用java源代码学数据结构<三>：ArrayList 详解
查看java文档可以知道，抽象类AbstractList有两个实现子类：Vector和ArrayList。二者的内部结构、基本方法大致相同.只不过Vector是线程安全的，ArrayList是不安全的。ArrayList相比于一般的Array，就是多了一个变长的功能，但相应的时间效率有所下降。
由于ArrayLIst和Vector很想，大部分代码基本一致，就不在提供详细的注释，可以对比参考Vector类的分析（http://blog.csdn.net/wanghao109/article/details/13162419）。
package java.util;/*1.ArrayList实现了List接口，允许包含null2.ArrayList和Vector基本一致，除了ArrayList是非同步的（unsynchronized） 要想使ArrayList多线程同步，可以封装ArrayList,如 List list = Collections.synchronizedList(new ArrayList(...));*/public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable{//序列号 private static final long serialVersionUID = 8683452581122892189L;//默认的capacity为10，和vector一样大小private static final int DEFAULT_CAPACITY = 10;//ArrayList类共享的数组对象，private static final Object[] EMPTY_ELEMENTDATA = {};/*1.ArrayList存储元素的数组,Arraylist的capacity等于数组的length2.对于所有一开始为空的ArrayList(就是指elementData == EMPTY_ELEMENTDATA), 当第一个元素被加入时，ArrayList扩充为DEFAULT_CAPACITY*/private transient Object[] elementData;//包含元素的数目private int size;//初始化大小为initialCapacity的ArrayListpublic ArrayList(int initialCapacity) { super(); if (initialCapacity < 0) throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity); this.elementData = new Object[initialCapacity]; }//无参构造函数public ArrayList() { super();//注意这里：默认使用静态数组EMPTY_ELEMENTDATA this.elementData = EMPTY_ELEMENTDATA; }//和vector类似，使用集合c初始化ArrayListpublic ArrayList(Collection<? extends E> c) { elementData = c.toArray(); size = elementData.length; // c.toArray might (incorrectly) not return Object[] (see 6260652)//函数使用说明参见Vector分析 if (elementData.getClass() != Object[].class) elementData = Arrays.copyOf(elementData, size, Object[].class); }/*如果ArrayList当前实际元素数目小于capacity，将vector缩小。常用于减少ArrayList的存储空间*/public void trimToSize() { modCount++; if (size < elementData.length) { elementData = Arrays.copyOf(elementData, size); } }/*1.增大ArrayList的大小，确保能存放至少minCapacity个元素*/public void ensureCapacity(int minCapacity) {/* 如果elementData等于EMPTY_ELEMENTDATA，表示ArrayList对象只初始化， 还未包含任何元素，那么将ArrayList扩展为DEFAULT_CAPACITY*/ int minExpand = (elementData != EMPTY_ELEMENTDATA) // any size if real element table ? 0 // larger than default for empty table. It's already supposed to be // at default size. : DEFAULT_CAPACITY; if (minCapacity > minExpand) { ensureExplicitCapacity(minCapacity); } }//内部函数，功能和vector类相似private void ensureCapacityInternal(int minCapacity) { if (elementData == EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); } ensureExplicitCapacity(minCapacity); }private void ensureExplicitCapacity(int minCapacity) { modCount++; // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); }/*The maximum size of array to allocate.有些VM需要在数组前加些头信息（header words ）*/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.private void grow(int minCapacity) { // overflow-conscious code int oldCapacity = elementData.length;//默认是增加原来oldcapacity的一半（重点） 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; }//实际元素数目，和vector方法类似public int size() { return size; }//是否为空public boolean isEmpty() { return size == 0; }//是否包含o对象public boolean contains(Object o) { return indexOf(o) >= 0; }////index为起始位置,返回-1表示不包含o对象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; }//反向查找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; }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(); } }public Object[] toArray() { return Arrays.copyOf(elementData, size); }//通过将对象置为null,来删除对象内存空间@SuppressWarnings("unchecked") 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; }@SuppressWarnings("unchecked") E elementData(int index) { return (E) elementData[index]; }public E get(int index) { rangeCheck(index); return elementData(index); }public E set(int index, E element) { rangeCheck(index); E oldValue = elementData(index); elementData[index] = element; return oldValue; }public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; }public void add(int index, E element) { rangeCheckForAdd(index); ensureCapacityInternal(size + 1); // Increments modCount!! System.arraycopy(elementData, index, elementData, index + 1, size - index); elementData[index] = element; size++; }public E remove(int index) { rangeCheck(index); modCount++; E oldValue = elementData(index); int numMoved = size - index - 1; if (numMoved > 0) System.arraycopy(elementData, index+1, elementData, index, numMoved); elementData[--size] = null; // clear to let GC do its work return oldValue; }public boolean remove(Object o) { if (o == null) { for (int index = 0; index < size; index++) if (elementData[index] == null) { fastRemove(index); return true; } } else { for (int index = 0; index < size; index++) if (o.equals(elementData[index])) { fastRemove(index); return true; } } return false; }/* * Private remove method that skips bounds checking and does not * return the value removed. */private void fastRemove(int index) { modCount++; int numMoved = size - index - 1; if (numMoved > 0) System.arraycopy(elementData, index+1, elementData, index, numMoved); elementData[--size] = null; // clear to let GC do its work }public void clear() { modCount++; // clear to let GC do its work for (int i = 0; i < size; i++) elementData[i] = null; size = 0; }public boolean addAll(Collection<? extends E> c) { Object[] a = c.toArray(); int numNew = a.length; ensureCapacityInternal(size + numNew); // Increments modCount System.arraycopy(a, 0, elementData, size, numNew); size += numNew; return numNew != 0; }public boolean addAll(int index, Collection<? extends E> c) { rangeCheckForAdd(index); Object[] a = c.toArray(); int numNew = a.length; ensureCapacityInternal(size + numNew); // Increments modCount int numMoved = size - index; if (numMoved > 0) System.arraycopy(elementData, index, elementData, index + numNew, numMoved); System.arraycopy(a, 0, elementData, index, numNew); size += numNew; return numNew != 0; }protected void removeRange(int fromIndex, int toIndex) { modCount++; int numMoved = size - toIndex; System.arraycopy(elementData, toIndex, elementData, fromIndex, numMoved); // clear to let GC do its work int newSize = size - (toIndex-fromIndex); for (int i = newSize; i < size; i++) { elementData[i] = null; } size = newSize; }private void rangeCheck(int index) { if (index >= size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); }private void rangeCheckForAdd(int index) { if (index > size || index < 0) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); }private String outOfBoundsMsg(int index) { return "Index: "+index+", Size: "+size; }public boolean removeAll(Collection<?> c) { return batchRemove(c, false); }public boolean retainAll(Collection<?> c) { return batchRemove(c, true); }//通过控制complement,来表示是删除还是保留private boolean batchRemove(Collection<?> c, boolean complement) { final Object[] elementData = this.elementData; int r = 0, w = 0; boolean modified = false; try { for (; r < size; r++)//通过控制complement,来表示是删除还是保留，具体实现 if (c.contains(elementData[r]) == complement) elementData[w++] = elementData[r]; } finally { // Preserve behavioral compatibility with AbstractCollection, // even if c.contains() throws. if (r != size) { System.arraycopy(elementData, r, elementData, w, size - r); w += size - r; } if (w != size) { // clear to let GC do its work for (int i = w; i < size; i++) elementData[i] = null; modCount += size - w; size = w; modified = true; } } return modified; }private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException{ // Write out element count, and any hidden stuff int expectedModCount = modCount; s.defaultWriteObject(); // Write out size as capacity for behavioural compatibility with clone() s.writeInt(size); // Write out all elements in the proper order. for (int i=0; i<size; i++) { s.writeObject(elementData[i]); } if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } } /** * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is, * deserialize it). */ private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { elementData = EMPTY_ELEMENTDATA; // Read in size, and any hidden stuff s.defaultReadObject(); // Read in capacity s.readInt(); // ignored if (size > 0) { // be like clone(), allocate array based upon size not capacity ensureCapacityInternal(size); Object[] a = elementData; // Read in all elements in the proper order. for (int i=0; i<size; i++) { a[i] = s.readObject(); } } }/*1.将ListIterator放到ArrayList的内部，既实现了ArrayList数据封装, 又可以使迭代器能访问ArrayList内部数据2.返回从index开始的迭代器*/public ListIterator<E> listIterator(int index) { if (index < 0 || index > size) throw new IndexOutOfBoundsException("Index: "+index); return new ListItr(index); }//默认返回从0开始的迭代器public ListIterator<E> listIterator() { return new ListItr(0); }public Iterator<E> iterator() { return new Itr(); }//类似于vectorprivate class Itr implements Iterator<E> { int cursor; // index of next element to return int lastRet = -1; // index of last element returned; -1 if no such//用于检查线程是否同步，如果线程不同步，它们两个的值不一样 int expectedModCount = modCount; public boolean hasNext() { return cursor != size; } @SuppressWarnings("unchecked") public E next() { checkForComodification(); int i = cursor; if (i >= size) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) throw new ConcurrentModificationException(); cursor = i + 1; return (E) elementData[lastRet = i]; } public void remove() { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { ArrayList.this.remove(lastRet); cursor = lastRet; lastRet = -1; expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } }//线程安全检查 final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } }//类似于Itrprivate class ListItr extends Itr implements ListIterator<E> { ListItr(int index) { super(); cursor = index; } public boolean hasPrevious() { return cursor != 0; } public int nextIndex() { return cursor; } public int previousIndex() { return cursor - 1; } @SuppressWarnings("unchecked") public E previous() { checkForComodification(); int i = cursor - 1; if (i < 0) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) throw new ConcurrentModificationException(); cursor = i; return (E) elementData[lastRet = i]; } public void set(E e) { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { ArrayList.this.set(lastRet, e); } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } public void add(E e) { checkForComodification(); try { int i = cursor; ArrayList.this.add(i, e); cursor = i + 1; lastRet = -1; expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } } public List<E> subList(int fromIndex, int toIndex) {//类型检查 subListRangeCheck(fromIndex, toIndex, size); return new SubList(this, 0, fromIndex, toIndex); } static void subListRangeCheck(int fromIndex, int toIndex, int size) { if (fromIndex < 0) throw new IndexOutOfBoundsException("fromIndex = " + fromIndex); if (toIndex > size) throw new IndexOutOfBoundsException("toIndex = " + toIndex); if (fromIndex > toIndex) throw new IllegalArgumentException("fromIndex(" + fromIndex + ") > toIndex(" + toIndex + ")"); }//实现了AbstractList的随机访问方法 private class SubList extends AbstractList<E> implements RandomAccess { private final AbstractList<E> parent; private final int parentOffset; private final int offset; int size; SubList(AbstractList<E> parent, int offset, int fromIndex, int toIndex) { this.parent = parent; this.parentOffset = fromIndex; this.offset = offset + fromIndex; this.size = toIndex - fromIndex; this.modCount = ArrayList.this.modCount; } public E set(int index, E e) { rangeCheck(index); checkForComodification(); E oldValue = ArrayList.this.elementData(offset + index); ArrayList.this.elementData[offset + index] = e; return oldValue; } public E get(int index) { rangeCheck(index); checkForComodification(); return ArrayList.this.elementData(offset + index); } public int size() { checkForComodification(); return this.size; } public void add(int index, E e) { rangeCheckForAdd(index); checkForComodification(); parent.add(parentOffset + index, e); this.modCount = parent.modCount; this.size++; } public E remove(int index) { rangeCheck(index); checkForComodification(); E result = parent.remove(parentOffset + index); this.modCount = parent.modCount; this.size--; return result; } protected void removeRange(int fromIndex, int toIndex) { checkForComodification(); parent.removeRange(parentOffset + fromIndex, parentOffset + toIndex); this.modCount = parent.modCount; this.size -= toIndex - fromIndex; } public boolean addAll(Collection<? extends E> c) { return addAll(this.size, c); } public boolean addAll(int index, Collection<? extends E> c) { rangeCheckForAdd(index); int cSize = c.size(); if (cSize==0) return false; checkForComodification(); parent.addAll(parentOffset + index, c); this.modCount = parent.modCount; this.size += cSize; return true; } public Iterator<E> iterator() { return listIterator(); } public ListIterator<E> listIterator(final int index) { checkForComodification(); rangeCheckForAdd(index); final int offset = this.offset; return new ListIterator<E>() { int cursor = index; int lastRet = -1; int expectedModCount = ArrayList.this.modCount; public boolean hasNext() { return cursor != SubList.this.size; } @SuppressWarnings("unchecked") public E next() { checkForComodification(); int i = cursor; if (i >= SubList.this.size) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (offset + i >= elementData.length) throw new ConcurrentModificationException(); cursor = i + 1; return (E) elementData[offset + (lastRet = i)]; } public boolean hasPrevious() { return cursor != 0; } @SuppressWarnings("unchecked") public E previous() { checkForComodification(); int i = cursor - 1; if (i < 0) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (offset + i >= elementData.length) throw new ConcurrentModificationException(); cursor = i; return (E) elementData[offset + (lastRet = i)]; } public int nextIndex() { return cursor; } public int previousIndex() { return cursor - 1; } public void remove() { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { SubList.this.remove(lastRet); cursor = lastRet; lastRet = -1; expectedModCount = ArrayList.this.modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } public void set(E e) { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { ArrayList.this.set(offset + lastRet, e); } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } public void add(E e) { checkForComodification(); try { int i = cursor; SubList.this.add(i, e); cursor = i + 1; lastRet = -1; expectedModCount = ArrayList.this.modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } final void checkForComodification() { if (expectedModCount != ArrayList.this.modCount) throw new ConcurrentModificationException(); } }; } public List<E> subList(int fromIndex, int toIndex) { subListRangeCheck(fromIndex, toIndex, size); return new SubList(this, offset, fromIndex, toIndex); } private void rangeCheck(int index) { if (index < 0 || index >= this.size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } private void rangeCheckForAdd(int index) { if (index < 0 || index > this.size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } private String outOfBoundsMsg(int index) { return "Index: "+index+", Size: "+this.size; } private void checkForComodification() { if (ArrayList.this.modCount != this.modCount) throw new ConcurrentModificationException(); } }}个人体会：
1.Vector是线程安全的，同步的,ArrayList是不安全，不同步的。要想使得ArrayList同步，可以使用List list = Collections.synchronizedList(new ArrayList(...))来进行封装
2.通过比较vector和arraylist的private void grow(int minCapacity)方法，可以发现vector每次增加原来容量的一倍，而ArrayList每次增加原来容量的一半
3.ArrayList和Vector类中都包含两个迭代器：普通迭代器Itr,链表迭代器ListItr，其实它们的内部功能都是使用集合的内部属性或方法来实现的，只是函数名的不同以及功能的不同而已
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