ArrayList源码学习

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    //序列化编号
    private static final long serialVersionUID = 8683452581122892189L;

    //默认容量
    private static final int DEFAULT_CAPACITY = 10;

    
   //初始化一个空数组
    private static final Object[] EMPTY_ELEMENTDATA = {};

    //默认空数组
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    //ArrayList的存储容器，不参与序列化
    transient Object[] elementData; // non-private to simplify nested class access

   //ArrayList中的元素容量
    private int size;

    //初始化带参的ArrayList
    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);
        }
    }

    //初始化无参的ArrayList
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
    
    //初始化带集合的ArrayList，如果不是Object类型的元素，那么就直接使用Arrays工具来复制数组
    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;
        }
    }

    //把ArrayList的大小改成现在所拥有的元素个数
    public void trimToSize() {
        modCount++;  //modCount的作用是用来记录ArrayList的修改次数
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }

    //如果数组是默认的空数组，则minExpand为0，否则则为10，如果最小容量大于最小扩展量，则将数组扩容为最小容量
    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);
        }
    }
    
    //如果数组是空数组，则在默认容量和参数minCapacity选最大值，作为minCapacity的值
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

    //集合进行了修改，modCount加1，如果原数组大小比参数minCapacity大，则进行扩容处理
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

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

//扩容操作
//先用oldCapacity记录原数组大小
//用newCapacity记录原数组扩容1.5倍的大小，其中>>表示二进制的向右移1位，效果相当于除于2
//如果newCapactiy比参数minCapacity小就使用minCapacity作为扩容之后数组容量的值
//如果minCapacity比设定好的数组的最大值还要大，就使用Integer.MAX_VALUE来作为新数组的容量
    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;
    }

    //返回数组存储元素的数目
    public int size() {
        return size;
    }

    //检测数组是否有元素
    public boolean isEmpty() {
        return size == 0;
    }

   //检测数组中是否包含特定元素o
    //如果返回的是一个大于-1的整数，说明存在这样的元素
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

//如果元素为null，那么就查找数组中是否有值为null的元素
//如果元素不为null，则查找是否有特定元素
//有则返回下标位置，无则返回-1
//此方法为从前往后地查找
    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;
    }

    //功用与indexOf(Object o)相同，但查找方向相反
    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;
    }

   //创建一个新的数组，并把之前数组的所有元素复制一遍，但是区别在于，modCount为0，即产生了一个新的数组
    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);
        }
    }

    //将ArrayList中的元素以数组的形式返回
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }
    
    //返回指定类型的数组，注意这里使用的不是Arrays.copyOf()，而是System.arraycopy()
    @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;
    }
    
    //返回指定位置的元素
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    //先对指定位置进行范围检测，然后再进行指定位置元素返回
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    //对指定位置的元素的值重设为element，返回重设前的数值
    public E set(int index, E element) {
        rangeCheck(index);

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

//此操作有对modCount进行自增
//添加元素的时候有对原数组大小进行判断，如果数组仍有位置，则插入新元素，如果没有就扩容
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

//先对指定位置进行检测，然后对数组容量进行检测
//将指定位置及其之后的元素都向后退一个位置
//再把新元素放到指定位置上
    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++;
    }

//移除指定位置的元素
//给数组多出一个空位子出来，用于进行GC处理
    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;
    }

   //移除第一个出现的指定元素
    //先找到指定元素在数组中的位置，然后再进行移除操作
    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;
    }

    //移除指定位置的元素
    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
    }

    //把集合中的元素全部清空
    public void clear() {
        modCount++;

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

        size = 0;
    }

    //将指定集合中的元素都添加进集合中
    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;
    }

    //把指定位置的集合元素全部添加进集合中
    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;
    }

    //把指定范围的元素都移除
    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;
    }

//检测指定位置是否在存有元素的数组范围内
//用于元素查找功能
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

   //检测指定位置是否合法
    //用于元素添加功能
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    //如果指定位置不合法，则返回提示信息
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

//先判断集合是否合法
//然后将不存在于指定集合中的元素全部移除
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }

    //把存在于指定集合中的元素从集合中全部移除
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }
    
    //这是一个有着类似开关装置的方法
    //如果complement为false，则移除所有指定集合中没有的元素
    //如果complement为true，则移除所有指定集合中有的元素
    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;
    }

    //流操作方法，将数组对象写入流中
    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();
        }
    }

   //流操作方法，将信息从流中读出
    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();
            }
        }
    }

   //迭代器，返回内部类实例
    public ListIterator<E> listIterator(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }

    //迭代器，返回内部类实例
    public ListIterator<E> listIterator() {
        return new ListItr(0);
    }

   //迭代器，返回内部类实例
    public Iterator<E> iterator() {
        return new Itr();
    }

}

/**
     * 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 = {};