java源码分析之ArrayList

package java.util;/** * 继承自 AbstractList ，实现List<E>（集合）, RandomAccess（标记接口，支持快速随机访问）, * Cloneable（实现对象的浅拷贝）, java.io.Serializable（序列化）接口 （非线程安全） */public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable {private static final long serialVersionUID = 8683452581122892189L;/** * 定义默认容量为 10 */private static final int DEFAULT_CAPACITY = 10;/** * 定义一个空集合 */private static final Object[] EMPTY_ELEMENTDATA = {};/** * 定义一个空的数组 */private transient Object[] elementData;/** * 定义一个值 用于表示集合的元素数量 （元素数量不一定等于集合容量） */private int size;/** * 构造具有指定初始容量的空集合 */public ArrayList(int initialCapacity) {super();if (initialCapacity < 0)throw new IllegalArgumentException("Illegal Capacity: " + initialCapacity);this.elementData = new Object[initialCapacity];}/** * 构造初始容量为 10 的空集合. */public ArrayList() {super();this.elementData = EMPTY_ELEMENTDATA;}/** * 按照集合迭代器返回的顺序构造包含指定集合元素的集合 */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);}/** * 去掉集合中的多余位置，优化存储（以实际元素个数为集合容量大小） */public void trimToSize() {modCount++;if (size < elementData.length) {elementData = Arrays.copyOf(elementData, size);}}/** * 增加ArrayList实例的容量，如果有必要，确保它至少能容纳的最小容量为参数指定元素个数。 */public void ensureCapacity(int minCapacity) {int minExpand = (elementData != EMPTY_ELEMENTDATA) ? 0 : DEFAULT_CAPACITY;if (minCapacity > minExpand) {ensureExplicitCapacity(minCapacity);}}// 判断集合默认容量与指定最小容量的大小（取大）private void ensureCapacityInternal(int minCapacity) {if (elementData == EMPTY_ELEMENTDATA) {minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);}ensureExplicitCapacity(minCapacity);}// 判断指定容量与元素多少，如容量不足，则扩容private void ensureExplicitCapacity(int minCapacity) {modCount++;if (minCapacity - elementData.length > 0)grow(minCapacity);}/** * 集合的最大空间大小 */private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;/** * 增加容量(每次1.5倍)，确保集合的最小容量能容纳指定元素数量。 */private void grow(int minCapacity) {int oldCapacity = elementData.length;// 注意此处扩充capacity的方式是将其向右一位再加上原来的数，实际上是扩充了1.5倍(原始容量x3)/2 + 1int newCapacity = oldCapacity + (oldCapacity >> 1);//如果还不够，则直接将minCapacity设置为当前容量 if (newCapacity - minCapacity < 0)newCapacity = minCapacity;//如果还不够，则直接将Interger的容量作为空间if (newCapacity - MAX_ARRAY_SIZE > 0)newCapacity = hugeCapacity(minCapacity);elementData = Arrays.copyOf(elementData, newCapacity);}/** * 返回指定集合空间的最大值（Integer的最大值） */private static int hugeCapacity(int minCapacity) {if (minCapacity < 0)throw new OutOfMemoryError();return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;}/** * 返回集合空间的实际大小 */public int size() {return size;}/** * 判断 集体是否为空 */public boolean isEmpty() {return size == 0;}/** * 判断集合是否包含指定元素 */public boolean contains(Object o) {return indexOf(o) >= 0;}/** * 返回指定元素在集合中首次出现的索引位置（正向匹配）如无，则返回 -1 指定元素为空，数组某个位置元素也为空，则返回该索引 */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;}/** * 返回指定元素最后出现的索引位置（反向匹配）如无，则返回 -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;}/** * 返回该arraylis实例浅拷贝 (元素本身不被复制)Object的 clone()方法 */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) {throw new InternalError();}}/** * 返回包含此集合中所有元素按适当顺序（从第一个到最后一个元素）组成的Object数组 */public Object[] toArray() {return Arrays.copyOf(elementData, size);}/** * 返回包含此集合中所有元素（从第一个到最后一个元素）组成的数组； * 如果传入数组的长度小于集合size，返回一个新的数组，大小为size，类型与传入数组相同。所传入数组长度与size相等， * 则将elementData复制到传入数组中并返回传入的数组。若传入数组长度大于size，除了复制elementData外， * 还将把返回数组的第size个元素置为空 */@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;}/** * 返回指定位置集合元素值 */@SuppressWarnings("unchecked")E elementData(int index) {return (E) elementData[index];}/** * 返回集合中指定位置的元素 */public E get(int index) {// 判断index是否越界rangeCheck(index);return elementData(index);}/** * 设置集合中指定位置的元素 */public E set(int index, E element) {rangeCheck(index);E oldValue = elementData(index);elementData[index] = element;return oldValue;}/** * 在集合末尾添加元素（同时增加集合容量） */public boolean add(E e) {ensureCapacityInternal(size + 1); // Increments modCount!!elementData[size++] = e;return true;}/** * 向集合指定位置添加元素（同时增加集合容量） */public void add(int index, E element) {rangeCheckForAdd(index);ensureCapacityInternal(size + 1);// 实现数组间的复制(源数组，源数组复制的起始位置，目标数据，目标数组放置的起始位置，复制的长度)// System.arraycopy只是把index位置及其后面的元素，拷贝到数组的index+1及其后面的位置中，// 也就是把index及其后面的元素全部后移一位。 最后，把新的元素放到数组的index位置System.arraycopy(elementData, index, elementData, index + 1, size - index);elementData[index] = element;size++;}/** * 移除指定位置的元素（同时减少集合容量：将最后一位赋值为Null,以便垃圾回收） */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 workreturn oldValue;}/** * 删除集合中某元素（如为空，则删除数组中值为空的值）返回true,删除成功 */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;}/* * 快速删除指定位置的元素 */private void fastRemove(int index) {modCount++;int numMoved = size - index - 1; //从 index+1 开始，后面的元素依次替换前面的元素if (numMoved > 0)System.arraycopy(elementData, index + 1, elementData, index, numMoved);//将最后一个元素设置为Null,以便垃圾回收elementData[--size] = null; // clear to let GC do its work}/** * 删除集合中所有的元素（设置所有值为null，以便垃圾回收） */public void clear() {modCount++;// clear to let GC do its workfor (int i = 0; i < size; i++)elementData[i] = null;size = 0;}/** * 按照指定集合中的元素顺序将所有元素添加到现有集合的末尾（如果操作过程中修改指定集合，操作为空） */public boolean addAll(Collection<? extends E> c) {Object[] a = c.toArray();int numNew = a.length;ensureCapacityInternal(size + numNew); // Increments modCountSystem.arraycopy(a, 0, elementData, size, numNew);size += numNew;return numNew != 0;}/** * 按照指定集合中的元素顺序将所有元素添加到现有集合的指定位置（如果操作过程中修改指定集合，操作为空） */public boolean addAll(int index, Collection<? extends E> c) {rangeCheckForAdd(index);// 将集合转换为数组Object[] a = c.toArray();int numNew = a.length;ensureCapacityInternal(size + numNew); // Increments modCountint numMoved = size - index;// 扩大集合容量if (numMoved > 0)System.arraycopy(elementData, index, elementData, index + numNew, numMoved);System.arraycopy(a, 0, elementData, index, numNew);size += numNew;return numNew != 0;}/** * 删除集合中fromIndex到toIndex之间的所有元素。将剩下的元素左移并减少它们的索引 */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 workint newSize = size - (toIndex - fromIndex);for (int i = newSize; i < size; i++) {elementData[i] = null;}size = newSize;}/** * 检查指定index是否越界 */private void rangeCheck(int index) {if (index >= size)throw new IndexOutOfBoundsException(outOfBoundsMsg(index));}/** * 判断指定index是否合理 */private void rangeCheckForAdd(int index) {if (index > size || index < 0)throw new IndexOutOfBoundsException(outOfBoundsMsg(index));}/** * 错误信息的详解 */private String outOfBoundsMsg(int index) {return "Index: " + index + ", Size: " + size;}/** * 删除集合中等于指定集合中的元素的元素 */public boolean removeAll(Collection<?> c) {return batchRemove(c, false);}/** * 移除集合中所有未包含在指定集合中的元素 */public boolean retainAll(Collection<?> c) {return batchRemove(c, true);}/** * 移除此集合中包含着指定集合里的元素（如果存在） */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 workfor (int i = w; i < size; i++)elementData[i] = null;modCount += size - w;size = w;modified = true;}}return modified;}/** * 将集合保存为一个流，即序列化，将 容量和元素值信息写入流中 */private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {// Write out element count, and any hidden stuffint expectedModCount = modCount;s.defaultWriteObject();// 写入容量s.writeInt(size);//写入第一个元素for (int i = 0; i < size; i++) {s.writeObject(elementData[i]);}if (modCount != expectedModCount) {throw new ConcurrentModificationException();}}/** * 反序列化，将一个流转换为一个ArrayList实例. */private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {elementData = EMPTY_ELEMENTDATA;// Read in size, and any hidden stuffs.defaultReadObject();// Read in capacitys.readInt(); // ignoredif (size > 0) {// 分配集合空间根据集合长度，而不是容量ensureCapacityInternal(size);Object[] a = elementData;// 按适当顺序读入所有元素for (int i = 0; i < size; i++) {a[i] = s.readObject();}}}/** * 返回一个从index开始迭代的实例（建立在AbstractList的遍历基础上） */public ListIterator<E> listIterator(int index) {if (index < 0 || index > size)throw new IndexOutOfBoundsException("Index: " + index);return new ListItr(index);}/** * 返回一个从从0开始的迭代实例 */public ListIterator<E> listIterator() {return new ListItr(0);}/** * 返回一个迭代器实例（包含hasNext,next,remove等操作） */public Iterator<E> iterator() {return new Itr();}/** * 迭代类，对集合进行迭代处理(内部私有类) */private class Itr implements Iterator<E> {int cursor; // index of next element to returnint lastRet = -1; // index of last element returned; -1 if no suchint expectedModCount = modCount;/** * 根据下一个索引值与空间大小判断是否有下一个元素 */public boolean hasNext() {return cursor != size;}/** * 返回下一个索引对应的元素 */@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();}}/** * 判断是否改变数组结构 */final void checkForComodification() {if (modCount != expectedModCount)throw new ConcurrentModificationException();}}/** * 内部迭代类，继承Itr，实现ListIterator */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();}}}/** * 返回集合中从fromIndex到toIndex的元素构成一个新的集合 */public List<E> subList(int fromIndex, int toIndex) {//判断参数是否越界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();}}}

总结：

1.ArrayList  是基于动态数组的 可以自动增长空间容量的集合.

2.对于随机访问集合中的元素，使用arrayList 效率比较高.

3.arrayList 插入或者删除元素都将对后面的剩余元素进行移动，操作量比较大（末尾插入不需要）.

4.通过索引进行遍历速度最快（for循环list.size()），通过迭代器遍历速度最慢（Itreator）.

5.非线性安全，无法自动同步.如需线性安全，可使用vector.

6.直接删除元素（remove(Object o)）需要遍历数组，如果删除索引（remove(int index)），不需要遍历数组.