ArrayList 源码分析

定义：

public class ArrayList<E> extends AbstractList<E>        implements List<E>, RandomAccess, Cloneable, java.io.Serializable{}

变量：

private static final int DEFAULT_CAPACITY = 10;          默认的初始容量private static final Object[] EMPTY_ELEMENTDATA = {};   空的数据容器private transient Object[] elementData;数据容器private int size;        数据量protected transient int modCount = 0 ;                  数据容器update的次数

构造方法：

 ① public ArrayList() {        super();        this.elementData = EMPTY_ELEMENTDATA;          数据容器为空的数据容器    }② public ArrayList(Collection<? extends E> c) {        elementData = c.toArray();        size = elementData.length;// c.toArray might (incorrectly) not return Object[] (see 6260652)      </span>        if (elementData.getClass() != Object[].class)            elementData = Arrays.copyOf(elementData, size, Object[].class);    }E 为数据的泛型， 将c中所有c.size的数据存放在容器中,size = c.size， 6260652是JDKbug库里面的一个bug编号，害怕上述操作无法转型成Object[] ，后面又copy了一次

方法：

 public void trimToSize() {        modCount++;        if (size < elementData.length) {            elementData = Arrays.copyOf(elementData, size);        }    }把elementData 的length 缩小为size

 public void ensureCapacity(int minCapacity) {        int minExpand = (elementData != EMPTY_ELEMENTDATA)            ? 0            : DEFAULT_CAPACITY;        if (minCapacity > minExpand) {            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);    }private static int hugeCapacity(int minCapacity) {        if (minCapacity < 0) // overflow            throw new OutOfMemoryError();        return (minCapacity > MAX_ARRAY_SIZE) ?            Integer.MAX_VALUE :            MAX_ARRAY_SIZE;    }是否扩容:① 如果minCapacity < minExpand  不扩容②minCapacity  >minExpand && minCapacity< =elementData.length   modCount++ ,不扩容③minCapacity  >minExpand && minCapacity >elementData.length     modCount++,elementData 扩容扩容的情况:   首先是原长度的2/3(奇数就是2/3上-1)是否>minCapacity ,minCapacity取较小值 ,如果minCapacity  - (Integer.MAX_VALUE- 8)>0 那么就调用hugeCapacity;如果 minCapacity超出了int的最大值会报内存溢出<span style="font-family: Arial, Helvetica, sans-serif;">当minCapacity较大且还不大于Integer.MAX_VALUE 我这边要报Requested array size exceeds VM limit，超出了我堆内存的设置大小，一般不会new 太大的数据，如果太大还是考虑一下程序逻辑吧</span>

public int size() {        return size;    }获取集合里面实际的数据量<pre name="code" class="java">public boolean isEmpty() {        return size == 0;    }是否没有实际的数据<pre name="code" class="java"> public boolean contains(Object o) {        return indexOf(o) >= 0;    }是否有元素o 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;    }遍历实际的数据量 找到o第一次出现的位置，找不到返回-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;    }反序遍历实际的数据量 找到o第一次出现的位置，找不到返回-1

public Object clone() {        try {            @SuppressWarnings("unchecked")                ArrayList<E> v = (ArrayList<E>) super.clone();            v.elementData = Arrays.copyOf(elementData, size);            v.modCount = 0;            return v;        } catch (CloneNotSupportedException e) {            // this shouldn't happen, since we are Cloneable            throw new InternalError();        }    }实现了Cloneable接口，重写了Object的conle(),length缩减为size,modCount =0 ,其他参数和原来一样.

关于clone详见：http://www.blogjava.net/orangelizq/archive/2007/10/17/153573.html