ArrayList，

ArrayList

ArrayList是基于底层数组的实现List。故对于LinkedList,ArrayListy善于随机访问数组，插入和删除则比较慢。

ArrayList的定义

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

• 从ArrayList<\E>可以看出它是支持泛型的，它继承自AbstractList，实现了List、RandomAccess、Cloneable、java.io.Serializable接口。

常量

private static final long serialVersionUID = 8683452581122892189L;
private static final int DEFAULT_CAPACITY = 10;
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
transient Object[] elementData; 
private int size;

• serialVersionUID ：Java的序列化机制是通过serialVersionUID来验证版本一致性。
• DEFAULT_CAPACITY ：数组初始容量。
• EMPTY_ELEMENTDATA：空数组，用以区别DEFAULTCAPACITY_EMPTY_ELEMENTDATA 。
• DEFAULTCAPACITY_EMPTY_ELEMENTDATA ：空数组，无参构造器被使用。
• size：表示数组的大小。

构造器

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);
        }
}

public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}


 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;
        }
    }

• ArrayList(int initialCapacity)：有参构造器，创建容量为initialCapacity的数组。
• ArrayList() ：无参构造器，elementData指向DEFAULTCAPACITY_EMPTY_ELEMENTDATA数组（容量为0但第一次对elementData使用add()方法时，会对其扩容，扩容默认大小为10）
• ArrayList(Collection< ? extends E> c)：创建的数组所含元素与c中的顺序、大小一致。

添加

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++;
}

private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

 private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

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

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);
    }

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;
    }

• 调用add(E e)或add(int index, E element)方法，首先会调用ensureCapacityInternal(int minCapacity)方法，确保数组有空间保存新添加的数组。

• ensureCapacityInternal(int minCapacity)方法，先判断数组elementData 是否由无参构造器创建，若是则在该数组第一次创建时把minCapacity赋值为10;

• ensureExplicitCapacity(int minCapacity)方法，会比较最小容量minCapacity与数组实际大小。若小则说明数组有足够空间保存新值，若大则说明数组没有足够空间保存新值且需要对数组扩容即调用grow(int minCapacity)方法。

• grow(int minCapacity)方法中newCapacity = oldCapacity + (oldCapacity >> 1);即newCapacity =1.5倍elementData.length;也就是在newCapacity - minCapacity < 0为假且不越界的情况下会把数组扩大至1.5倍elementData.length;

删除

public E remove(int index) {
        rangeCheck(index);//检查是否越界

        modCount++;//Fail-Fast 机制 
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
//        确认边界并把index后的元素向前移一位;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
//        把数组最后一位设置为null;原本元素即在GC时被回收
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
}

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
    }

 public void clear() {
        modCount++;

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

        size = 0;
    }

public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }
protected void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = size - toIndex;
//        把toIndex后的元素移动到fromIndex后
        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;
    }
    public static void arraycopy(Object srsc,//源数组
        int srcPos,//源数组要复制的起始位置
        Object dest,//目的数组
        int destPos,//目的数组放置的起始位置
        int length) //复制的长度

• System.arraycopy()方法把srsc第srcPos后的元素复制到dest数组第destpos位置;

• remove(int index)方法，先检查是否越界后把index后的元素向前移一位，从而完成删除功能;

• remove(Object o)方法，先找到o元素后由于o元素下标在范围内不会造成越界，故使用fastRemove(int index)方法快速删除;
• fastRemove(int index)方法，跳过了越界检查故remove(int index)快;
• Clear()方法把elementData数组中每个元素设为null;GC时会适当的回收;
• removeRange(int fromIndex, int toIndex)方法把elementData数组下标fromIndex到toIndex的元素删除，运用到了System.arraycopy()方法;

Fail-Fast机制–modCount

• 在多线程下，由于增加、删除操作会造成线程不安全，故用modCount来保障，若modCount与expectedModCount不一致则会造成异常，从而保障数据安全;

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();
            }
        }
 @Override
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> consumer) {
            Objects.requireNonNull(consumer);
            final int size = ArrayList.this.size;
            int i = cursor;
            if (i >= size) {
                return;
            }
            final Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length) {
                throw new ConcurrentModificationException();
            }
            while (i != size && modCount == expectedModCount) {
                consumer.accept((E) elementData[i++]);
            }
            // update once at end of iteration to reduce heap write traffic
            cursor = i;
            lastRet = i - 1;
            checkForComodification();
        }

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