ArrayList

ArrayList是基于数组实现的，是一个动态数组，其容量能自动增长，类似于C语言中的动态申请内存，动态增长内存。

• ArrayList不是线程安全的，只能用在单线程环境下
• 多线程环境下可以考虑用Collections.synchronizedList(List l)函数返回一个线程安全的ArrayList类
• 也可以使用concurrent并发包下的CopyOnWriteArrayList类
• ArrayList实现了Serializable接口，因此它支持序列化，能够通过序列化传输
• 实现了RandomAccess接口，支持快速随机访问，实际上就是通过下标序号进行快速访问
• 实现了Cloneable接口，能被克隆

根据JDK版本的不同 ，构造方法也不同

/**
*   JDK 1.8
*/
/**
 * Default initial capacity.
 */
private static final int DEFAULT_CAPACITY = 10;
/**
 * 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 = {};
/**
 * Shared empty array instance used for empty instances.
 */
private static final Object[] EMPTY_ELEMENTDATA = {};

 /**
  * 上面这个对象数组就是其存储元素的数据结构，前面有一个java关键字transient
  * 这个关键字是去序列化的意思，即，在这个类序列化后保存到磁盘或者输出到输出流的时候
  * 这个对象数组是不被保存或者输出的。(这个不是下面的翻译，对transient解释)
  * 
  * 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

/**
 * ArrayList带容量大小的构造函数。  
 * 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);
   }
}

/**
 * 无参构造方法构造的ArrayList的默认返回空数组
 * Constructs an empty list with an initial capacity of ten.
 */
public ArrayList() {
   this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

/**
 * 带有Collection参数的构造方法
 * 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;
   }
}


/**
* JDK 1.7/1.6
*/
/** 
* 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) {  
     super();  
     if (initialCapacity < 0)  
         throw new IllegalArgumentException("Illegal Capacity: "+  
                                            initialCapacity);  
     this.elementData = new Object[initialCapacity];  
 }  

 /** 
  * 无参构造直接返回了this（10）;默认10 这也是与1.8不同的地方
  * Constructs an empty list with an initial capacity of ten. 
  */  
 public ArrayList() {  
     this(10);  
 }  

 /** 
  * Constructs a list containing the elements of the specified 
  * collection, in the order they are returned by the collection's 
  * iterator. 
  * 在这1.8也多了一个判断
  * @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();  
     size = elementData.length;  
     // c.toArray might (incorrectly) not return Object[] (see 6260652)  
     if (elementData.getClass() != Object[].class)  
         elementData = Arrays.copyOf(elementData, size, Object[].class);  
 }  

为什么用到 transient？

这就跟这个ArrayList的特性有关，我们知道ArrayList的容量，也就是这个数组的容量，一般都是预留一些容量，等到容量不够时再拓展，那么就会出现容量还有冗余的情况，如果这时候进行序列化，整个数组都会被序列化，连后面没有意义空元素的也被序列化。这些是不应该被存储的。所以java的设计者，就为这个类提供了一个writeObject方法，在实现了Serializable接口的类，如果这个类提供了writeObject方法，那么在进行序列化的时候就会通过writeObject方法进行序列化，所以ArrayList的writeObject方法就会显式的为每个实际的数组元素进行序列化，只序列化有用的元素。

为什么源码中大量地调用了Arrays.copyof()和System.arraycopy()方法?

public static <T> T[] copyOf(T[] original, int newLength) {
    return (T[]) copyOf(original, newLength, original.getClass());
}
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;
}

最后都调用了System.arraycopy（）方法。

/**
* 该方法被标记了native，调用了系统的C/C++代码，在JDK中是看不到的，
* 但在openJDK中可以看到其源码。该函数实际上最终调用了C语言的memmove()函数，
* 因此它可以保证同一个数组内元素的正确复制 和移动，比一般的复制方法的实现效率要高很多，
* 很适合用来批量处理数组。Java强烈推荐在复制大量数组元素时用该方法，以取得更高的效率。
* 这也说明了ArrayList 与数组
*/  
public static native void arraycopy(Object src,  int  srcPos,
                                        Object dest, int destPos,
                                        int length);

JDK1.6 开始扩容办法也不一样

/** 
 * JDK1.6
 * Increases the capacity of this <tt>ArrayList</tt> instance, if 
 * necessary, to ensure that it can hold at least the number of elements 
 * specified by the minimum capacity argument. 
 * 
 * @param   minCapacity   the desired minimum capacity 
 */  
 public void ensureCapacity(int minCapacity) {  
     modCount++;  
     int oldCapacity = elementData.length;  
     if (minCapacity > oldCapacity) {  
         Object oldData[] = elementData;  
         int newCapacity = (oldCapacity * 3)/2 + 1;  
         if (newCapacity < minCapacity)  
             newCapacity = minCapacity;  
         // minCapacity is usually close to size, so this is a win:  
         elementData = Arrays.copyOf(elementData, newCapacity);  
     }  
 } 
 /**
  * JDK1.8 
  */
 /**
  * The maximum size of array to allocate.
  * Some VMs reserve some header words in an array.
  * Attempts to allocate larger arrays may result in
  * OutOfMemoryError: Requested array size exceeds VM limit
  */
  private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
 /**
  * Increases the capacity to ensure that it can hold at least the
  * number of elements specified by the minimum capacity argument.
  *
  * @param minCapacity the desired minimum capacity
  */
  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);
  }
   private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
   }

• 1.7JDK开始使用的是位运算
• 在算出newCapacity时，其没有和ArrayList所定义的MAX_ARRAY_SIZE作比较，为什么没有进行比较呢，原因是jdk1.6没有定义这个MAX_ARRAY_SIZE最大容量，也就是说，其没有最大容量限制的，但是jdk1.7以上做了一个改进，进行了容量限制。

Java 集合中常见 checkForComodification()方法的作用? modCount和expectedModCount作用?

ArrayList 快速访问

ArrayList基于数组实现，可以通过下标索引直接查找到指定位置的元素，因此查找效率高，但每次插入或删除元素，就要大量地移动元素，插入删除元素的效率低。

/**
* 先检查索引 然后校验操作正确
*/
public E get(int var1) {
   this.rangeCheck(var1);
    return this.elementData(var1);
}