源码分析之List（二）ArrayList

　

• 基于数组实现，是一个动态数组，其容量能自动增长。
• ArrayList不是线程安全的，建议在单线程中使用，多线程可以选择Vector或CopyOnWriteArrayList。
• 实现了RandomAccess接口，可以通过下标序号进行快速访问。
• 实现了Cloneable接口，能被克隆。
• 实现了Serializable接口，支持序列化。

 

ArrayList源码分析

　　展示部分源码：

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
    /**
     * Default initial capacity. 默认初始容量
     */
    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. 使用DEFAULT**的集合，添加第一个元素时，扩展到默认容量
     */
    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;

    /**
     * Constructs an empty list with the specified initial capacity. 有参构造函数
     * @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. 无参构造函数
     */
    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. 有参构造函数，集合转成ArrayList
     * @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();
        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;
        }
    }
    /**
     * 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  最大数组大小
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
     * Returns the number of elements in this list.
     * @return the number of elements in this list
     */
    public int size() {
        return size;
    }

    /**
     * Returns <tt>true</tt> if this list contains no elements.
     * @return <tt>true</tt> if this list contains no elements
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * Returns <tt>true</tt> if this list contains the specified element. More formally, returns <tt>true</tt> if and only if this list contains
     * at least one element <tt>e</tt> such that  <tt>(o==null ? e==null : o.equals(e))</tt>.
     * @param o element whose presence in this list is to be tested
     * @return <tt>true</tt> if this list contains the specified element
     */
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    /**
     * 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 <tt>i</tt> such that  <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, 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 <tt>i</tt> such that  <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, 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;
    }

    /**
     * Returns a shallow copy of this <tt>ArrayList</tt> instance.  (The elements themselves are not copied.) 浅拷贝
     * @return a clone of this <tt>ArrayList</tt> instance
     */
    public Object clone() {
        try {
            ArrayList<?> v = (ArrayList<?>) super.clone();
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);
        }
    }

    /**
     * Returns an array containing all of the elements in this list in proper sequence (from first to last element).
     * @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.  数组长度大于list长度，多余的会被赋值null
     * @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*/
    @SuppressWarnings("unchecked")
    public <T> 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;
    }

    // Positional Access Operations
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    /**
     * 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 elementData(index);
    }

    /**
     * Replaces the element at the specified position in this list with  the specified element. set方法会返回旧值
     *
     * @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;
    }

    /**
     * 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;
    }

    /**
     * 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). 在指定位置增加元素,后面元素顺次后移
     *
     * @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++;
    }

    /**
     * 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;
    }

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

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

        size = 0;
    }

    /**
     * @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.  
     *
     * @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*/
    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;
    }

    /**
     * Removes from this list all of the elements whose index is between {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.*/
    protected void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = size - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);

        // clear to let GC do its work
        int newSize = size - (toIndex-fromIndex);
        for (int i = newSize; i < size; i++) {
            elementData[i] = null;
        }
        size = newSize;
    }/**
     * Removes from this list all of its elements that are contained in the specified collection.
     *
     * @param c collection containing elements to be removed from this list
     * @return {@code true} if this list changed as a result of the call
     * @see Collection#contains(Object)
     */
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }

    /**
     * Retains only the elements in this list that are contained in the specified collection.  In other words, removes from this list all
     * of its elements that are not contained in the specified collection. 从列表中删除所有未包含在指定集合中的元素
     *
     * @param c collection containing elements to be retained in this list
     * @return {@code true} if this list changed as a result of the call
     * @see Collection#contains(Object)
     */
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }

    private boolean batchRemove(Collection<?> c, boolean complement) {
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;
        boolean modified = false;
        try {
            for (; r < size; r++)
                if (c.contains(elementData[r]) == complement)
                    elementData[w++] = elementData[r];
        } finally {
            // Preserve behavioral compatibility with AbstractCollection, even if c.contains() throws.
            if (r != size) {
                System.arraycopy(elementData, r, elementData, w, size - r);
                w += size - r;
            }
            if (w != size) {
                // clear to let GC do its work
                for (int i = w; i < size; i++)
                    elementData[i] = null;
                modCount += size - w;
                size = w;
                modified = true;
            }
        }
        return modified;
    }

    /**
     * Save the state of the <tt>ArrayList</tt> instance to a stream (that is, serialize it).*/
    private void writeObject(java.io.ObjectOutputStream s)throws java.io.IOException{
        // Write out element count, and any hidden stuff
        int expectedModCount = modCount;
        s.defaultWriteObject();

        // Write out size as capacity for behavioural compatibility with clone()
        s.writeInt(size);

        // Write out all elements in the proper order.
        for (int i=0; i<size; i++) {
            s.writeObject(elementData[i]);
        }

        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }

    /**
     * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is, deserialize it).
     */
    private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {
        elementData = EMPTY_ELEMENTDATA;

        // Read in size, and any hidden stuff
        s.defaultReadObject();
        // Read in capacity
        s.readInt(); // ignored

        if (size > 0) {
            // be like clone(), allocate array based upon size not capacity
            ensureCapacityInternal(size);

            Object[] a = elementData;
            // Read in all elements in the proper order.
            for (int i=0; i<size; i++) {
                a[i] = s.readObject();
            }
        }
    }


    @Override
    @SuppressWarnings("unchecked")
    public void sort(Comparator<? super E> c) {
        final int expectedModCount = modCount;
        Arrays.sort((E[]) elementData, 0, size, c);
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }
}

　　总结：

• ArrayList 实际上是通过一个数组去保存数据的。当我们使用无参构造函数构造ArrayList时，则ArrayList的默认容量大小是10。
• 当ArrayList容量不足以容纳全部元素时，ArrayList会重新设置容量：新的容量=“(原始容量x3)/2 + 1”;如果设置后的新容量还不够，则直接把新容量设置为传入的参数。容量最大不超过 Integer.MAX_VALUE - 8
• ArrayList基于数组实现，可以通过下标索引直接查找到指定位置的元素，因此查找效率高，但每次插入或删除元素，就要大量地移动元素，插入删除元素的效率低。
• Object[] toArray()方法。该方法有可能会抛出java.lang.ClassCastException异常。toArray()返回的是 Object[] 数组，Java不支持向下转型。(例如，将Object[]转换为的Integer[])
• System.arraycopy()方法。该方法被标记了native，调用了系统的C/C++代码，该函数实际上最终调用了C语言的memmove()函数，因此它可以保证同一个数组内元素的正确复制和移动，比一般的复制方法的实现效率要高很多，很适合用来批量处理数组