ArrayList 源码解析

Posted 2020-07-06 StubbornAnt

本解析源码来自JDK1.7

ArrayList 概要

• 继承Collection接口，实现了随机存取，自动扩容
• 内部使用数组进行存储，size表示List中元素的个数

            private transient Object[] elementData;
            private int size;

• 允许null值
• 与Vector基本相同，但是ArrayList是非同步的，可以使用 List list = Collections.synchronizedList(new ArrayList(...))做同步
• ArrayList的线性操作比LinkedList的操作的常数项系数要小
• 有fast-fail机制

一些常量

DEFAULT_CAPACITY = 10
Object[] EMPTY_ELEMENTDATA = {}
MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8

实现接口

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable  

• List 包含了ArrayList的主要操作方法的描述
• RandomAccess 标志接口，表明随机访问效率高
• Cloneable 标志接口，表明可以进行深拷贝
• Serializable 可序列化

构造函数

主要有三种构造方式
- 给定初始大小，创建initialCapacity的数组
- 不给定大小，使用空数组
- 给定另个一Collection c，将集合c的转换为数组，拷贝数组作为成员数组，原集合大小作为size

    public ArrayList(int initialCapacity) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
    } 
    public ArrayList() {
        super();
        this.elementData = EMPTY_ELEMENTDATA;
    }
    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);
    } 

List 接口主要方法

public interface List<E> extends Collection<E> {
    // Query Operations
    int size();
    boolean isEmpty();
    boolean contains(Object o);
    Iterator<E> iterator();
    Object[] toArray();
    <T> T[] toArray(T[] a);
    // Modification Operations
    boolean add(E e);
    boolean remove(Object o);
    // Bulk Modification Operations
    boolean containsAll(Collection<?> c);
    boolean addAll(Collection<? extends E> c);
    boolean addAll(int index, Collection<? extends E> c);
    boolean removeAll(Collection<?> c);
    boolean retainAll(Collection<?> c);
    void clear();
    // Comparison and hashing
    boolean equals(Object o);
    int hashCode();
    // Positional Access Operations
    E get(int index);
    E set(int index, E element);
    void add(int index, E element);
    E remove(int index);
    // Search Operations
    int indexOf(Object o);
    int lastIndexOf(Object o);
    // List Iterators
    ListIterator<E> listIterator();
    ListIterator<E> listIterator(int index);
    // View
    List<E> subList(int fromIndex, int toIndex);
}

存入元素

主要方法

• set(int index, E element) 更新指定位置元素
• add(E e) 末尾添加元素
• add(int i,E e) 指定位置后添加元素
• addAll(Collection c) 末尾批量添加元素
• addAll(int i, Collection) 指定位置批量添加元素

主要操作

• 检查插入index的合法性 rangecheck（index）
  
  • rangeCheck 只检查index是否**大于等于**size，不检查index是否小于零，因为rangeCheck后总是数组操作，所以会抛出ArrayIndexOutOfBoundsException，而不是IndexOutOfBoundsExcepiton
  • rangeCheckForAdd 检查index是否**大于**size或小于0，抛出IndexOutOfBoundsExcepiton
  • 异常信息为 Index：index，Size：size
• 确保数组有足够容量 ensureCapacityInternal（minCapacity）
• 将数据拷贝到数组中
  利用Arrays.copyof（originalArray，length）来进行扩容，length指定新数组长度，如果新数组大于原数组长度，其余部分补null

    public E set(int index, E element) {
        rangeCheck(index);
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    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++;
    }
    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;
    }
    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;
    }      
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }  
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }  
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        ensureExplicitCapacity(minCapacity);
    }  

容量扩展

• ensureCapacityInternal 如果数组为空，就扩展数组为max（默认大小，给定的minCapacity）大小，否则扩展到给定最小值大小
• 检查是否有足够容量，如果没有调用grow(minCapacity)进行扩容
• 默认扩展到1.5倍
• 如果扩展1.5倍后还是小于给定最小值，就要扩展到的大小为给定最小值
• 如果要扩展到的大小大于MAX_ARRAY_SIZE，就扩展到 Integer.MAX_VALUE

NOTE
- 共有的扩展数组方法 ensureCapacity（minCapacity）
ArrayList提供了共有的保证容量的方法，一般情况下我们是不用管扩容的事情，但是当我们预计要插入的元素个数比较多时，且数量可估计时，手动确保List容量可以避免频繁的扩容，造成的数据频繁的拷贝，从而提高效率
- 默认初始化会将数组初始化为空，存入元素时直接扩展到DefaultCapacity
如果List为空，只有minCapacity大于DefaultCapacity时才需要扩容，如果List不为空，大于零就需要扩容。
- 扩展的相反操作 trimToSize(int size)
如果设定的size小于数组的长度，用Arrays.copyof(array,length)复制创建一个给定大小的数组

    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        ensureExplicitCapacity(minCapacity);
    }  
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }  
    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;
    }  
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != EMPTY_ELEMENTDATA)
            // any size if real element table
            ? 0
            // larger than default for empty table. It‘s already supposed to be
            // at default size.
            : DEFAULT_CAPACITY;
        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }  

读取元素 get(index)

• 首先进行index合法性检查，注意不检查小于零的情况，而是由数组抛出ArrayIndexOutOfBoundsException
• 进行类型转换，返回

    public E get(int index) {
        rangeCheck(index);
        return elementData(index);
    } 
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    } 

查找

• 采用逐个遍历的方法进行查找
• indexOf从前往后找，lastIndexOf从后往前找，
• contains（e） 调用 indexOf进行查找
  NOTE
  由于允许null值，所以null需要单独处理，而不能用elementData[i].equals(o)

    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;
    } 
    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;
    }
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    } 

删除元素

删除元素主要过程：检查参数合法性 其后面元素前移，然后把最后面的元素全部置为null。
- remove（index） 保存原index值用来返回，index后的元素向前拷贝，最后值赋值null以回收

    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;
    }  

• remove（object） 遍历找到对应元素，不需要进行index合法性检查，直接删除

    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 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
    }  

• removeAll（collection），retainAll（collection）
  
  • removeAll（collection） 与 retainAll（collection）是两个相反的过程，一个保留给定集合包含的元素，相当于求交集，一个删除给定集合包含的元素，相当于求差集
  • batchRemove 使用双指针的方法，将要保留的值赋值到前面，然后将后面的值置null
    NOTE
    其中finally语句中，r<\size 的情况是由于Collection中的contains方法有可能抛出NullPointerException和ClassCastException，如果出现异常，就将当前w之后的值复制到r之后，不再检查是否包含，而后r之后的值依旧赋值为null

    public boolean removeAll(Collection<?> c) {
        return batchRemove(c, false);
    } 
    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) {
                // 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;
    }

Fast-Fail机制

• 源码中凡是修改List结构（插入，删除，打乱顺序，调整容量，不包含set更新元素），都会涉及到modCount++
• 在ArrayList类创建迭代器之后，除非通过迭代器自身remove或add对列表结构进行修改，否则在其他线程中以任何形式对列表进行修改，迭代器马上会抛出异常，快速失败。
• 该机制通过检查modCount的值来确定是否迭代过程中有其他线程对列表进行修改

        private void checkForComodification() {
            if (ArrayList.this.modCount != this.modCount)
                throw new ConcurrentModificationException();
        }  

复制方法 Clone

浅拷贝，拷贝引用，而非创建新对象，如果引用指向的对象改变也会跟着改变

    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();
        }
    }  

转换为数组 toArray

• toArray（）返回新的Object数组
• toArray(T[]) 如果实参数组长度小于List，返回一个新的T类型数组，如果实参数组大于List，执行复制拷贝，将紧邻的后一个置为null（在list不含null时，便于检查List长度），返回实参数组

    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    } 
    @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;
    } 

序列化方法

• elementData声明为transient是不会被序列化写入的，所以序列化分两步，首先执行通用序列化，然后将数组中的元素逐一读入，或写出
• 在进行写入的过程中有fast-fail机制，即这个过程中有对list的修改会造成失败
• 虽然size单独写入了，但是在readObject中这个值并没有用到，为了与clone方法适配

    private transient Object[] elementData;  
    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();
        }
    }
    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

• 迭代器作为ArrayList的内部类，可以直接访问修改ArrayList
• ArrayList实现了两种迭代器，其中ListIterator除了实现基本的Iterator方法（hasNext，Next，remove），还包含更丰富的方法
• ListIterator额外实现的方法
  
  • listIterator(index) 可以指定开始遍历的位置
  • hasPrevious 有没有前驱
  • previous 返回前驱
  • add 实现添加元素
  • set 更新上次访问的元素
  • previousIndex() nextIndex() 返回下一个，和上一个位置
    NOTE
    remove，add，set方法都是改变上次被访问元素位置进行操作，连续调用两次以上就会出现问题

    public ListIterator<E> listIterator(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }
    public ListIterator<E> listIterator() {
        return new ListItr(0);
    }
    public Iterator<E> iterator() {
        return new 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();
        }
    }
    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();
            }
        }
    }  

子视图 SubList

• 首先进行位置检查，如果合法，就调用新建SubList对象
• ArrayList将自身作为参数传递，也就是说，对SubList的操作其实是对原ArrayList的操作，SubList的方法跟ArrayList相近，只是需要进行index的转换，加上fromIndex
• SubList 只会返回ListIterator，其ListIterator对象通过匿名内部类的方式定义

    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();
        }
    }