【Java8源码分析】集合框架-ArrayList
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一、总结
（1）ArrayList随机元素时间复杂度O(1)，插入删除操作需大量移动元素，效率较低
（2）为了节约内存，当新建容器为空时，会共享Object[] EMPTY_ELEMENTDATA = {}和 Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}空数组
（3）容器底层采用数组存储，每次扩容为1.5倍
（4）ArrayList的实现中大量地调用了Arrays.copyof()和System.arraycopy()方法，其实Arrays.copyof()内部也是调用System.arraycopy()。System.arraycopy()为Native方法
（5）两个ToArray方法
Object[] toArray()方法。该方法有可能会抛出java.lang.ClassCastException异常<T> T[] toArray(T[] a)方法。该方法可以直接将ArrayList转换得到的Array进行整体向下转型
（6）ArrayList可以存储null值
（7）ArrayList每次修改（增加、删除）容器时，都是修改自身的modCount；在生成迭代器时，迭代器会保存该modCount值，迭代器每次获取元素时，会比较自身的modCount与ArrayList的modCount是否相等，来判断容器是否已经被修改，如果被修改了则抛出异常（fast-fail机制）。
二、源码和注解
package java.util;public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable{ private static final long serialVersionUID = 8683452581122892189L; // 默认初始大小 private static final int DEFAULT_CAPACITY = 10; // 指定大小的空的ArrayList共享此对象 private static final Object[] EMPTY_ELEMENTDATA = {}; // 默认大小的空的ArrayList共享 private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; // 存储容器，为数组 transient Object[] elementData; // 存放容器大小 private int size; // 传入初始大小 public ArrayList(int initialCapacity) { if (initialCapacity > 0) { this.elementData = new Object[initialCapacity]; } else if (initialCapacity == 0) { this.elementData = EMPTY_ELEMENTDATA; } else { throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity); } } // 无參构造 public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; } // 由Collection构造 public ArrayList(Collection<? extends E> c) { elementData = c.toArray(); if ((size = elementData.length) != 0) { if (elementData.getClass() != Object[].class) elementData = Arrays.copyOf(elementData, size, Object[].class); } else { this.elementData = EMPTY_ELEMENTDATA; } } // 将当前容量设为实际元素个数 public void trimToSize() { modCount++; if (size < elementData.length) { elementData = (size == 0) ? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size); } } // 确保容器容量，当容器非空时，不得小于默认大小 public void ensureCapacity(int minCapacity) { int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) ? 0 : DEFAULT_CAPACITY; if (minCapacity > minExpand) { ensureExplicitCapacity(minCapacity); } } private void ensureCapacityInternal(int minCapacity) { if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); } ensureExplicitCapacity(minCapacity); } // 注意此方法在每次添加元素时调用，确保容量足够，且更改modCount private void ensureExplicitCapacity(int minCapacity) { modCount++; if (minCapacity - elementData.length > 0) grow(minCapacity); } // 数组最大元素个数，因为部分虚拟机会为数组预留了头部，故这里减8保证不会内存溢出 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; // 扩容为minCapacity或者旧容器大小*1.5中较大的一个 private void grow(int minCapacity) { int oldCapacity = elementData.length; int newCapacity = oldCapacity + (oldCapacity >> 1); if (newCapacity - minCapacity < 0) newCapacity = minCapacity; if (newCapacity - MAX_ARRAY_SIZE > 0) newCapacity = hugeCapacity(minCapacity); elementData = Arrays.copyOf(elementData, newCapacity); } private static int hugeCapacity(int minCapacity) { if (minCapacity < 0) // 溢出 throw new OutOfMemoryError(); return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; } public int size() { return size; } public boolean isEmpty() { return size == 0; } // 遍历所有元素的方法，效率较低 public boolean contains(Object o) { return indexOf(o) >= 0; } 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; } // 浅拷贝！！ public Object clone() { try { ArrayList<?> v = (ArrayList<?>) 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(e); } } // 该方法有可能会抛出java.lang.ClassCastException异常， // 如果直接用向下转型的方法，将整个ArrayList集合转变为指定类型的Array数组，便会抛出该异常 public Object[] toArray() { return Arrays.copyOf(elementData, size); } // 该方法可以直接将ArrayList转换得到的Array进行整体向下转型 @SuppressWarnings("unchecked") public <T> T[] toArray(T[] a) { if (a.length < size) 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]; } // 随机访问 public E get(int index) { rangeCheck(index); return elementData(index); } // 随机存储 public E set(int index, E element) { rangeCheck(index); E oldValue = elementData(index); elementData[index] = element; return oldValue; } // 末尾添加元素 public boolean add(E e) { ensureCapacityInternal(size + 1); // 改变modCount elementData[size++] = e; return true; } // 随机位置添加元素，需复制后续元素，效率较低 public void add(int index, E element) { rangeCheckForAdd(index); ensureCapacityInternal(size + 1); // 复制插入位置的后续元素 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); // 设置不访问的元素为null，触发gc elementData[--size] = null; return oldValue; } // 删除第一个出现的元素 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; } // 快速删除，在确保下标正确的情况下，不检查下标范围 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; } // 清楚所有元素，设置为null，但存储的elementData容量不变 public void clear() { modCount++; 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); 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) { Objects.requireNonNull(c); return batchRemove(c, false); } public boolean retainAll(Collection<?> c) { Objects.requireNonNull(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; } // java.io.Serializable的写入函数 // 将ArrayList的“容量，所有的元素值”都写入到输出流中 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(); } } // java.io.Serializable的读取函数：根据写入方式读出 // 先将ArrayList的“容量”读出，然后将“所有的元素值”读出 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(); } } }}三、参考
http://blog.csdn.net/ns_code/article/details/35568011
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