自己动手写写：ArrayList源码浅析
了解你所使用的东西，最直接有效的方式莫过于源码切入的方式！
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最近会写一个源码分析的系列文章！这篇文章先从最常用的例子ArrayList下手剖析！
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一. ArrayList
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下面是ArrayList的类结构
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
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1. 两个重要的成员变量
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/** * The array buffer into which the elements of the ArrayList are stored. * The capacity of the ArrayList is the length of this array buffer. */ private transient Object[] elementData; /** * The size of the ArrayList (the number of elements it contains). * * @serial */ private int size;
我们知道ArrayList的内部真实的存储结构是数组，正是此elmentDate; size很明显就是ArrayList的长度（这可不是数组的长度）。
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2. 三个构造函数
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/** * Constructs an empty list with the specified initial capacity. * * @param initialCapacity the initial capacity of the list * @exception 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]; }? ?initialCapacity是初始化数组长度的参数。
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/** * Constructs an empty list with an initial capacity of ten. */ public ArrayList() { this(10); }?默认初始化数组的长度为10
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/** * 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(); size = elementData.length; // c.toArray might (incorrectly) not return Object[] (see 6260652) if (elementData.getClass() != Object[].class) elementData = Arrays.copyOf(elementData, size, Object[].class); }?上面这个构造函数也没啥好说的，使用另外一个Collection初始化，就是将数据c的内容copy到elementData中。
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3. 几个重要的方法
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/** * Inserts the specified element at the specified position in this * list. Shifts the element currently at that position (if any) and * any subsequent elements to the right (adds one to their indices). * * @param index index at which the specified element is to be inserted * @param element element to be inserted * @throws IndexOutOfBoundsException {@inheritDoc} */ public void add(int index, E element) { if (index > size || index < 0) throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size); ensureCapacity(size + 1); // Increments modCount!! System.arraycopy(elementData, index, elementData, index + 1, size - index); elementData[index] = element; size++; }这个add方法的作用就是将此element插入到数组下表为index下，如果超出当前size会报错的。
其中ensureCapacity(size + 1); 的作用是什么呢？我们来看一下这个方法的内容！
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/** * 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); } }?没错，这个方法就是扩充elementData数组的长度所用。新增一条数据后，如果发现当前elementData数组的长度不够时，会扩充elementData数组，扩充后的elementData数组的长度是原elementData的长度*3/2 + 1后的长度。ps:为啥扩充了一半左右，还不清楚。
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?看得出来，ArrayList的内存就是维护了一个数组，通过不断的新建长度更长的数组并复制数据来完成的！这也就决定了ArrayList的插入速度在需要扩容的时候会比较慢，但是索引查询的数组是相当的快！ps:扩建数组的代价相对而言还是较大的，对于能够预估容量的情况下可以直接初始化一定容量的数组。
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/** * 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} */ public E get(int index) { RangeCheck(index); return (E)elementData[index]; }? ?根据索引获得对象，没啥好说的！其中RangeCheck(index)是检查下表是否越界!
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/** * 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} */ public E remove(int index) { RangeCheck(index); modCount++; E oldValue = (E)elementData[index]; int numMoved = size - index - 1; if (numMoved > 0) System.arraycopy(elementData, index + 1, elementData, index, numMoved); elementData[--size] = null; // Let gc do its work return oldValue; }?根据索引移除对象的方法。就是将index后面的所有对象向前移动一位，并将elementData[--size] = null; // Let gc do its work
其中modCount参数是父类AbstractList中定义的，详情如下：
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/** * The number of times this list has been <i>structurally modified</i>. * Structural modifications are those that change the size of the * list, or otherwise perturb it in such a fashion that iterations in * progress may yield incorrect results. * * <p>This field is used by the iterator and list iterator implementation * returned by the {@code iterator} and {@code listIterator} methods. * If the value of this field changes unexpectedly, the iterator (or list * iterator) will throw a {@code ConcurrentModificationException} in * response to the {@code next}, {@code remove}, {@code previous}, * {@code set} or {@code add} operations. This provides * <i>fail-fast</i> behavior, rather than non-deterministic behavior in * the face of concurrent modification during iteration. * * <p><b>Use of this field by subclasses is optional.</b> If a subclass * wishes to provide fail-fast iterators (and list iterators), then it * merely has to increment this field in its {@code add(int, E)} and * {@code remove(int)} methods (and any other methods that it overrides * that result in structural modifications to the list). A single call to * {@code add(int, E)} or {@code remove(int)} must add no more than * one to this field, or the iterators (and list iterators) will throw * bogus {@code ConcurrentModificationExceptions}. If an implementation * does not wish to provide fail-fast iterators, this field may be * ignored. */ protected transient int modCount = 0; ?这里注释也说明的很清楚了，modCount的含义就modify count（list的修改次数），这是一个可选的参数，子类完全可以不操作这个成员变量，但是如果你想提供一个 fail-fast iterators，你就需要在每次修改时modCount++。
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/** * Returns the index of the first occurrence of the specified element * in this list, or -1 if this list does not contain the element. * More formally, returns the lowest index <tt>i</tt> such that * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, * or -1 if there is no such index. */ 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; }查找容器内是否包含o对象并返回第一次找到的索引。这个也没啥好的办法呀，直接遍历一遍呗！?
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/** * Returns an array containing all of the elements in this list * in proper sequence (from first to last element). * * <p>The returned array will be "safe" in that no references to it are * maintained by this list. (In other words, this method must allocate * a new array). The caller is thus free to modify the returned array. * * <p>This method acts as bridge between array-based and collection-based * APIs. * * @return an array containing all of the elements in this list in * proper sequence */ public Object[] toArray() { return Arrays.copyOf(elementData, size); }?返回数组形式的数据，也没啥好的。
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/** * Returns an array containing all of the elements in this list in proper * sequence (from first to last element); the runtime type of the returned * array is that of the specified array. If the list fits in the * specified array, it is returned therein. Otherwise, a new array is * allocated with the runtime type of the specified array and the size of * this list. * * <p>If the list fits in the specified array with room to spare * (i.e., the array has more elements than the list), the element in * the array immediately following the end of the collection is set to * <tt>null</tt>. (This is useful in determining the length of the * list <i>only</i> if the caller knows that the list does not contain * any null elements.) * * @param a the array into which the elements of the list are to * be stored, if it is big enough; otherwise, a new array of the * same runtime type is allocated for this purpose. * @return an array containing the elements of the list * @throws ArrayStoreException if the runtime type of the specified array * is not a supertype of the runtime type of every element in * this list * @throws NullPointerException if the specified array is null */ public <T> T[] toArray(T[] a) { if (a.length < size) // Make a new array of a's runtime type, but my contents: return (T[])Arrays.copyOf(elementData, size, a.getClass()); System.arraycopy(elementData, 0, a, 0, size); if (a.length > size) a[size] = null; return a; }?这个方法也是返回数组形式的数据，如果a.length < size 则按照a的运行时类型新建一个的数组，并把elementData的数组全部copy进去返回此数组，否则将elementData的数组copy进数组里面，并且将其他索引处置null.
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当然还有一些其他的方法，这里就不再分析了，看看都懂得！下个章节介绍LinkedList的内容！
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ps:以上是基于1.6版本的类库源码分析!
这不是源码么 。
这是源码啊，没说不是啊！ 。