ArrayList是集合的一种实现，实现了接口List，List接口继承了Collection接口。Collection是所有集合类的父类。ArrayList使用非常广泛，不论是数据库表查询，excel导入解析，还是网站数据爬取都需要使用到，了解ArrayList原理及使用方法显得非常重要。
一. 定义一个ArrayList //默认创建一个ArrayList集合 List<String> list = new ArrayList<>(); //创建一个初始化长度为100的ArrayList集合 List<String> initlist = new ArrayList<>(100); //将其他类型的集合转为ArrayList List<String> setList = new ArrayList<>(new HashSet());我们读一下源码，看看定义ArrayList的过程到底做了什么？
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable { /** * Default initial capacity. */ private static final int DEFAULT_CAPACITY = 10; /** * Shared empty array instance used for empty instances. */ private static final Object[] EMPTY_ELEMENTDATA = {}; /** * Shared empty array instance used for default sized empty instances. We * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when * first element is added. */ private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; /** * 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 /** * The size of the ArrayList (the number of elements it contains). * * @serial */ private int size; /** * 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; } } }其实源码里面已经很清晰了，ArrayList非线程安全，底层是一个Object[]，添加到ArrayList中的数据保存在了elementData属性中。
当调用new ArrayList<>()时，将一个空数组{}赋值给了elementData，这个时候集合的长度size为默认长度0；
当调用new ArrayList<>(100)时，根据传入的长度，new一个Object[100]赋值给elementData，当然如果玩儿的话，传了一个0，那么将一个空数组{}赋值给了elementData；
当调用new ArrayList<>(new HashSet())时，根据源码，我们可知，可以传递任何实现了Collection接口的类，将传递的集合调用toArray()方法转为数组内赋值给elementData;
注意：在传入集合的ArrayList的构造方法中，有这样一个判断
if (elementData.getClass() != Object[].class)，
给出的注释是：c.toArray might (incorrectly) not return Object[] (see 6260652)，即调用toArray方法返回的不一定是Object[]类型，查看ArrayList源码
public Object[] toArray() { return Arrays.copyOf(elementData, size);}我们发现返回的确实是Object[]，那么为什么还会有这样的判断呢？
如果有一个类CustomList继承了ArrayList，然后重写了toArray()方法呢。。
public class CustomList<E> extends ArrayList { @Override public Integer [] toArray() { return new Integer[]{1,2}; }; public static void main(String[] args) { Object[] elementData = new CustomList<Integer>().toArray(); System.out.println(elementData.getClass()); System.out.println(Object[].class); System.out.println(elementData.getClass() == Object[].class); } }执行结果：
class [Ljava.lang.Integer; class [Ljava.lang.Object; false接着说，如果传入的集合类型和我们定义用来保存添加到集合中值的Object[]类型不一致时，ArrayList做了什么处理？读源码看到，调用了Arrays.copyOf(elementData, size, Object[].class);，继续往下走
public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) { T[] copy = ((Object)newType == (Object)Object[].class) ? (T[]) new Object[newLength] : (T[]) Array.newInstance(newType.getComponentType(), newLength); System.arraycopy(original, 0, copy, 0, Math.min(original.length, newLength)); return copy; }我们发现定义了一个新的数组，将原数组的数据拷贝到了新的数组中去。
二. ArrayList常用方法ArrayList有很多常用方法，add，addAll，set，get，remove，size，isEmpty等
首先定义了一个ArrayList,
List<String> list = new ArrayList<>(10); list.add('牛魔王'); list.add('蛟魔王'); ... list.add('美猴王');Object[] elementData中数据如下：