java数据结构之ArrayList源码分析

ArrayList的源码对它的定义为Resizable-array implementation of the List interface,ArrayList是一种动态数组,它可以动态的增加和减少元素,灵活的设置数组的大小。我们知道,数组定义后其大小就无法变化了,那么ArrayList如是如何实现动态数组这个功能的呢?

下面我们就从ArrayList的源码来分析:

## 1、ArrayList的初始化

public class ArrayList extends AbstractList
        implements List, RandomAccess, Cloneable, java.io.Serializable
{
    /**
     * 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.
     */
    // Android-note: Also accessed from java.util.Collections
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;
}
首先,ArrayList的默认容量是10,它里面实际存储数据的是数组elementData

我们再来看下ArrayList的构造函数:

    /**
     * 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) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * 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 c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

可以看到,当我们调用ArrayList的无参构造函数时,elementData等于默认容量下的空数组DEFAULTCAPACITY_EMPTY_ELEMENTDATA;而当我们调用ArrayList(int capacity)的构造函数时,capacity如果合法的话将会覆盖默认容量并给elementData赋予对应的初始值;如果是调用ArrayList(Collection c)的构造方法的话,ArrayList直接取出Collection中的数组并拷贝给elementData。

到这里ArrayList的初始化过程我们已经知道了,其初始化主要就是数组elementData的初始化。
## 2、ArrayList的数据添加

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return true (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    /**
     * 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) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));

        ensureCapacityInternal(size + 1);  // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }

ArrayList的数据添加有两个重载的方法,add(E e)是通过 elementData[size++] = e将数据添加到数组末端,add(int index, E element)是将通过System.arraycopy(elementData, index, elementData, index + 1,size - index)先将elementData[index]及后面的所有数据整体向后移一位,然后通过elementData[index] = element;实现将数据添加到指定位置。而在添加数据之前都会调用ensureCapacityInternal来确保ArrayList的容量能够容得下即将添加的数据。

下面我们来看下ensureCapacityInternal是如何做到这点的。

    /**
     * Increases the capacity of this ArrayList 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 != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
            // any size if not default element table
            ? 0
            // larger than default for default 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 == 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);
    }
    /**
     * 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;
    }

可以看到,ensureCapacityInternal首先判断ArrayList是否时走的无参构造函数-若是的话则此时对应的默认容量是10,而elementData应该等于DEFAULTCAPACITY_EMPTY_ELEMENTDATA,因此minCapacity就等于10和minCapacity中的最大值了,然后调用ensureExplicitCapacity来判断ArrayList是否需要拓容,若要求的容量minCapacity大于当前容量则调用grow来进行拓容。
  grow是这样进行拓容的:先将新容量newCapacity设定为当前容量的1.5倍,然后再看newCapacity是否能满足需求容量minCapacity,如果不满足则直接将newCapacity设为minCapacity,最后还要判断当前要求的容量是否大于MAX_ARRAY_SIZE,若大于的话执行newCapacity = hugeCapacity(minCapacity);在hugeCapacity中首先判断minCapacity是否过大——即是否大于Integer.MAX_VALUE(一个大于Integer.MAX_VALUE的值——比如由long强制转换为int赋给一个int类型的minCapacity时会截断从而产生负值-如Byte.MAX_VALUE(127) +1赋值给一个byte变量会得到-128),如若minCapacity未超过Integer.MAX_VALUE的话则根据其是否大于MAX_ARRAY_SIZE来取值。至此,grow通过将新容量设置为当前容量的1.5倍并经过一系列判断得到最终newCapacity,然后调用 elementData = Arrays.copyOf(elementData, newCapacity)将数组大小拓展为newCapacity。
  上面介绍的是ArrayList添加一个单一的元素,下面我们看下ArrayList直接添加Collection:

    /**
     * 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 true if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection 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 true 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 c) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(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;
    }

其实addAll(Collection c) 和addAll(int index, Collection c)对应add(E e)和add(int index, E e),只不过这两个方法添加的是collection中的数组,在添加之前同样会调用ensureCapacityInternal来确保后面的添加不会超出容量,addAll(Collection c)是直接通过 System.arraycopy(a, 0, elementData, size, numNew);将collection中的数组添加到elementData的尾部;addAll(int index, Collection c)是先通过System.arraycopy(elementData, index, elementData, index + numNew,numMoved);将elementData[index]及后面所有数据整体后移numNew位以给新插入的数组预留位置,然后调用System.arraycopy(a, 0, elementData, index, numNew);将collection中的数组a拷贝到预留出来的位置上,就这样实现了collection的添加。

从上面ArryList的数据添加过程我们已经知道了原理,下面我们来看看ArrayList的数据移除。

## 3、ArrayList的数据移除

    /**
     * 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) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));

        modCount++;
        E oldValue = (E) 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
     * i such that
     * (o==null ? get(i)==null : o.equals(get(i)))
     * (if such an element exists).  Returns true 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 true 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
    }

    /**
     * Removes all of the elements from this list.  The list will
     * be empty after this call returns.
     */
    public void clear() {
        modCount++;

        // clear to let GC do its work
        for (int i = 0; i < size; i++)
            elementData[i] = null;

        size = 0;
    }

移除ArrayList的某一个元素调用remove(Object o),该方法会找出object的位置index并通过fastRemove(int index)来真正移除该元素,移除的核心代码是System.arraycopy(elementData, index+1, elementData, index,numMoved);通过将elementData[index+1]以及其后的所有数据整体向前移一位并将elementData[size-1]置为null,这样先前在index位置的object被覆盖从而不存在了而且数组elementData[size-1]位置也空出来一个空位置可供添加新元素。调用clear()会清除掉elementData中所有的元素。

再来看看ArrayList的数据更新:
## 4、ArrayList的数据更新

    /**
     * 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) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));

        E oldValue = (E) elementData[index];
        elementData[index] = element;
        return oldValue;
    }

数据需要更新的话调用set(int index, E element)将指定位置index处的元素替换为element,该接口很简单,这里就不多介绍了。

最后我们看下ArrayList的数据查询。

## 5、ArrayList的数据获取

    /**
     * Returns an array containing all of the elements in this list
     * in proper sequence (from first to last element).
     *
     * 

The returned array will be "safe" in that no references to it are * maintained by this list. (In other words, this method must allocate * a new array). The caller is thus free to modify the returned array. * *

This method acts as bridge between array-based and collection-based * APIs. * * @return an array containing all of the elements in this list in * proper sequence */ public Object[] toArray() { return Arrays.copyOf(elementData, size); } /** * Returns an array containing all of the elements in this list in proper * sequence (from first to last element); the runtime type of the returned * array is that of the specified array. If the list fits in the * specified array, it is returned therein. Otherwise, a new array is * allocated with the runtime type of the specified array and the size of * this list. * *

If the list fits in the specified array with room to spare * (i.e., the array has more elements than the list), the element in * the array immediately following the end of the collection is set to * null. (This is useful in determining the length of the * list only if the caller knows that the list does not contain * any null elements.) * * @param a the array into which the elements of the list are to * be stored, if it is big enough; otherwise, a new array of the * same runtime type is allocated for this purpose. * @return an array containing the elements of the list * @throws ArrayStoreException if the runtime type of the specified array * is not a supertype of the runtime type of every element in * this list * @throws NullPointerException if the specified array is null */ @SuppressWarnings("unchecked") public T[] toArray(T[] a) { if (a.length < size) // Make a new array of a's runtime type, but my contents: return (T[]) Arrays.copyOf(elementData, size, a.getClass()); System.arraycopy(elementData, 0, a, 0, size); if (a.length > size) a[size] = null; return a; } /** * 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) { if (index >= size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); return (E) elementData[index]; } ...... /** * Returns the index of the first occurrence of the specified element * in this list, or -1 if this list does not contain the element. * More formally, returns the lowest index i such that * (o==null ? get(i)==null : o.equals(get(i))), * or -1 if there is no such index. */ public int indexOf(Object o) { if (o == null) { for (int i = 0; i < size; i++) if (elementData[i]==null) return i; } else { for (int i = 0; i < size; i++) if (o.equals(elementData[i])) return i; } return -1; } /** * Returns the index of the last occurrence of the specified element * in this list, or -1 if this list does not contain the element. * More formally, returns the highest index i such that * (o==null ? get(i)==null : o.equals(get(i))), * or -1 if there is no such index. */ public int lastIndexOf(Object o) { if (o == null) { for (int i = size-1; i >= 0; i--) if (elementData[i]==null) return i; } else { for (int i = size-1; i >= 0; i--) if (o.equals(elementData[i])) return i; } return -1; } ......

首先toArray()和toArray(T[] a) 都是返回ArrayList的数组elementData的一份拷贝,因此对返回的数组进行操作不行影响到elementData本身;get(int index)中如何index合法将返回ArrayList的数组elementData中索引为index的元素-即elementData[index]。indexOf(Object o) 和lastIndexOf(Object o)是检索Object在ArrayList中的位置,只是indexOf是顺序检索数组,而lastIndexOf是逆序检索。

## 6、ArrayList的fail-fast机制

因为ArrayList是非线程安全的,因此采用fail-fast机制来控制对Iterator的使用,从而避免导致程序更严重的错误。ArrayList使用modCount来表示修改的次数,当我们使用iterator()来获取Iterator时,Iterator开始的expectedModCount等于modCount,此时我们进行Iterator的遍历或删除等操作是可以正常运行的,但是倘若此时有另一个线程将要修改ArrayList的数据结构-如添加或者删除操作时modCount会进行+1,在此之后Iterator再进行操作时将会检测到modCount != expectedModCount,进而抛出ConcurrentModificationException异常并退出。

    /**
     * Returns an iterator over the elements in this list in proper sequence.
     *
     * 

The returned iterator is fail-fast. * * @return an iterator over the elements in this list in proper sequence */ public Iterator iterator() { return new Itr(); } /** * An optimized version of AbstractList.Itr */ private class Itr implements Iterator { // Android-changed: Add "limit" field to detect end of iteration. // The "limit" of this iterator. This is the size of the list at the time the // iterator was created. Adding & removing elements will invalidate the iteration // anyway (and cause next() to throw) so saving this value will guarantee that the // value of hasNext() remains stable and won't flap between true and false when elements // are added and removed from the list. protected int limit = ArrayList.this.size; int cursor; // index of next element to return int lastRet = -1; // index of last element returned; -1 if no such int expectedModCount = modCount; public boolean hasNext() { return cursor < limit; } @SuppressWarnings("unchecked") public E next() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); int i = cursor; if (i >= limit) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) throw new ConcurrentModificationException(); cursor = i + 1; return (E) elementData[lastRet = i]; } public void remove() { if (lastRet < 0) throw new IllegalStateException(); if (modCount != expectedModCount) throw new ConcurrentModificationException(); try { ArrayList.this.remove(lastRet); cursor = lastRet; lastRet = -1; expectedModCount = modCount; limit--; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } @Override @SuppressWarnings("unchecked") public void forEachRemaining(Consumer consumer) { Objects.requireNonNull(consumer); final int size = ArrayList.this.size; int i = cursor; if (i >= size) { return; } final Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) { throw new ConcurrentModificationException(); } while (i != size && modCount == expectedModCount) { consumer.accept((E) elementData[i++]); } // update once at end of iteration to reduce heap write traffic cursor = i; lastRet = i - 1; if (modCount != expectedModCount) throw new ConcurrentModificationException(); } }

至此,ArrayList的数据处理原理我们已经分析完了,

总的来说,ArrayList的动态数组功能主要是通过ensureCapacityInternal和System.arraycopy来实现的。首先通过ensureCapacityInternal来确保下一步的数据操作不会超出ArrayList的容量,然后通过System.arraycopy来进行数组元素的变迁。

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