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李大辉 · 更新于 2018-10-21 01:00:46

ArrayList 的实现原理

ArrayList 概述

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。

ArrayList 的实现

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:

实现的接口

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。

底层使用数组实现

/**
* 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;

构造方法


    /**
     * 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:

  1. public ArrayList()可以构造一个默认初始容量为10的空列表;
  2. public ArrayList(int initialCapacity)构造一个指定初始容量的空列表;
  3. 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。

存储

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)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。

读取

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));
}

删除

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&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;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。

调整数组容量

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。

Fail-Fast 机制

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。