Java 的 ArrayList 的底层数据结构

1. 数据结构--ArrayList源码摘要

ublic 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 = {};

    /**
     * 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 == EMPTY_ELEMENTDATA will be expanded to
     * DEFAULT_CAPACITY when the first element is added.
     */
    private transient Object[] elementData;

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

ArrayList 的底层最重要的两个属性：Object 数组和 size 属性。

2. ArrayList 的底层数组的调整

add方法--ArrayList源码摘要

/**
     * 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;
    }

grow方法--ArrayList源码摘要

/**

* Increases the capacity of this <tt>ArrayList</tt> instance, if

* necessary, to ensure that it can hold at least the number of elements

* specified by the minimum capacity argument.

*

* @param   minCapacity   the desired minimum capacity

*/

public void ensureCapacity(int minCapacity) {

int minExpand = (elementData != EMPTY_ELEMENTDATA)

// any size if real element table

? 0

// larger than default for empty table. It's already supposed to be

// at default size.

: DEFAULT_CAPACITY;

if (minCapacity > minExpand) {

ensureExplicitCapacity(minCapacity);

}

}

private void ensureCapacityInternal(int minCapacity) {

if (elementData == 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);

}

/**

* The maximum size of array to allocate.

* Some VMs reserve some header words in an array.

* Attempts to allocate larger arrays may result in

* OutOfMemoryError: Requested array size exceeds VM limit

*/

private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

/**

* 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);

}

private static int hugeCapacity(int minCapacity) {

if (minCapacity < 0) // overflow

throw new OutOfMemoryError();

return (minCapacity > MAX_ARRAY_SIZE) ?

Integer.MAX_VALUE :

MAX_ARRAY_SIZE;

}

2点结论：

a. ArrayList 是通过将底层 Object 数组复制的方式（System.arraycopy方法）来处理数组的增长；

b. 当ArrayList 的容量不足时，其扩充容量的方式：先将容量扩充至当前容量的1.5倍，若还不够，则将容量扩充至当前需要的数量（grow方法）。

remove 方法--ArrayList源码摘要

/**
     * 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; // clear to 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;

}

/*

* Private remove method that skips bounds checking and does not

* return the value removed.

*/

private void fastRemove(int index) {

modCount++;

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

}

这也就解释了 ArrayList 的特点：增加、删除和移动元素的效率低（数组复制过程消耗资源较多）； 而查找元素和更新元素的效率高。

3. ArrayList与Vector的区别

1） vector 是线程同步的，所以它也是线程安全的，而arraylist 是线程异步的，是不安全的。如果不考虑到线程的安全因素，一般用 arraylist效率比较高。

2）如果集合中的元素的数目大于目前集合数组的长度时，vector 增长率为目前数组长度的100%, 而arraylist 增长率为目前数组长度的50% .如果在集合中使用数据量比较大的数据，用vector有一定的优势。

3）如果查找一个指定位置的数据，vector和arraylist使用的时间是相同的，都是O(1) ,这个时候使用vector和arraylist都可以。

而如果移动一个指定位置的数据花费的时间为O(n-i)n为总长度，这个时候就应该考虑到使用linklist ,因为它移动一个指定位置的数据所花费的时间为0(1),而查询一个指定位置的数据时花费的时间为0(i)。