The new element is inserted before the implicit
* cursor: a subsequent call to {@code next} would be unaffected, and a
* subsequent call to {@code previous} would return the new element.
* (This call increases by one the value that would be returned by a
* call to {@code nextIndex} or {@code previousIndex}.)
*/
void add(E e);
}
一个集合在遍历过程中进行插入删除操作很容易造成错误，特别是无序队列，是无法在遍历过程中进行这些操作的。但是List是一个有序集合，所以在这实现了一个ListIteractor，可以在遍历过程中进行元素操作，并且可以双向访问。
ArrayList
就Java文档的解释，整理出以下几点特点：
- ArrayList是一个实现了List接口的可变数组
- 可以插入null
- 它的size, isEmpty, get, set, iterator,add这些方法的时间复杂度是O(1),如果add n个数据则时间复杂度是O(n).
- ArrayList不是synchronized的。
然后我们来简单看下ArrayList源码实现。这里只写部分源码分析。
所有元素都是保存在一个Object数组中，然后通过size控制长度。
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
private static final long serialVersionUID = 8683452581122892189L;
/**
* 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.
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
*/
private int size;
}
这时候看下add的代码分析
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
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);
}
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);
}
其实在每次add的时候会判断数据长度，如果不够的话会调用Arrays.copyOf，复制一份更长的数组，并把前面的数据放进去。
我们再看下remove的代码是如何实现的。
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;
}
其实就是直接使用System.arraycopy把需要删除index后面的都往前移一位然后再把最后一个去掉。
LinkedList
LinkedList是一个链表维护的序列容器。和ArrayList都是序列容器，一个使用数组存储，一个使用链表存储。
数组和链表
- 查找方面。数组的效率更高，可以直接索引出查找，而链表必须从头查找。
- 插入删除方面。特别是在中间进行插入删除，这时候链表体现出了极大的便利性，只需要在插入或者删除的地方断掉链然后插入或者移除元素，然后再将前后链重新组装，但是数组必须重新复制一份将所有数据后移或者前移。
- 在内存申请方面，当数组达到初始的申请长度后，需要重新申请一个更大的数组然后把数据迁移过去才行。而链表只需要动态创建即可。
如上LinkedList和ArrayList的区别也就在此。根据使用场景选择更加适合的List。
源码解析。
LinkedList源码的属性结构
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
transient int size = 0;
/**
* Pointer to first node.
* Invariant: (first == null && last == null) ||
* (first.prev == null && first.item != null)
*/
transient Node<E> first;
/**
* Pointer to last node.
* Invariant: (first == null && last == null) ||
* (last.next == null && last.item != null)
*/
transient Node<E> last;
/**
* Constructs an empty list.
*/
public LinkedList() {
}
}
节点Node的定义,它在LinkedList中是一个静态内部类。
private static class Node<E> {
E item;
Node<E> next;
Node<E> prev;
Node(Node<E> prev, E element, Node<E> next) {
this.item = element;
this.next = next;
this.prev = prev;
}
}
每个LinkedList中会持有链表的头指针和尾指针
列举最基本的插入和删除的链表操作
private void linkFirst(E e) {
final Node<E> f = first;
final Node<E> newNode = new Node<>(null, e, f);
first = newNode;
if (f == null)
last = newNode;
else
f.prev = newNode;
size++;
modCount++;
}
void linkLast(E e) {
final Node<E> l = last;
final Node<E> newNode = new Node<>(l, e, null);
last = newNode;
if (l == null)
first = newNode;
else
l.next = newNode;
size++;
modCount++;
}
void linkBefore(E e, Node<E> succ) {
// assert succ != null;
final Node<E> pred = succ.prev;
final Node<E> newNode = new Node<>(pred, e, succ);
succ.prev = newNode;
if (pred == null)
first = newNode;
else
pred.next = newNode;
size++;
modCount++;
}
private E unlinkFirst(Node<E> f) {
// assert f == first && f != null;
final E element = f.item;
final Node<E> next = f.next;
f.item = null;
f.next = null; // help GC
first = next;
if (next == null)
last = null;
else
next.prev = null;
size--;
modCount++;
return element;
}
private E unlinkLast(Node<E> l) {
// assert l == last && l != null;
final E element = l.item;
final Node<E> prev = l.prev;
l.item = null;
l.prev = null; // help GC
last = prev;
if (prev == null)
first = null;
else
prev.next = null;
size--;
modCount++;
return element;
}
E unlink(Node<E> x) {
// assert x != null;
final E element = x.item;
final Node<E> next = x.next;
final Node<E> prev = x.prev;
if (prev == null) {
first = next;
} else {
prev.next = next;
x.prev = null;
}
if (next == null) {
last = prev;
} else {
next.prev = prev;
x.next = null;
}
x.item = null;
size--;
modCount++;
return element;
}
上面方法就是链表的核心，头尾中间插入，头尾中间删除。其他对外的调用都是围绕这几个方法进行操作的，同时LinkedList还实现了Deque接口，Deque接口是继承Queue的。所以LinkedList还支持队列的pop，push，peek操作。
总结
ArrayList 数组形式访问List链式集合数据，元素可重复，访问元素较快 数组
LinkedList 链表方式的List链式集合，元素可重复，元素的插入删除较快 双向链表