1、简述
ArrayList底层的实现是使用了数组保存所有的数据，所有的操作本质上是对数组的操作，每一个ArrayList实例都有一个默认的容量（数组的大小，默认是10），随着
对ArrayList不断增加元素，默认的数组会不断的向新数组进行拷贝，由于ArrayList的内部是通过对数组的操作实现的，所以它是线程不安全的
2、实现
a、构造方法：
AyyarList一共提供了三种构造方法：
/** * 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 */ 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); } } /** * Constructs an empty list with an initial capacity of ten. */ public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; } /** * 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 */ 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; } }
在jdk1.8中，ArrayList的无参构造方法默认的是创建了一个空的数组，只有当你第一次添加是时候才会设置它的默认长度为10 ，在jdk1.6中无参构造方法默认
的就是创建一个长度为10 的空数组
b、定义内部数组：
/** * The array buffer into which the elements of the ArrayList are stored. * The capacity of the ArrayList is the length of this array buffer. Any * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA * will be expanded to DEFAULT_CAPACITY when the first element is added. */ transient Object[] elementData; // non-private to simplify nested class access
关于transient关键字的说明可以参考我的另外一篇博客 java学习笔记之对象序列化
3、ArrayList的操作
增加操作：
add(E e)：
/** * 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}) */ public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; }
调用add(E e)方法时首先会调用ensureCapacityInternal(int minCapacity)去判断是否需要对集合进行扩容，然后默认的将新插入的对象放到内部数组的末尾，
当内部数组需要扩容时，每次直接将数组的长度值原来的两倍，这种操作的代价是很高的，所以在使用过程中我们尽量避免数组的扩容，当可以预知数组长度的时候
可以在构造的时候久指定其长度
add(int index, E element)
/** * 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} */ 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++; }
在调用add(int index, E element)方法想ArrayList中插入一条数据时，这个方法内部先去判断传入的下表是否大于数组的长度。大于的话就会
抛出IndexOutOfBoundsException异常，插入的下表小于数组的长度的时候，再去判断数组是否需要扩容，最后再调用System.arraycopy方法将数组下表大于传入
的index的元素全部后移以为，并将插入的元素放到index位置
addAll(Collection<? extends E> c)
/** * 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 */ 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; }
在调用addAll(Collection<? extends E> c) 插入一个集合时，这个方法内部先去判断增加这个集合数组是否需要扩容，然后调用
arraycopy(Object src, int srcPos,Object dest, int destPos,int length)方法将新增加的集合放到数组的末尾
更新操作：
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; }
在调用set（int index,E e）方法修改里面的值时，方法内部先去检查index下表是都超过数组的最大长度，然后再检查是否有其他的线程对这个对象的长度
进行修改了(所以是线程不安全的，多线程同时操作容易直接抛异常)，最后是直接替换数组中下表index对应的值
删除操作：
/** * 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} */ 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; }
在调用remove(int index)删除ArrayList中的数据时，首先校验传入的下标index是大于数组的长度，然后取出将要被删除的数并判断下标index之后是否还有元素，
如果有的话将下标之后的元素全部往前移动一位，并最终将删除的元素的值返回