这是博主第二次读ArrayList 源码，第一次是在很久之前了，当时读起来有些费劲，记得那时候HashMap的源码还是哈希表+链表的数据结构。
时隔多年，再次阅读起来ArrayList感觉还蛮简单的，但是HashMap已经不是当年的HashMap了，所以下一篇会写HashMap的。
起因：最近写了一个简单的文件校验方法，然后犯了一些比较低级的错误，博主的师兄在进行发布CR时，提出了一些建议，博主感觉羞愧难当，特此记录一下，诸君共勉。代码以及建议如下，已做脱敏处理：
/**
* 修改前
*/
public String checkFileWithJveye() {
//建立 sftp链接，获取目录下所有文件列表集合
List<String> remoteSftpFileList = getRemoteSftpFileList();
//获取服务器已下载列表文件集合（以XXX结尾的）
List<String> localFileList = getLocalFileList();
//已经存在的文件-remove
for (int i = 0; i < remoteSftpFileList.size(); i++) {
if (localFileList.contains(remoteSftpFileList.get(i))) {
remoteSftpFileList.remove(i);
}
}
return remoteSftpFileList.toString();
}
/**
* 师兄的批注：
* Master @XXXX 大约 6 小时之前
* 不应该在循环内进行 list.remove(index) 操作，当remove一个元素之后，
* list会调整顺序，size() 会重新计算len，但是i还是原来的值，
* 导致list有些值没有被循环到。推荐使用迭代器 list.iterator()，
* 或者将返回的类型改为set，空间换时间，这样速度也能快些。
* 讨论中师兄从源码方面解释了ArrayList使用的是遍历Remove，而HashSet直接通过Hash值进行Remove效率更高。
*/
/**
* 修改后
*/
public String checkFileWithJveye2() {
//建立 sftp链接，获取目录下所有文件列表集合
Set<String> remoteSftpFiles = getRemoteSftpFileList();
//获取服务器已下载列表文件集合（以XXX结尾的）
Set<String> localFiles = getLocalFileList();
//已经存在的文件-remove
for (Iterator<String> it = remoteSftpFiles.iterator(); it.hasNext(); ) {
String fileName = it.next();
if (localFiles.contains(fileName)) {
it.remove();
}
}
//返回未获取文件的列表
return remoteSftpFiles.toString();
}因此博主重读了ArrayList等源代码，都是复制的源代码，方便阅读，省去了很多年东西（本来东西也不多），只有简单的增删改，继承关系图如下：
代码如下：
import java.util.*;
/**
* ArrayList简单的增删改查
* @param <E>
*/
public class Bag<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable{
/**
* Default initial capacity.
* 默认的初始化容量
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
* EMPTY_CAPACITY
*/
//private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* Shared empty array instance used for default sized empty instances. We
* distinguish(辨别，分清) this from EMPTY_ELEMENTDATA to know how much to inflate(增长) when
* first element is added.
* DEFAULT_CAPACITY
*/
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.
*
* non-private to simplify nested class access 非私有简化嵌套类的使用。
*/
transient Object[] elementData;
/**
* The size of the ArrayList (the number of elements it contains).
* 集合中元素的个数
* @serial
*/
private int size;
/**
* 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
* 最大容量
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* Constructs an empty list with an initial capacity of ten.
* 初始化构造方法-这里只保留了一个
*/
public Bag() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* 增
* 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})
*/
@Override
public boolean add(E e) {
// Increments modCount!!
ensureCapacityInternal(size + 1);
elementData[size++] = e;
return true;
}
public void ensureCapacityInternal(int minCsap){
minCsap=minCsap>=DEFAULT_CAPACITY?minCsap:DEFAULT_CAPACITY;
if (minCsap - elementData.length > 0){
grow(minCsap);
}
}
/**
* 查
* Returns the element at the specified position in this list.
* @param index index of the element to return
* @return the element at the specified position in this list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
@Override
public E get(int index) {
if (index >= size){
throw new IndexOutOfBoundsException("Index: "+index+", Size: "+size());
}
return elementData(index);
}
// Positional Access Operations
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
/**
* Increases the capacity to ensure that it can hold at least the
* number of elements specified by(由。。。指定) 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) {
// overflow
if (minCapacity < 0){
throw new OutOfMemoryError
("Required array size too large");
}
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
/**
*删
* 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}
*/
@Override
public E remove(int index) {
//验证index
rangeCheck(index);
E oldValue = elementData(index);
int numMoved = size - index - 1;
if (numMoved > 0){
System.arraycopy(elementData, index+1, elementData, index, numMoved);
}
// clear to let GC do its work?
elementData[--size] = null;
return oldValue;
}
private void rangeCheck(int index) {
if (index >= size){
throw new IndexOutOfBoundsException("Index:+index+, Size: +size()");
}
}
/**
* 删
* Removes the first occurrence of the specified element from this list,
* if it is present. If the list does not contain the element, it is
* unchanged. More formally, removes the element with the lowest index
* <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
* (if such an element exists). Returns <tt>true</tt> if this list
* contained the specified element (or equivalently, if this list
* changed as a result of the call).
*
* @param o element to be removed from this list, if present
* @return <tt>true</tt> if this list contained the specified element
*/
@Override
public boolean remove(Object o){
if(o==null){
for (int i=0;i<elementData.length;i++){
if(elementData[i]==null){
fastRemove(i);
return true;
}
}
}else{
for (int i=0;i<elementData.length;i++){
if(o.equals(elementData[i])){
fastRemove(i);
return true;
}
}
}
return false;
}
/**
* arrayCopy( arr1, 2, arr2, 5, 10);
* 意思是;将arr1数组里从索引为2的元素开始, 复制到数组arr2里的索引为5的位置, 复制的元素个数为10个.
*/
private void fastRemove(int index){
int numMoved = size - index - 1;
if (numMoved > 0){
System.arraycopy(elementData, index+1, elementData, index, numMoved);
}
// clear to let GC do its work
elementData[--size] = null;
}
/**
* Returns the number of elements in this list.
*
* @return the number of elements in this list
*/
@Override
public int size() {
return size;
}
/**
* Returns <tt>true</tt> if this list contains no elements.
*
* @return <tt>true</tt> if this list contains no elements
*/
@Override
public boolean isEmpty() {
return size == 0;
}
}