ArrayList是Java的集合类，底层数据结构是动态数组，查询快，增删慢，按照插入的先后顺序保存元素。现在来看看它的源码实现。 先看看它的成员变量：
/** * 序列号，序列化时使用 */ private static final long serialVersionUID = 8683452581122892189L; /** * 数组默认容量 */ private static final int DEFAULT_CAPACITY = 10; /** * 默认构造数组里的空数组 */ private static final Object[] EMPTY_ELEMENTDATA = {}; /** * 默认构造数组里的空数组 */ private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; /** * 存储元素的底层数组 */ transient Object[] elementData; // non-private to simplify nested class access /** * 集合内元素的数量 */ private int size;然后是构造方法： 常用的前两种，主要是要初始化存储元素的数组。
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; } }新增元素有两种方法，一种添加到集合尾部，一种是在指定位置添加元素，在指定位置添加元素时，需要把原来在那个位置及其往后的元素整体后移一个位置，用到System.arraycopy(Object src, int srcPos, Object dest, int destPos, int length)方法，即把源数组从指定位置开始的元素复制到新数组的指定位置，复制length个元素过去。
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++; }这里要提下，每次添加都需要对elementData数组的容量进行判断，确保有足够空间添加新元素。即如下方法：
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); }如果现在的最小所需容量已经大于数组的长度了，就需要进行扩容，调用Arrays.copyOf(Object[] original, int newLength)方法进行扩容，将旧数组元素按照新的数组长度填充元素，并返回新长度的数组。
get方法查找指定位置索引的元素，由于底层数据结构是数组，直接从elementData里返回即可，时间复杂度为O(1)。
@SuppressWarnings("unchecked") E elementData(int index) { return (E) elementData[index]; } public E get(int index) { rangeCheck(index); return elementData(index); }根据元素进行查找其在集合中的位置(前面提到集合元素是安装插入顺序排列的)。需要遍历数组，时间复杂度是O(N)。
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; }直接把数组对应位置的元素修改即可，还有一种迭代器遍历时修改，后面另外再说。
public E set(int index, E element) { rangeCheck(index); E oldValue = elementData(index); elementData[index] = element; return oldValue; }删除元素时要把删除元素所在位置后面的元素都往前挪，还是用到System.arraycopy(Object src, int srcPos, Object dest, int destPos, int length)方法；根据元素删除时则需要遍历数组。
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; } 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 }ArrayList的核心方法就这几个，后面再讲下ArrayList的迭代器遍历。
