ArrayList 源码解析
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本解析源码来自JDK1.7
ArrayList 概要
- 继承Collection接口，实现了随机存取，自动扩容
- 内部使用数组进行存储，size表示List中元素的个数
private transient Object[] elementData; private int size;- 允许null值
- 与Vector基本相同，但是ArrayList是非同步的，可以使用
List list = Collections.synchronizedList(new ArrayList(...))做同步 - ArrayList的线性操作比LinkedList的操作的常数项系数要小
- 有fast-fail机制
一些常量
- 没有指定ArrayList初始容量，首次添加元素时ArrayList会扩展到长度为10
- 没有指定ArrayList初始容量，且没有添加元素之前，内部数组为空
- ArrayList的长度扩展是不会超过Integer.MAX_VALUE-8
DEFAULT_CAPACITY = 10 Object[] EMPTY_ELEMENTDATA = {}MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8 实现接口
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable - List 包含了ArrayList的主要操作方法的描述
- RandomAccess 标志接口，表明随机访问效率高
- Cloneable 标志接口，表明可以进行深拷贝
- Serializable 可序列化
构造函数
主要有三种构造方式
- 给定初始大小，创建initialCapacity的数组
- 不给定大小，使用空数组
- 给定另个一Collection c，将集合c的转换为数组，拷贝数组作为成员数组，原集合大小作为size
public ArrayList(int initialCapacity) { super(); if (initialCapacity < 0) throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity); this.elementData = new Object[initialCapacity]; } public ArrayList() { super(); this.elementData = EMPTY_ELEMENTDATA; } 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); } List 接口主要方法
public interface List<E> extends Collection<E> { // Query Operations int size(); boolean isEmpty(); boolean contains(Object o); Iterator<E> iterator(); Object[] toArray(); <T> T[] toArray(T[] a); // Modification Operations boolean add(E e); boolean remove(Object o); // Bulk Modification Operations boolean containsAll(Collection<?> c); boolean addAll(Collection<? extends E> c); boolean addAll(int index, Collection<? extends E> c); boolean removeAll(Collection<?> c); boolean retainAll(Collection<?> c); void clear(); // Comparison and hashing boolean equals(Object o); int hashCode(); // Positional Access Operations E get(int index); E set(int index, E element); void add(int index, E element); E remove(int index); // Search Operations int indexOf(Object o); int lastIndexOf(Object o); // List Iterators ListIterator<E> listIterator(); ListIterator<E> listIterator(int index); // View List<E> subList(int fromIndex, int toIndex);}存入元素
主要方法
- set(int index, E element) 更新指定位置元素
- add(E e) 末尾添加元素
- add(int i,E e) 指定位置后添加元素
- addAll(Collection c) 末尾批量添加元素
- addAll(int i, Collection) 指定位置批量添加元素
主要操作
- 检查插入index的合法性 rangecheck（index）
- rangeCheck 只检查index是否**大于等于**size，不检查index是否小于零，因为rangeCheck后总是数组操作，所以会抛出ArrayIndexOutOfBoundsException，而不是IndexOutOfBoundsExcepiton
- rangeCheckForAdd 检查index是否**大于**size或小于0，抛出IndexOutOfBoundsExcepiton
- 异常信息为 Index：index，Size：size
- 确保数组有足够容量 ensureCapacityInternal（minCapacity）
- 将数据拷贝到数组中
利用Arrays.copyof（originalArray，length）来进行扩容，length指定新数组长度，如果新数组大于原数组长度，其余部分补null
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 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; } 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; } private void ensureCapacityInternal(int minCapacity) { if (elementData == EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); } ensureExplicitCapacity(minCapacity); } 容量扩展
- ensureCapacityInternal 如果数组为空，就扩展数组为max（默认大小，给定的minCapacity）大小，否则扩展到给定最小值大小
- 检查是否有足够容量，如果没有调用grow(minCapacity)进行扩容
- 默认扩展到1.5倍
- 如果扩展1.5倍后还是小于给定最小值，就要扩展到的大小为给定最小值
- 如果要扩展到的大小大于MAX_ARRAY_SIZE，就扩展到 Integer.MAX_VALUE
NOTE
- 共有的扩展数组方法 ensureCapacity（minCapacity）
ArrayList提供了共有的保证容量的方法，一般情况下我们是不用管扩容的事情，但是当我们预计要插入的元素个数比较多时，且数量可估计时，手动确保List容量可以避免频繁的扩容，造成的数据频繁的拷贝，从而提高效率
- 默认初始化会将数组初始化为空，存入元素时直接扩展到DefaultCapacity
如果List为空，只有minCapacity大于DefaultCapacity时才需要扩容，如果List不为空，大于零就需要扩容。
- 扩展的相反操作 trimToSize(int size)
如果设定的size小于数组的长度，用Arrays.copyof(array,length)复制创建一个给定大小的数组
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); } 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; } public void ensureCapacity(int minCapacity) { 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); } } 读取元素 get(index)
- 首先进行index合法性检查，注意不检查小于零的情况，而是由数组抛出ArrayIndexOutOfBoundsException
- 进行类型转换，返回
public E get(int index) { rangeCheck(index); return elementData(index); } @SuppressWarnings("unchecked") E elementData(int index) { return (E) elementData[index]; } 查找
- 采用逐个遍历的方法进行查找
- indexOf从前往后找，lastIndexOf从后往前找，
- contains（e） 调用 indexOf进行查找
NOTE
由于允许null值，所以null需要单独处理，而不能用elementData[i].equals(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; } 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 boolean contains(Object o) { return indexOf(o) >= 0; } 删除元素
删除元素主要过程：检查参数合法性 其后面元素前移，然后把最后面的元素全部置为null。
- remove（index） 保存原index值用来返回，index后的元素向前拷贝，最后值赋值null以回收
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; } - remove（object） 遍历找到对应元素，不需要进行index合法性检查，直接删除
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 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 } - removeAll（collection），retainAll（collection）
- removeAll（collection） 与 retainAll（collection）是两个相反的过程，一个保留给定集合包含的元素，相当于求交集，一个删除给定集合包含的元素，相当于求差集
- batchRemove 使用双指针的方法，将要保留的值赋值到前面，然后将后面的值置null
NOTE
其中finally语句中，r<size的情况是由于Collection中的contains方法有可能抛出NullPointerException和ClassCastException，如果出现异常，就将当前w之后的值复制到r之后，不再检查是否包含，而后r之后的值依旧赋值为null
public boolean removeAll(Collection<?> c) { return batchRemove(c, false); } public boolean retainAll(Collection<?> c) { return batchRemove(c, true); } 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++) 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; }Fast-Fail机制
- 源码中凡是修改List结构（插入，删除，打乱顺序，调整容量，不包含set更新元素），都会涉及到modCount++
- 在ArrayList类创建迭代器之后，除非通过迭代器自身remove或add对列表结构进行修改，否则在其他线程中以任何形式对列表进行修改，迭代器马上会抛出异常，快速失败。
- 该机制通过检查modCount的值来确定是否迭代过程中有其他线程对列表进行修改
private void checkForComodification() { if (ArrayList.this.modCount != this.modCount) throw new ConcurrentModificationException(); } 复制方法 Clone
浅拷贝，拷贝引用，而非创建新对象，如果引用指向的对象改变也会跟着改变
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(); } } 转换为数组 toArray
- toArray（）返回新的Object数组
- toArray(T[]) 如果实参数组长度小于List，返回一个新的T类型数组，如果实参数组大于List，执行复制拷贝，将紧邻的后一个置为null（在list不含null时，便于检查List长度），返回实参数组
public Object[] toArray() { return Arrays.copyOf(elementData, size); } @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; } 序列化方法
- elementData声明为transient是不会被序列化写入的，所以序列化分两步，首先执行通用序列化，然后将数组中的元素逐一读入，或写出
- 在进行写入的过程中有fast-fail机制，即这个过程中有对list的修改会造成失败
- 虽然size单独写入了，但是在readObject中这个值并没有用到，为了与clone方法适配
private transient Object[] elementData; 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(); } } 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(); } } } 迭代器
- 迭代器作为ArrayList的内部类，可以直接访问修改ArrayList
- ArrayList实现了两种迭代器，其中ListIterator除了实现基本的Iterator方法（hasNext，Next，remove），还包含更丰富的方法
- ListIterator额外实现的方法
- listIterator(index) 可以指定开始遍历的位置
- hasPrevious 有没有前驱
- previous 返回前驱
- add 实现添加元素
- set 更新上次访问的元素
- previousIndex() nextIndex() 返回下一个，和上一个位置
NOTE
remove，add，set方法都是改变上次被访问元素位置进行操作，连续调用两次以上就会出现问题
public ListIterator<E> listIterator(int index) { if (index < 0 || index > size) throw new IndexOutOfBoundsException("Index: "+index); return new ListItr(index); } public ListIterator<E> listIterator() { return new ListItr(0); } public Iterator<E> iterator() { return new Itr(); } private 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 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(); } } final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } } private 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(); } } } 子视图 SubList
- 首先进行位置检查，如果合法，就调用新建SubList对象
- ArrayList将自身作为参数传递，也就是说，对SubList的操作其实是对原ArrayList的操作，SubList的方法跟ArrayList相近，只是需要进行index的转换，加上fromIndex
- SubList 只会返回ListIterator，其ListIterator对象通过匿名内部类的方式定义
public List<E> subList(int fromIndex, int toIndex) { 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 + ")"); } 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(); } } 1 0
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