ArrayList源码阅读----JDK1.8

    //定义一个默认的长度10
    private static final int DEFAULT_CAPACITY = 10;
    //定义空的数组
    private static final Object[] EMPTY_ELEMENTDATA = {};
    //定义数组用来存储放入ArrayList的元素
    private transient Object[] elementData;
    //定义记录ArrayList中元素的个数
    private int size;

    //构造方法--创建指定长度的数组的ArrayList
    public ArrayList(int initialCapacity) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
    }
    //创建ArrayList数组长度为0的ArrayList,当向其中添加第一个元素的时候，扩展数组的长度为10--默认数组长度
    public ArrayList() {
        super();
        this.elementData = EMPTY_ELEMENTDATA;
    }
    //创建含有指定集合元素的ArrayList，通过集合的toArray()方法返回一个数组，让ArrayList的数组指向该数组
    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);
    }
    //当ArrayList的元素个数小于数组长度时候，将数组的长度减小为ArrayList中元素的个数
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = Arrays.copyOf(elementData, size);
        }
    }
    // 向ArrayList中添加一个元素，添加前先检查数组的容量，是否需要扩容，再添加元素进去
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    //注意：minCapavity为添加元素时ArrayList需要最小的容量--确定需要的最小容量
    private void ensureCapacityInternal(int minCapacity) {
           if (elementData == EMPTY_ELEMENTDATA) {
               minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
           }

           ensureExplicitCapacity(minCapacity);
    }
    //判断数组是否要进行扩容量，需要的容量大于数组长度就需要扩容
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

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

    }
    //控制内存溢出使用，对最大容量的限制，最大的数组长度为整型最大值减8
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    //扩展容量：先根据原来的容量扩展为原来的1.5倍，再和最小需要的容量比较，假如还是比需要的小，则将最小需要的赋值给数组新容量
　　　假如新容量比最大的容量大，则再次扩容，扩容完成后将数组原来的内容，复制到新的数组中，其实本质实现还是利用的本地方法
    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);
    }
    //再次进行扩容
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

 //按照下标进行添加，涉及到下标的先检查下标是否越界，检测容量，移动元素，添加元素
     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++;
     }
     //判断下标是否越界
     private void rangeCheckForAdd(int index) {
         if (index > size || index < 0)
             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
     }
     //添加集合中所有元素进入ArrayList,先检查容量，在复制，修改元素个数
     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;
     }
     //从特定下标开始，将集合中的元素加入到ArrayList里面
     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;
      }
     //删除指定下标的元素
      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;
       }
      //按照对象的内容删除，空和非空两种情况
      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;
        }
      //将特定的下标的元素设置为指定的元素
      public E set(int index, E element) {
             rangeCheck(index);

             E oldValue = elementData(index);
             elementData[index] = element;
             return oldValue;
       }
      //是否包含特定的元素
      public boolean contains(Object o) {
             return indexOf(o) >= 0;
      }
      //返回特定对象第一次出现的下标，空或非空
      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;
      }
      //特定对象最后一次出现的下标
      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;
       }
      //返回包含ArrayList元素的数组
      public Object[] toArray() {
             return Arrays.copyOf(elementData, size);
      }
      //返回一条特定的子ArrayList，SubList在其中是以内部类的形式存在
      public List<E> subList(int fromIndex, int toIndex) {
          subListRangeCheck(fromIndex, toIndex, size);
          return new SubList(this, 0, fromIndex, toIndex);
      }