ArrayList底层实现原理

时间 2019-11-30

原文原文链接

ArrayList底层实现原理

ArrayList 的实现原理

ArrayList 概述

ArrayList 能够理解为动态数组，用 MSDN 中的说法，就是 Array 的复杂版本。与 Java 中的数组相比，它的容量能动态增加。ArrayList 是 List 接口的可变数组的实现。实现了全部可选列表操做，并容许包括 null 在内的全部元素。除了实现 List 接口外，此类还提供一些方法来操做内部用来存储列表的数组的大小。（此类大体上等同于 Vector 类，除了此类是不一样步的。）
每一个 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。它老是至少等于列表的大小。随着向 ArrayList 中不断添加元素，其容量也自动增加。自动增加会带来数据向新数组的从新拷贝，所以，若是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，应用程序也可使用 ensureCapacity 操做来增长 ArrayList 实例的容量，这能够减小递增式再分配的数量。
注意，此实现不是同步的。若是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做，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。）
咱们先学习了解其内部的实现原理，才能更好的理解其应用。
java

ArrayList 的实现

对于 ArrayList 而言，它实现 List 接口、底层使用数组保存全部元素。其操做基本上是对数组的操做。下面咱们来分析 ArrayList 的源代码：
数组

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，咱们便可以经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，能经过序列化去传输。
数据结构

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：并发

public ArrayList()能够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。
app

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。然后，取出原 index 位置的值，而且将新的 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 方法增加容量。dom

/**
     * 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。函数

/**
     * 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。学习

/**
     * 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加等因素。
ui

读取

这个方法就比较简单了，ArrayList 可以支持随机访问的缘由也是很显然的，由于它内部的数据结构是数组，而数组自己就是支持随机访问。该方法首先会判断输入的index值是否越界，而后将数组的 index 位置的元素返回便可。this

/**
* 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。须要注意的是该方法的返回值并不相同，以下：

/**
     * 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做，也会致使被移除的元素之后的全部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，咱们看到，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，若是超出，数组将会进行扩容，以知足添加数据的需求。数组扩容有两个方法，其中开发者能够经过一个 public 的方法ensureCapacity(int minCapacity)来增长 ArrayList 的容量，而在存储元素等操做过程当中，若是遇到容量不足，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);
    }

从上述代码中能够看出，数组进行扩容时，会将老数组中的元素从新拷贝一份到新的数组中，每次数组容量的增加大约是其原容量的 1.5 倍（从int newCapacity = oldCapacity + (oldCapacity >> 1)这行代码得出）。这种操做的代价是很高的，所以在实际使用时，咱们应该尽可能避免数组容量的扩张。当咱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，以免数组扩容的发生。或者根据实际需求，经过调用ensureCapacity 方法来手动增长 ArrayList 实例的容量。

Fail-Fast 机制

ArrayList 也采用了快速失败的机制，经过记录 modCount 参数来实现。在面对并发的修改时，迭代器很快就会彻底失败，而不是冒着在未来某个不肯定时间发生任意不肯定行为的风险。关于 Fail-Fast 的更详细的介绍，我在以前将 HashMap 中已经提到。