Java容器List接口

List接口是Java中经常用到的接口，如果对具体的List实现类的特性不了解的话，可能会导致程序性能的下降，下面从原理上简单的介绍List的具体实现：

可以看到，List继承了Collection接口，而Collection接口继承了Iterable接口。其中还有AbstractCollection和AbstractList的实现，用于List对象的公共部分代码的复用：

public interface List<E> extends Collection<E> {
    // Query Operations

public interface Collection<E> extends Iterable<E> {
    // Query Operations

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
    /**
     * Sole constructor.  (For invocation by subclass constructors, typically
     * implicit.)
     */
    protected AbstractList() {
    }

public abstract class AbstractCollection<E> implements Collection<E> {
    /**
     * Sole constructor.  (For invocation by subclass constructors, typically
     * implicit.)
     */
    protected AbstractCollection() {
    } 

具体这三个接口定义的方法如下：

现在来看看List接口的具体特性：

1、Vector

Vector类是通过数组实现的，支持线程的同步，即某一时刻只有一个线程能够写Vector，避免多线程同时写而引起的不一致性，但实现同步需要很高的花费，因此，访问它的效率不是很高。

现在来看一下Vector的成员变量和其中部分的构造方法：

public class Vector<E>
    extends AbstractList<E>
    implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    /**
     * The array buffer into which the components of the vector are
     * stored. The capacity of the vector is the length of this array buffer,
     * and is at least large enough to contain all the vector's elements.
     *
     * <p>Any array elements following the last element in the Vector are null.
     *
     * @serial
     */
    protected Object[] elementData;

    /**
     * The number of valid components in this {@code Vector} object.
     * Components {@code elementData[0]} through
     * {@code elementData[elementCount-1]} are the actual items.
     *
     * @serial
     */
    protected int elementCount;

    /**
     * The amount by which the capacity of the vector is automatically
     * incremented when its size becomes greater than its capacity.  If
     * the capacity increment is less than or equal to zero, the capacity
     * of the vector is doubled each time it needs to grow.
     *
     * @serial
     */
    protected int capacityIncrement;

    /** use serialVersionUID from JDK 1.0.2 for interoperability */
    private static final long serialVersionUID = -2767605614048989439L;

    /**
     * Constructs an empty vector with the specified initial capacity and
     * capacity increment.
     *
     * @param   initialCapacity     the initial capacity of the vector
     * @param   capacityIncrement   the amount by which the capacity is
     *                              increased when the vector overflows
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    public Vector(int initialCapacity, int capacityIncrement) {
    super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
    this.elementData = new Object[initialCapacity];
    this.capacityIncrement = capacityIncrement;
    }

对数据结构有一定了解的人都知道，使用数组作为存储底层，当存储空间不够了的时候，是会重新分配一块更大的内存空间，然后把原空间的数据全部都copy过去，最后释放原空间，这样会导致程序性能的下降，或者是GC的消耗（Vector是默认扩展1倍）。所以我们对处理的数据量要有一定的预估，初始化Vector的时候指定容量大小。由于对Vector所有动作都添加synchronized关键字，同样会导致执行的性能下降。

下面是对Vector添加元素的时候的代码：

    /**
     * Appends the specified element to the end of this Vector.
     *
     * @param e element to be appended to this Vector
     * @return {@code true} (as specified by {@link Collection#add})
     * @since 1.2
     */
    public synchronized boolean add(E e) {
    modCount++;
    ensureCapacityHelper(elementCount + 1);
    elementData[elementCount++] = e;
        return true;
    }

    /**
     * This implements the unsynchronized semantics of ensureCapacity.
     * Synchronized methods in this class can internally call this
     * method for ensuring capacity without incurring the cost of an
     * extra synchronization.
     *
     * @see #ensureCapacity(int)
     */
    private void ensureCapacityHelper(int minCapacity) {
    int oldCapacity = elementData.length;
    if (minCapacity > oldCapacity) {
        Object[] oldData = elementData;
        int newCapacity = (capacityIncrement > 0) ?
        (oldCapacity + capacityIncrement) : (oldCapacity * 2);
            if (newCapacity < minCapacity) {
        newCapacity = minCapacity;
        }
            elementData = Arrays.copyOf(elementData, newCapacity);
    }
    }

2、Stack

Stack 类表示后进先出（LIFO）的对象堆栈。它通过五个操作对类 Vector 进行了扩展，同样是线程安全的 ，允许将向量视为堆栈。它提供了通常的 push 和 pop 操作，以及取堆栈顶点的 peek 方法、测试堆栈是否为空的 empty 方法、在堆栈中查找项并确定到堆栈顶距离的 search 方法。当操作的集合不存在元素的时候，抛出EmptyStackException。

public
class Stack<E> extends Vector<E> {
    /**
     * Creates an empty Stack.
     */
    public Stack() {
    }

3、ArrayList

同样，ArrayList内部是通过数组实现的，它允许对元素进行快速随机访问。数组的缺点是每个元素之间不能有间隔，当数组大小不满足时需要增加存储能力，就要讲已经有数组的数据复制到新的存储空间中。当从ArrayList的中间位置插入或者删除元素时，需要对数组进行复制、移动、代价比较高（ArrayList在内存不够时默认是扩展50% + 1个）。因此，它适合随机查找和遍历，不适合插入和删除。由于没有添加同步，所以是非线程安全的，执行效率也要比Vector高很多

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;

    /**
     * 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;

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

    /**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param   initialCapacity   the initial capacity of the list
     * @exception 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];
    }

    public void ensureCapacity(int minCapacity) {
    modCount++;
    int oldCapacity = elementData.length;
    if (minCapacity > oldCapacity) {
        Object oldData[] = elementData;
        int newCapacity = (oldCapacity * 3)/2 + 1;
            if (newCapacity < minCapacity)
        newCapacity = minCapacity;
            // minCapacity is usually close to size, so this is a win:
            elementData = Arrays.copyOf(elementData, newCapacity);
    }
    }

4、LinkedList

LinkedList是用双向链表结构存储数据的，很适合数据的动态插入和删除，随机访问和遍历速度比较慢。另外，他还提供了List接口中没有定义的方法，专门用于操作表头和表尾元素，可以当作堆栈、队列和双向队列使用。

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
    private transient Entry<E> header = new Entry<E>(null, null, null);
    private transient int size = 0;

    /**
     * Constructs an empty list.
     */
    public LinkedList() {
        header.next = header.previous = header;
    }

    Entry(E element, Entry<E> next, Entry<E> previous) {
        this.element = element;
        this.next = next;
        this.previous = previous;
    }
    }

总结：根据不同实现类的特性，选择对应的数据结构，能提高程序的运行效率 。需要注意的是，一条线程在进行list列表元素迭代的时候，另外一条线程如果想要在迭代过程中，想要对元素进行操作的时候，比如满足条件添加新元素，会发生ConcurrentModificationException并发修改异常。当然后来版本的CopyOnWrite容器解决了该问题，后续会介绍。