java读书笔记：ArrayList源码详解(基于jdk1.8)

 //继承AbstractSequentialList，实现list接口，支持随机访问，支持克隆，支持序列化。public class ArrayList<E> extends AbstractList<E>        implements List<E>, RandomAccess, Cloneable, java.io.Serializable{    private static final long serialVersionUID = 8683452581122892189L;    /**     * The array buffer into which the elements of the ArrayList are stored.     * The capacity of the ArrayList is the length of this array buffer.     */    //存放object类型的数组    private transient Object[] elementData;    /**     * The size of the ArrayList (the number of elements it contains).     *     * @serial     */    //数组大小    private int size;    /**     * 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     */   //带初始化大小的构造器    public ArrayList(int initialCapacity) {        super();        if (initialCapacity < 0)            throw new IllegalArgumentException("Illegal Capacity: "+                                               initialCapacity);        this.elementData = new Object[initialCapacity];    }    /**     * Constructs an empty list with an initial capacity of ten.     */   //空的构造器，默认为10    public ArrayList() {        this(10);    }    /**     * 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     */   //初始化值的构造器    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);    }    /**     * Trims the capacity of this <tt>ArrayList</tt> instance to be the     * list's current size.  An application can use this operation to minimize     * the storage of an <tt>ArrayList</tt> instance.     */    //将arraylist数组调整为实际大小，这是对扩容方式（每次扩容为原来大小的1.5倍）的一个修正    public void trimToSize() {        modCount++;        int oldCapacity = elementData.length;        if (size < oldCapacity) {            elementData = Arrays.copyOf(elementData, size);        }    } * @param      src      the source array.     * @param      srcPos   starting position in the source array.     * @param      dest     the destination array.     * @param      destPos  starting position in the destination data.     * @param      length   the number of array elements to be copied.     * @exception  IndexOutOfBoundsException  if copying would cause     *               access of data outside array bounds.     * @exception  ArrayStoreException  if an element in the <code>src</code>     *               array could not be stored into the <code>dest</code> array     *               because of a type mismatch.     * @exception  NullPointerException if either <code>src</code> or     *               <code>dest</code> is <code>null</code>.     */      //arraycopy这个方法的功能就是，将一个数组添加到另一个数组中，添加的数组从srcPos位置开始复制     //被添加的数组，从destpos处开始接收添加数组的元素，添加的length长度个    public static native void arraycopy(Object src,  int  srcPos,                                        Object dest, int destPos,                                        int length);      /**     * 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     */   //扩容检测    public void ensureCapacity(int minCapacity) {        if (minCapacity > 0)            ensureCapacityInternal(minCapacity);    }   //扩容检测    private void ensureCapacityInternal(int minCapacity) {      //计时器        modCount++;        // overflow-conscious code        if (minCapacity - elementData.length > 0)           //扩容方法            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        //老的容量大小        int oldCapacity = elementData.length;         //准备扩容的大小，右移符号相当于除，一位就是除以二的一次方，因为计算机是二进制        //这句话的意思就是预扩容的大小等于原来的1.5倍，        //具体扩容1.5倍原因可以参考 : http://stackoverflow.com/questions/3336291/logic-used-in-ensurecapacity-method-in-arraylist        int newCapacity = oldCapacity + (oldCapacity >> 1);        //比较预扩容和实际需要最小容量的大小        if (newCapacity - minCapacity < 0)            newCapacity = minCapacity;      //预扩容和integer最大值-8的比较，这是个天文数字，估计普通人一辈子都用不到      //MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;        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);    }       //与最大容量比较，返回值是integer最大值-8或者integer最大值，这地方很拗口，不知道bloch大神怎么想的？    private static int hugeCapacity(int minCapacity) {        if (minCapacity < 0) // overflow            throw new OutOfMemoryError();        return (minCapacity > MAX_ARRAY_SIZE) ?            Integer.MAX_VALUE :            MAX_ARRAY_SIZE;    }    /**     * 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&nbsp;?&nbsp;e==null&nbsp;:&nbsp;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&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,     * or -1 if there is no such index.     */    //查找元素第一次出现的位置，可以理解为从左向右查找    public int indexOf(Object o) {        //元素为null时        if (o == null) {            for (int i = 0; i < size; i++)                if (elementData[i]==null)                    return i;        } else {         //有实际值时            for (int i = 0; i < size; i++)                //使用equals，比较的是值和类型都要一样                if (o.equals(elementData[i]))                    return i;        }       //否则返回 -1        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&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,     * or -1 if there is no such index.     */   //最后一次出现的位置，可以理解为从右向左查找，和前一个方法放一起形成一个二分法    public int lastIndexOf(Object o) {         //为空时        if (o == null) {            for (int i = size-1; i >= 0; i--)                if (elementData[i]==null)                    return i;        } else {         //有值时            for (int i = size-1; i >= 0; i--)                if (o.equals(elementData[i]))                    return i;        }        //否则返回 -1         return -1;    }    /**     * Returns a shallow copy of this <tt>ArrayList</tt> instance.  (The     * elements themselves are not copied.)     *     * @return a clone of this <tt>ArrayList</tt> instance     */        //克隆，关键是Arrays.copyOf(elementData, size)这个方法    public Object clone() {        try {            @SuppressWarnings("unchecked")                ArrayList<E> v = (ArrayList<E>) 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();        }    }    /**     * Returns an array containing all of the elements in this list     * in proper sequence (from first to last element).     *     * <p>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.     *     * <p>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     */   //返回一个新的arraylist数组    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.     *     * <p>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     * <tt>null</tt>.  (This is useful in determining the length of the     * list <i>only</i> 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")     //返回T类型的arrayList数组    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")    //得到index位置的元素    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}     */     调用elementData方法，返回index位置的元素    public E get(int index) {      //检查index位置的元素        rangeCheck(index);        return elementData(index);    }    /**     * 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}     */    //将index位置的元素替换为element，并返回原来的值    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})     */     //增加元素    public boolean add(E e) {        //扩容        ensureCapacityInternal(size + 1);  // Increments modCount!!         //将数组的末尾引用指向增加的元素        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}     */    //将元素增加到指定位置，之后的元素整体向后面移动一位    public void add(int index, E element) {        rangeCheckForAdd(index);        ensureCapacityInternal(size + 1);  // Increments modCount!!       //这里是关键，意思是将旧的数组从index位置copy到新的数组里      //存放从index+1开始，向后copy size-index个位置，其实就相当于将index位置空出来      //原来的元素整体向后移动一位。        System.arraycopy(elementData, index, elementData, index + 1,                         size - index);       //index位置的引用指向新加进来的元素        elementData[index] = element;      //容量加一        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}     */     //删除index位置的元素，并返回它，index后的元素总体向前移动一位    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);        //数组移动后的最后一位的值设为null，等待gc回收        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     */    //删除第一次出现的值    public boolean remove(Object o) {        //值为空时，araylist允许值为空        if (o == null) {            for (int index = 0; index < size; index++)                if (elementData[index] == null) {                    fastRemove(index);                    return true;                }        } else {           //值不为空时            for (int index = 0; index < size; index++)                if (o.equals(elementData[index])) {                    fastRemove(index);                    return true;                }        }        //否则返回false        return false;    }    /*     * Private remove method that skips bounds checking and does not     * return the value removed.     */   //快速删除    private void fastRemove(int index) {        modCount++;        int numMoved = size - index - 1;        if (numMoved > 0)            System.arraycopy(elementData, index+1, elementData, index,                             numMoved);          ////数组移动后的最后一位的值设为null，等待gc回收        elementData[--size] = null; // 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++;         //清除        // Let gc do its work        for (int i = 0; i < size; i++)            elementData[i] = null;        size = 0;    }    /**     * 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     */     //将全部的object元素加入到arraylist数组中    public boolean addAll(Collection<? extends E> c) {      //得到一个包含所有元素的arraylist数组        Object[] a = c.toArray();        //得到长度，为复制做准备        int numNew = a.length;       //扩容        ensureCapacityInternal(size + numNew);  // Increments modCount         //将a数组从第一个开始copy，copy到elementData，从size处开始存放，存放numNew个        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     */      //将数组从index位置开始存放，返回成功或者失败    public boolean addAll(int index, Collection<? extends E> c) {        rangeCheckForAdd(index);        //得到一个临时的arraylist        Object[] a = c.toArray();       //要增加的长度        int numNew = a.length;       //扩容        ensureCapacityInternal(size + numNew);  // Increments modCount       //从何处开始移动        int numMoved = size - index;        if (numMoved > 0)              //元素从index + numNew位置整体向后移动numMoved位            System.arraycopy(elementData, index, elementData, index + numNew,                             numMoved);        System.arraycopy(a, 0, elementData, index, numNew);        size += numNew;        return numNew != 0;    }    //后面的都是迭代和一些辅助性功能，就不写解读了。    /**     * Removes from this list all of the elements whose index is between     * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.     * Shifts any succeeding elements to the left (reduces their index).     * This call shortens the list by {@code (toIndex - fromIndex)} elements.     * (If {@code toIndex==fromIndex}, this operation has no effect.)     *     * @throws IndexOutOfBoundsException if {@code fromIndex} or     *         {@code toIndex} is out of range     *         ({@code fromIndex < 0 ||     *          fromIndex >= size() ||     *          toIndex > size() ||     *          toIndex < fromIndex})     */    protected void removeRange(int fromIndex, int toIndex) {        modCount++;        int numMoved = size - toIndex;        System.arraycopy(elementData, toIndex, elementData, fromIndex,                         numMoved);        // Let gc do its work        int newSize = size - (toIndex-fromIndex);        while (size != newSize)            elementData[--size] = null;    }    /**     * 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));    }    /**     * A version of rangeCheck used by add and addAll.     */    private void rangeCheckForAdd(int index) {        if (index > size || index < 0)            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));    }    /**     * Constructs an IndexOutOfBoundsException detail message.     * Of the many possible refactorings of the error handling code,     * this "outlining" performs best with both server and client VMs.     */    private String outOfBoundsMsg(int index) {        return "Index: "+index+", Size: "+size;    }    /**     * 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     * @throws ClassCastException if the class of an element of this list     *         is incompatible with the specified collection     * (<a href="Collection.html#optional-restrictions">optional</a>)     * @throws NullPointerException if this list contains a null element and the     *         specified collection does not permit null elements     * (<a href="Collection.html#optional-restrictions">optional</a>),     *         or if the specified collection is null     * @see Collection#contains(Object)     */    public boolean removeAll(Collection<?> 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.     *     * @param c collection containing elements to be retained in this list     * @return {@code true} if this list changed as a result of the call     * @throws ClassCastException if the class of an element of this list     *         is incompatible with the specified collection     * (<a href="Collection.html#optional-restrictions">optional</a>)     * @throws NullPointerException if this list contains a null element and the     *         specified collection does not permit null elements     * (<a href="Collection.html#optional-restrictions">optional</a>),     *         or if the specified collection is null     * @see Collection#contains(Object)     */    public boolean retainAll(Collection<?> 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) {                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).     *     * @serialData The length of the array backing the <tt>ArrayList</tt>     *             instance is emitted (int), followed by all of its elements     *             (each an <tt>Object</tt>) in the proper order.     */    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 array length        s.writeInt(elementData.length);        // 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 {        // Read in size, and any hidden stuff        s.defaultReadObject();        // Read in array length and allocate array        int arrayLength = s.readInt();        Object[] a = elementData = new Object[arrayLength];        // Read in all elements in the proper order.        for (int i=0; i<size; i++)            a[i] = s.readObject();    }    /**     * Returns a list iterator over the elements in this list (in proper     * sequence), starting at the specified position in the list.     * The specified index indicates the first element that would be     * returned by an initial call to {@link ListIterator#next next}.     * An initial call to {@link ListIterator#previous previous} would     * return the element with the specified index minus one.     *     * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.     *     * @throws IndexOutOfBoundsException {@inheritDoc}     */    public ListIterator<E> listIterator(int index) {        if (index < 0 || index > size)            throw new IndexOutOfBoundsException("Index: "+index);        return new ListItr(index);    }    /**     * Returns a list iterator over the elements in this list (in proper     * sequence).     *     * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.     *     * @see #listIterator(int)     */    public ListIterator<E> listIterator() {        return new ListItr(0);    }    /**     * Returns an iterator over the elements in this list in proper sequence.     *     * <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>.     *     * @return an iterator over the elements in this list in proper sequence     */    public Iterator<E> iterator() {        return new Itr();    }    /**     * An optimized version of AbstractList.Itr     */    private class Itr implements Iterator<E> {        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 != size;        }        @SuppressWarnings("unchecked")        public E next() {            checkForComodification();            int i = cursor;            if (i >= size)                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();            checkForComodification();            try {                ArrayList.this.remove(lastRet);                cursor = lastRet;                lastRet = -1;                expectedModCount = modCount;            } catch (IndexOutOfBoundsException ex) {                throw new ConcurrentModificationException();            }        }        final void checkForComodification() {            if (modCount != expectedModCount)                throw new ConcurrentModificationException();        }    }    /**     * An optimized version of AbstractList.ListItr     */    private class ListItr extends Itr implements ListIterator<E> {        ListItr(int index) {            super();            cursor = index;        }        public boolean hasPrevious() {            return cursor != 0;        }        public int nextIndex() {            return cursor;        }        public int previousIndex() {            return cursor - 1;        }        @SuppressWarnings("unchecked")        public E previous() {            checkForComodification();            int i = cursor - 1;            if (i < 0)                throw new NoSuchElementException();            Object[] elementData = ArrayList.this.elementData;            if (i >= elementData.length)                throw new ConcurrentModificationException();            cursor = i;            return (E) elementData[lastRet = i];        }        public void set(E e) {            if (lastRet < 0)                throw new IllegalStateException();            checkForComodification();            try {                ArrayList.this.set(lastRet, e);            } catch (IndexOutOfBoundsException ex) {                throw new ConcurrentModificationException();            }        }        public void add(E e) {            checkForComodification();            try {                int i = cursor;                ArrayList.this.add(i, e);                cursor = i + 1;                lastRet = -1;                expectedModCount = modCount;            } catch (IndexOutOfBoundsException ex) {                throw new ConcurrentModificationException();            }        }    }    /**     * Returns a view of the portion of this list between the specified     * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.  (If     * {@code fromIndex} and {@code toIndex} are equal, the returned list is     * empty.)  The returned list is backed by this list, so non-structural     * changes in the returned list are reflected in this list, and vice-versa.     * The returned list supports all of the optional list operations.     *     * <p>This method eliminates the need for explicit range operations (of     * the sort that commonly exist for arrays).  Any operation that expects     * a list can be used as a range operation by passing a subList view     * instead of a whole list.  For example, the following idiom     * removes a range of elements from a list:     * <pre>     *      list.subList(from, to).clear();     * </pre>     * Similar idioms may be constructed for {@link #indexOf(Object)} and     * {@link #lastIndexOf(Object)}, and all of the algorithms in the     * {@link Collections} class can be applied to a subList.     *     * <p>The semantics of the list returned by this method become undefined if     * the backing list (i.e., this list) is <i>structurally modified</i> in     * any way other than via the returned list.  (Structural modifications are     * those that change the size of this list, or otherwise perturb it in such     * a fashion that iterations in progress may yield incorrect results.)     *     * @throws IndexOutOfBoundsException {@inheritDoc}     * @throws IllegalArgumentException {@inheritDoc}     */    public List<E> subList(int fromIndex, int toIndex) {        subListRangeCheck(fromIndex, toIndex, size);        return new SubList(this, 0, fromIndex, toIndex);    }    static void subListRangeCheck(int fromIndex, int toIndex, int size) {        if (fromIndex < 0)            throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);        if (toIndex > size)            throw new IndexOutOfBoundsException("toIndex = " + toIndex);        if (fromIndex > toIndex)            throw new IllegalArgumentException("fromIndex(" + fromIndex +                                               ") > toIndex(" + toIndex + ")");    }    private class SubList extends AbstractList<E> implements RandomAccess {        private final AbstractList<E> parent;        private final int parentOffset;        private final int offset;        int size;        SubList(AbstractList<E> parent,                int offset, int fromIndex, int toIndex) {            this.parent = parent;            this.parentOffset = fromIndex;            this.offset = offset + fromIndex;            this.size = toIndex - fromIndex;            this.modCount = ArrayList.this.modCount;        }        public E set(int index, E e) {            rangeCheck(index);            checkForComodification();            E oldValue = ArrayList.this.elementData(offset + index);            ArrayList.this.elementData[offset + index] = e;            return oldValue;        }        public E get(int index) {            rangeCheck(index);            checkForComodification();            return ArrayList.this.elementData(offset + index);        }        public int size() {            checkForComodification();            return this.size;        }        public void add(int index, E e) {            rangeCheckForAdd(index);            checkForComodification();            parent.add(parentOffset + index, e);            this.modCount = parent.modCount;            this.size++;        }        public E remove(int index) {            rangeCheck(index);            checkForComodification();            E result = parent.remove(parentOffset + index);            this.modCount = parent.modCount;            this.size--;            return result;        }        protected void removeRange(int fromIndex, int toIndex) {            checkForComodification();            parent.removeRange(parentOffset + fromIndex,                               parentOffset + toIndex);            this.modCount = parent.modCount;            this.size -= toIndex - fromIndex;        }        public boolean addAll(Collection<? extends E> c) {            return addAll(this.size, c);        }        public boolean addAll(int index, Collection<? extends E> c) {            rangeCheckForAdd(index);            int cSize = c.size();            if (cSize==0)                return false;            checkForComodification();            parent.addAll(parentOffset + index, c);            this.modCount = parent.modCount;            this.size += cSize;            return true;        }        public Iterator<E> iterator() {            return listIterator();        }        public ListIterator<E> listIterator(final int index) {            checkForComodification();            rangeCheckForAdd(index);            final int offset = this.offset;            return new ListIterator<E>() {                int cursor = index;                int lastRet = -1;                int expectedModCount = ArrayList.this.modCount;                public boolean hasNext() {                    return cursor != SubList.this.size;                }                @SuppressWarnings("unchecked")                public E next() {                    checkForComodification();                    int i = cursor;                    if (i >= SubList.this.size)                        throw new NoSuchElementException();                    Object[] elementData = ArrayList.this.elementData;                    if (offset + i >= elementData.length)                        throw new ConcurrentModificationException();                    cursor = i + 1;                    return (E) elementData[offset + (lastRet = i)];                }                public boolean hasPrevious() {                    return cursor != 0;                }                @SuppressWarnings("unchecked")                public E previous() {                    checkForComodification();                    int i = cursor - 1;                    if (i < 0)                        throw new NoSuchElementException();                    Object[] elementData = ArrayList.this.elementData;                    if (offset + i >= elementData.length)                        throw new ConcurrentModificationException();                    cursor = i;                    return (E) elementData[offset + (lastRet = i)];                }                public int nextIndex() {                    return cursor;                }                public int previousIndex() {                    return cursor - 1;                }                public void remove() {                    if (lastRet < 0)                        throw new IllegalStateException();                    checkForComodification();                    try {                        SubList.this.remove(lastRet);                        cursor = lastRet;                        lastRet = -1;                        expectedModCount = ArrayList.this.modCount;                    } catch (IndexOutOfBoundsException ex) {                        throw new ConcurrentModificationException();                    }                }                public void set(E e) {                    if (lastRet < 0)                        throw new IllegalStateException();                    checkForComodification();                    try {                        ArrayList.this.set(offset + lastRet, e);                    } catch (IndexOutOfBoundsException ex) {                        throw new ConcurrentModificationException();                    }                }                public void add(E e) {                    checkForComodification();                    try {                        int i = cursor;                        SubList.this.add(i, e);                        cursor = i + 1;                        lastRet = -1;                        expectedModCount = ArrayList.this.modCount;                    } catch (IndexOutOfBoundsException ex) {                        throw new ConcurrentModificationException();                    }                }                final void checkForComodification() {                    if (expectedModCount != ArrayList.this.modCount)                        throw new ConcurrentModificationException();                }            };        }        public List<E> subList(int fromIndex, int toIndex) {            subListRangeCheck(fromIndex, toIndex, size);            return new SubList(this, offset, fromIndex, toIndex);        }        private void rangeCheck(int index) {            if (index < 0 || index >= this.size)                throw new IndexOutOfBoundsException(outOfBoundsMsg(index));        }        private void rangeCheckForAdd(int index) {            if (index < 0 || index > this.size)                throw new IndexOutOfBoundsException(outOfBoundsMsg(index));        }        private String outOfBoundsMsg(int index) {            return "Index: "+index+", Size: "+this.size;        }        private void checkForComodification() {            if (ArrayList.this.modCount != this.modCount)                throw new ConcurrentModificationException();        }    }}